Imcce.fr - Nouvelles astronomiques

Zoom on Near-Earth Asteroids

2010-01-20

Within the Solar System, surfaces of the atmosphereless bodies are strongly influenced by factors often associated with the expression "space weathering”(*). Recent scientific evidence has shown that the aging process surface due to space weather has a very strong momentum (of the order of one million years) relative to the age of the solar system (about 4.5 billion years).

In the case of small bodies, there is a dichotomy between the asteroids that cross the orbit of the Earth and those of the Main Belt. The spectral response surfaces of a class of Near-Earth Asteroids (the taxonomic class Q) shows minerals whose physical properties are less affected by the space weathering(**). Mechanisms such as close approaches with the Earth, Mars or Venus, or collisions with other small asteroids are proposed to produce the re-surfacing of Near-Earth Asteroids.

A French-American team examined the issue of proximity between NEOs and the terrestrial planets. The spectra of a sample of one hundred asteroids have been analyzed and the results were corroborated with their orbital dynamics.

The conclusion of this research is that the Earth can produce a significant impact for the re-surfacing processes, if the asteroid passes at least a distance of 16 Earth radii from the planet. This distance is approximately one quarter of the Earth-Moon distance. Seismic waves generated by the close passage are able to "shake up" the surface of the object so that rocks and regoliths are reorganizing. The reorganization of the surface of the asteroid is revealed by the spectral measurements. Having suffered less exposure to space weather, minerals resulting from the re-surfacing will show spectra which are in better agreement with laboratory spectra of ordinary chondrite meteorites.

In 2029, the asteroid 99942 Apophis, whose diameter is estimated at about 270 meters, will pass close to Earth. This passage will take place at a distance of about 42,000 km and will not affect our planet. However, during this close passage, the asteroid will be located inside the perimeter of the theoretical limit discussed above, and vibrations will be strong enough to produce its re-surfacing. Spectroscopic observations of the close approach of 99942 Apophis will be possible in 2029, will confirm these assumptions and will contribute to the validation of these results.

(*) Space weather includes several factors such as the solar wind, the cosmic rays, and the impacts with micro-meteoroids.
(**) The spectrum of Q-type asteroids is the best analogue for most meteorites that exist in collections (ordinary chondrite meteorites).

French team: Francesca DeMeo, Sihane Merouane (Observatoire de Paris), Alessandro Morbidelli (Observatoire de la Cote d'Azur), Pierre Vernazza (ESTEC and Observatoire de Paris), Richard Binzel (MIT IMCCE, Observatoire de Paris), Mirel Birlan (IMCCE, CNRS 8028, Observatoire de Paris).

Contact: Mirel Birlan (email : Mirel.Birlan@imcce.fr)

Asteroid 2867 Steins seen by Rosetta

2010-01-18

The ESA spacecraft Rosetta en route to the comet 67P/Churyumov-Greasimenko approached on September 5, 2008 a small main belt asteroid of 4.6 km across, 2867 Steins. Images taken by the onboard camera OSIRIS show an asteroid resembling a cut diamond. Its surface is covered by shallow craters. A string of 7 small craters identical, regularly spaced, adorns one of the facets of the asteroid.

The shape is probably due to an effect of resurfacing due to the YORP effect. This effect is due to asymmetrical reradiating of solar radiation by any body of irregular shape. A particularly imposing equatorial bulge contributes to the appearance of diamond. YORP spin-up could have caused landsliding of mid-latitude surface material towards the equatorial bulge.

Le calendrier musulman et le calcul des dates du mois de Ramadan

2009-08-21

Les musulmans utilisent deux sortes de calendriers lunaires.

Un calendrier perpétuel qui est basé sur la lunaison moyenne, il alterne six mois de 30 jours et six mois de 29 jours pour les années communes de 354 jours et sept mois de 30 jours et cinq mois de 29 jours pour les années abondantes de 355 jours (les deux derniers mois ayant 30 jours). Onze années abondantes sont judicieusement réparties sur une période de trente ans parmi dix-neuf années communes. Ce calendrier suit remarquablement bien la lunaison moyenne. En effet, il se décale d'un jour par rapport à la lunaison moyenne au bout de 30902 lunaisons soit environ 2575 années lunaires. Le jour calendaire commence le soir au crépuscule.

Un calendrier religieux qui est basé sur l'observation du premier croissant de Lune. Ce calendrier est par nature local, car les conditions d'observation dépendent du lieu d'observation et de l'époque à laquelle l'observation a lieu. Ainsi les premiers croissants de Lune sont difficilement observables dans l'hémisphère nord aux fortes latitudes pour les lunaisons proches de l'équinoxe d'automne alors qu'elles sont facilement observables dans l'hémisphère sud à la même époque. On a le phénomène inverse au voisinage de l'équinoxe de printemps. La longueur du mois ne pouvant avoir plus de trente jours, la nuit commençant au soir du 29e jour est la nuit du doute. Si le croissant est visible, le mois finissant a 29 jours et le nouveau mois commence le soir même. Si le croissant n'est pas visible, le mois finissant a 30 jours et le mois suivant commence le lendemain soir. Le début du mois de rang n dépend donc du début du mois de rang n°1. La prédiction du début et de la fin du mois de jeûne de Ramadan s'appuie sur ce principe et sur des critères de visibilité du premier croissant.

Finalement c'est l'autorité religieuse, le Conseil Français du Culte Musulman, qui décide des dates du début et de fin des mois en s'appuyant sur les prédictions de visibilité ou sur la visibilité effective du croissant.

En 2009 les deux croissants de Lune, correspondant au début et à la fin du mois de Ramadan de l'an 1430 de l'Hégire, seront visibles en France métropolitaine deux jours après les nouvelles Lunes d'août et de septembre : le soir du 22 août et le soir du 20 septembre 2009. L'observation du 21 août sera possible uniquement dans le sud du pays avec des aides optiques et deviendra de plus en plus difficile au fur et à mesure que les villes seront situées plus au nord. De même l'observation du 20 septembre à l'oeil nu ne sera possible que dans le sud de la France et nécessitera une aide optique au fur et à mesure que l'on montera en latitude.

Solstice d'été

2009-06-17

Dans l´hémisphère nord le solstice d´été correspond à l´instant où la longitude apparente du Soleil est égale à 90 degrés et il marque l´entrée dans cette saison. Notre calendrier (le calendrier grégorien) est construit de manière à rester proche d´une date fixe pour le début des saisons. La date du solstice d´été en 2009 est le 21 juin à 5h 45min 32s UTC soit à 7h 45min 32s TLF (temps légal français). Dans le calendrier grégorien créé en 1582, le solstice d´été peut survenir le 19, 20, 21 ou 22 juin. Il est survenu un 20 juin en 1896 et il tombe à nouveau à cette date en 2008. Il est survenu un 22 juin en 1975 et tombera à nouveau à cette date en 2203, 2207, 2211 et 2215 puis en 2302. Le solstice d´été tombera un 19 juin en 2488 et ce sera la première fois depuis la création du calendrier grégorien. Au XXe siècle les solstice d'été sont tombés exclusivement le 21 juin (64) et le 22 juin (36) alors qu'au XXIe siècle le solstice d'été tombera exclusivement le 20 juin (47) et le 21 juin (53). Le jour du solstice d´été, le Soleil passe à midi au zénith du tropique du Cancer. Plus au nord ce phénomène ne se produit jamais. C´est le jour où pour un lieu donné de l´hémisphère nord, la durée du jour est maximum. Le solstice d´été dans l´hémisphère nord correspond au solstice d´hiver dans l´hémisphère sud.

La direction de La Mecque grâce au Soleil

2009-05-27

La Mecque est situé entre les tropiques, les coordonnées de la Ka`ba sont les suivantes : longitude : 39°49' 34,18" est et latitude : 21° 25' 21,04" nord. Deux fois par an le Soleil passe au plus près du zénith de la ville aux dates où la déclinaison du Soleil est proche de la latitude du lieu. Ces jours, à l'instant du passage au méridien du Soleil à La Mecque le Soleil joue le même rôle que l'étoile polaire vis-à-vis du pôle nord, il suffit donc pour déterminer la direction de la Mecque d'observer la direction du Soleil à l'instant de son passage au méridien de La Mecque.

Date	Heure du passage au Méridien en UTC	Hauteur du Soleil
25/05/2009	9h 17m 37,3s	89° 34' 56,4"S
26/05/2009	9h 17m 43,8s	89° 45' 24,4"S
27/05/2009	9h 17m 50,7s	89° 55' 30,4"S
28/05/2009	9h 17m 58,1s	89° 54' 45,7"N
29/05/2009	9h 18m 06,0s	89° 45' 24,3"N

Heure UTC et hauteur du Soleil des passages au méridien de La Mecque

En 2009, le jour où le Soleil passe au plus proche du zénith est le 27 mai à 9h 17m 51s UTC, il suffit d'observer la direction du Soleil à cet instant pour avoir la direction de la Mecque. C'est également valable pour les deux jours au voisinage de cette date.
On remarquera qu'après cette date le Soleil culmine au nord.

The solar wind is the cause of rapid ageing of the asteroids

2009-04-22

A European team including researchers of Paris Observatory has just highlighted the influence of the solar wind on young surfaces of asteroids. The solar wind in less than one million years ages their surface giving them a very old appearance. They also showed that the observation of near Earth asteroids having a young surface can be explained by a renewal of materials of their surface due to the gravitational tidal forces at the time of their passage close to the Earth, thus compensating for the ageing effect of the solar wind. These results are published in the Nature magazine of April 23, 2009.

Why the materials constituting the surface of the asteroids reflect in a different way the light when they are in space or in laboratory, appearing thus redder and darker in space? This different behavior is mainly due to the interaction of the interplanetary medium with the surface of the asteroids. This surface is deteriorated by the solar wind and the micrometeorites present in the interplanetary medium.

This team studied young families of asteroids formed from violent collisions and having thus a young surface. These asteroids submitted to the impacts of the interplanetary medium see their surface evolving very quickly, in approximately 1 million years, to arrive at surfaces having average colors identical to those of old asteroids. This rapid evolution supports the role of the solar wind as principal element in the transformation of their surfaces. The composition plays also a part, the asteroids including olivine evolving more quickly.

Figure 1 : Evolution of the color of asteroids as a function of their age, assuming that these objects have the same composition. The initial color of an asteroid when it has just undergone a catastrophic collision (which breaks it up into various fragments) should be similar to the color of the meteorites measured in laboratory. The observation of young asteroids (age < 1 million years) reveals that their color has been strongly modified in this short time, giving them the appearance of old surfaces. Only the solar wind (red arrow) can act so quickly on their color, as demonstrated by laboratory experiments. In a second time, micrometeorites impacts could age surfaces with a longer time scale. © Nature.

This team was also interested in the asteroids crossing the orbit of the Earth (NEA: Near-Earth Asteroids) which for some have a color which reveals a surface virgin of any deterioration. All these objects were formed by collisions since more than 100 million years. Thus, recent collisions cannot explain their apparent youth and it is necessary to find another process being able to renovate the matter on the surface. These asteroids could undergo gravitational effects with the approach of the Earth or another telluric planet (tidal effects) tending to dredge up on the surface the internal material, which was not exposed to the solar wind and thus to give this effect of ~renovation~. It will be necessary to confirm this hypothesis in observing the spectral color of the asteroids of the same type present in the principal belt, between Mars and Jupiter, which should then be redder. Reference

Solar wind as the origin of rapid reddening of asteroid surfaces
P. Vernazza, R. P. Binzel, A. Rossi, M. Fulchignoni & M. Birlan
Nature 23/04/2009.

Contact

Pierre Vernazza (ESA, et Observatoire de Paris, LESIA)
Marcello Fulchignoni (Observatoire de Paris, LESIA, et Paris VII)
Mirel Birlan (Observatoire de Paris, IMCCE, et CNRS)

To photograph the sunset on the Grand Canal in Versailles

2009-04-06

However the azimuth of the axis is not perfectly orientated East-West, it deviates from approximately 22° what changes considerably the dates when this phenomenon is observable. The altitude of the floor in front of the frontage of the palace of Versailles turned towards the Grand canal is 142m. The horizon in the direction of the Grand canal is heightened because of a hill of 128 meters altitude, located at 10650 meters.

The azimuth of the axis of the Grand canal is 111°49' 56 " , thus approximately 111°50 '; (approximately 292 ° northern). Taking into account the atmospheric refraction and these various parameters, we calculated that the center of the Sun lies down in 2008 in the axis of the Grand canal at the following dates:

Sunset in the axis of the Grand Canal	Hour of sunset (TLF)*	Azimuth
The 26th april 2009	20h 55min 26s	111° 22' 58"
The 27th april 2009	20h 56min 54s	111° 53' 14"
The 28th april 2009	20h 58min 21s	112° 23' 11"
* TLF = French legal time

More details:
Be cautious: if the horizon is badly estimated, these results can be shifted by one day or two. The same if one goes up in altitude, for example if one observes from the first floor of the castle, the results can also be shifted by one day.

Moreover these calculations were made with a certain value of the horizontal atmospheric refraction, but the refraction varies with the atmospheric conditions (pressure, temperature...) the times of the sunsets can thus vary slightly with respect to the predicted values.

We gave these values to the nearest second of time but it is only the internal precision of calculation, the precision of the prediction, taking into account the uncertainties on the refraction, is about one minute of time.

If one studies the data above, one notes that the azimuth of the center of the Sun at sunset never falls exactly into the axis from the channel, it is close to this axis for the median dates (on April 27 and on August 14), it is slightly to the south (towards the left) when the azimuth is lower than 111°49' 56 "and slightly to the north (towards the right) when the azimuth is higher than 111°49' 56".

In all the cases it is possible to take a photograph in the axis of the Grand canal but the center of the Sun will be more or less high with respect to the horizon. One can also exploit the height of the observer while going down or while going up compared to the terrace of the frontage. If one goes up, the center of the Sun will be higher, if one goes down, the center of the Sun will be lower.

Caution: If the Sun at sunset dazzles you, do not look at it directly: it is still too high on the horizon. In this case prevent photographing it without filter, you are likely to damage your camera and your sight if you use an apparatus with a reflex optics.

Paris Astronomers "Catch" Passing Asteroid

2009-03-09

The small asteroid 2009 DD45 which whizzed past Earth on Monday was "caught" by a team of astronomers working at the Paris Observatory. This close encounter asteroid was discovered on 27 February 2009, by Rob McNaught at Siding Spring Observatory, Australia. On 2 March 2009, at 15h Paris Local Time, the asteroid was passing at its closest distance from the Earth, about 0.00048 AU (approximately 64,000 km). For a short period of time of about two hours, the brightness of the object allowed spectroscopic observations.

How could a Paris Observatory team measure this asteroid, which passed the opposite side of the Earth, soaring over Tahiti? A special facility of the Observatory, the Centre d'Observation à Distance en Astronomie à Meudon (CODAM) allowed the Paris team to command the 3 metre diameter telescope IRTF, a highly specialized telescope located at Mauna Kea, Hawaii. From Meudon, the Paris team was able to use the Hawaii telescope to track down the space intruder and capture its light into the telescope's near-infrared spectrograph, SpeX.

The resulting spectral colors, measured over the infrared wavelengths 0.8-2.5 micron, revealed characteristic signatures of the minerals olivine and pyroxene (commonly found in both meteorites and Earth rocks), placing the asteroid into a category that astronomers refer to as the S-class. By knowing this class, the team can associate 2009 DD45 with other small S-class asteroids which have measured values for their reflectivity; about 36% of the sunlight hitting the asteroid is reflected (results from Delbo et al. 2003, Icarus 166, 116).From the brightness of the asteroid, and its reflectivity, the Paris team could deduce the mean diameter to be 19 +/- 4 meters.

Contact persons:

Richard Binzel (Massachusetts Institute of Technology, invited researcher LESIA, IMCCE, Observatoire de Paris)

Francesca DeMeo (LESIA, Observatoire de Paris)

Mirel Birlan (IMCCE, Observatoire de Paris)

Summer time

Mon, 09 Mar 2009 00:00:00 +0100

In accordance with the decree of April 3rd, 2001 of the Ministry for the economy, Finances and of Industry, relating to the French standard time, the period from summer time for the year 2009 begins last Sunday from March to 2a.m. morning. Therefore, in the night on March 28th to 29th 2009, at 2a.m. it will be 3a.m.

Spring equinox

2009-03-09

In the Northern hemisphere, the spring equinox corresponds to the moment when apparent longitude of the Sun is equal to 0 degree (the direction of the Sun is then that of the gamma point, origin of celestial longitudes). Our calendar (the Gregorian calendar) is built so as to preserve a fixed date for the seasons beginning. The date of spring equinox is, in 2008, March 20th, at 11h 44min UTC (12h44min Paris standard time).

Since creation of the Gregorian calendar (1582) the spring equinox is on March 19th, 20th or 21th. At XIXth and XXth centuries it always is on March 20th or 21th. In the past, it was on March 19th in 1652, 1656, 1660, 1664, 1668, 1672, 1676, 1680, 1684, 1685, 1688, 1689, 1692, 1693, 1696, 1697, 1780, 1784, 1788, 1792 and 1796. It will again fall on March 19th in 2044. At the equinox day, if we ignore atmospheric refraction, duration of the night is equal to the duration of the day. It is also the day when Sun rises full East and lie down full West. The spring equinox in the Northern hemisphere corresponds to the autumn equinox in the Southern hemisphere.

Comète LULIN

2009-02-09

Lulin sera la comète la plus brillante de l'année, elle sera facilement observable à la fin du mois pendant la nouvelle Lune. La comète a été initialement décrite comme un astéroïde lors de sa découverte le 11 juillet 2007 par Chi-Sheng Lin avec le télescope de 41cm de l'observatoire de Lulin (Nantou, Taiwan). Le 17 juillet J.Young (Table Mountain Observatory, Californie) a noté une coma de 3 secondes de degré avec un noyau central brillant.

Cette comète va passer à 0,4 UA de la Terre le 24 février 2009, elle aura alors une magnitude proche de 5 et sera donc visible à l'oeil nu. La meilleur période d'observation est centrée sur cette date qui correspond à la fois à son opposition et à la nouvelle Lune. Elle sera alors observable toute la nuit dans la constellation du Lion. L'observation sera facilitée par l'utilisation de jumelle. Elle restera visible durant le printemps. Comme son orbite est proche de l'écliptique, une anti-queue, composée de poussière éjectée par la comète, sera visible.

Lever du Soleil dans l'axe de l'arche de l'Arc de Triomphe

2009-02-02

Comme chaque année début février, il est possible d'observer le lever du Soleil juste sous l'arche de l'Arc de Triomphe. Cette observation se fait depuis l'avenue de la Grande Armée. Le seul lieu propice à cette observation est le terre-plein situé au centre du square de la porte Maillot. Depuis ce lieu, le diamètre apparent du Soleil est lègèrement plus petit que le diamètre apparent de l'arche de l'Arc de Triomphe, mais si l'on s'éloigne plus de l'Arc de Triomphe, la ligne de visée dans la direction de l'arche recontre des feux de signalisation disgracieux.
Le tableau ci-dessous fournit les dates et instants de prises de vue du Soleil dans l'axe de l'arche de l'Arc de Triomphe depuis le square de la Porte Maillot. Les instants sont en Temps universel. Ajouter une heure pour avoir l'heure légale en France métropolitaine.
ATTENTION : même à faible altitude l'observation directe du Soleil peut être dangereuse pour la vue (et pour votre appareil photo). Pour les photos, il y a risque de surexposition, pensez à prendre un filtre si votre appareil photo ne permet pas de faire de très courtes expositions.

Jour	Instant du lever du centre du Soleil (UTC)	Période où le centre du Soleil passe par l'axe de l'arche (UTC)	Variation de la hauteur du centre du Soleil durant cette période (UTC)
04/02	7h 25m 02s	7h 26m 18s à 7h26m 38s	9' 52" à 12' 29"
05/02	7h 23m 31s	7h 27m 22s à 7h 27m 42s	30' 20" à 33' 00"
06/02	7h 21m 59s	7h 28m 26s à 7h 28m 46s	51' 29" à 54' 11"
07/02	7h 20m 25s	7h 29m 30s à 7h 29m 50s	1° 13' 16" à 1° 16' 00"

Webcast from Novosibirsk of the total eclipse of the Sun on August 1, 2008

2008-07-31

Partial eclipse of the Moon on 2008, August 16th

2008-07-17

This partial eclipse by the umbra will be visible partly from France, starting from the begining of the umbra till the end of the penumbra. Only the begining of the penumbra will not be visible. This table give the general circumstances of this eclipse. Time is given in Coordinated Universal Time (add 2 hours to get the French legal time).

To photograph the sunset on the Grand Canal in Versailles

2008-07-17

One reads sometimes in some journals that the axes of the Grand canal of the palace of Versailles are orientated East-West and North-South. Some deduced, with good reason, that the Sun was to lie down in the axis of the Grand canal at the equinoxes. However the azimuth of the axis is not perfectly orientated East-West, it deviates from approximately 22° what changes considerably the dates when this phenomenon is observable. The altitude of the floor in front of the frontage of the palace of Versailles turned towards the Grand canal is 142m. The horizon in the direction of the Grand canal is heightened because of a hill of 128 meters altitude, located at 10650 meters. The azimuth of the axis of the Grand canal is 111°49' 56 " , thus approximately 111°50 '; (approximately 292 ° northern). Taking into account the atmospheric refraction and these various parameters, we calculated that the center of the Sun lies down in 2008 in the axis of the Grand canal at the following dates: Sunset in the axis of the Grand Canal Hour of sunset (TLF)*Azimuth The 13th August 2008 21h 5m 14s112° 18' 24" The 14th August2008 21h 3m 28s111° 48' 54" The 15th August 2008 21h 1m 40s111° 19' 08" * TLF = French legal time More details: Be cautious: if the horizon is badly estimated, these results can be shifted by one day or two. The same if one goes up in altitude, for example if one observes from the first floor of the castle, the results can also be shifted by one day. Moreover these calculations were made with a certain value of the horizontal atmospheric refraction, but the refraction varies with the atmospheric conditions (pressure, temperature...) the times of the sunsets can thus vary slightly with respect to the predicted values. We gave these values to the nearest second of time but it is only the internal precision of calculation, the precision of the prediction, taking into account the uncertainties on the refraction, is about one minute of time. If one studies the data above, one notes that the azimuth of the center of the Sun at sunset never falls exactly into the axis from the channel, it is close to this axis for the median dates (on April 27 and on August 14), it is slightly to the south (towards the left) when the azimuth is lower than 111°49' 56 "and slightly to the north (towards the right) when the azimuth is higher than 111°49' 56". In all the cases it is possible to take a photograph in the axis of the Grand canal but the center of the Sun will be more or less high with respect to the horizon. One can also exploit the height of the observer while going down or while going up compared to the terrace of the frontage. If one goes up, the center of the Sun will be higher, if one goes down, the center of the Sun will be lower. Caution: If the Sun at sunset dazzles you, do not look at it directly: it is still too high on the horizon. In this case prevent photographing it without filter, you are likely to damage your camera and your sight if you use an apparatus with a reflex optics.

The sun under the Arc de Triomphe

2008-07-17

Each year in May, photographing the Sun lying down under the arc of Triumphal arch is possible. When we are at the right distance from the Triumphal Arch, the apparent diameter of the arch is equal to the apparent diameter of the Sun. For sunset, two specific locations are in general chosen, Place de la Concorde and Place Clemenceau. As seen from Place de la Concorde the apparent diameter of the arch is a little too small. The Sun, too large, is very slightly masked by the arch, but we benefit from a photograph catching the whole Champs-élysées. On the other hand, the second position, from Place Clemenceau, gives an apparent diameter of the arch nearly identical to the apparent diameter of Sun. It will be possible to photograph the Sun from the Rond-point des Champs-élysées Clemenceau on August 2nd, 3rd and 4th at the evening.

Dates	Sunset at Paris
August 2nd 2008	at 21h 18,9m standard time
August 3rd 2008	at 21h 17,4m standard time
August 4th 2008	at 21h 15,9m standard time

Calculation of the visibility of the first crescent of the Moon in August-September 2008

2008-07-17

For France in 2008, the two crescents of the Moon will be visible in metropolitan France two days after the new Moons on August and September, at evening on September 1st and at evening on October 1st . The observation on September 1st will be possible with the help of optics and will be more difficult when the observer will be higher in the North. Possible observation with naked eye will occur only under optimum conditions.

Earth in aphelion on July 4th

2008-07-02

The orbit of the Earth-Moon barycentre around the Sun is, at first approximation, an ellipse. The Earth-Sun distance is not constant and presents a minimum (perihelion) and a maximum (aphelion). The Earth will pass to its aphelion on July 4th, 2008 at 7h 40min 54s UTC. The Earth-Sun distance will be then of 1,016753512046ua is exactly 152,104160419735Mkm . This maximum value is not the same year after year because the Earth orbit is not a perfect ellipse but an ellipse perturbed by the gravitational effects. It can appear paradoxical that this maximum distance from the Sun is reached whereas we are in summer and the weather is hot in our hemisphere. This is due to the inclination of the rotation axis of the Earth with respect to the perpendicular of its orbit plan (the ecliptic). At the summer solstice (on June 20th in 2008), a date close to the passage of the Earth at aphelion, the Sun passes to the zenith of the tropic of Cancer. In Northern hemisphere the elevation of the Sun on the horizon and the duration of its illumination are then maximum. This effect contributes more to the climate than the distance to the Sun. The seasons are reversed between Northern hemisphere and Southern hemisphere. However it is false to believe that for our hemisphere, the fact that the Earth is at aphelion in summer and at perihelion in winter (towards January the 4th) would moderate the contrast of the seasons then would amplify it in southern hemisphere. It is also erroneous of saying that the day of the summer solstice is the day when the Sun culminates with its higher elevation in the Northern hemisphere! That is true only for the places located at the North of the Cancer tropic. The date of passage of the Earth in perihelion and that of passage in aphelion advance in our calendar. In this way in approximately 9.800 years, the Earth will pass in perihelion on June 21th at the time of the summer solstice.

Summer solstice on June 20th

2008-06-10

In the northern hemisphere the summer solstice corresponds to the date when the apparent longitude of the Sun is equal to 90 degrees and it marks the beginning of this season. Our calendar (the Gregorian calendar) is built so as to remain close to a fixed date for the beginning of the seasons. The date of summer solstice in 2008 is on June 20th at 23h 59min UTC. In the Gregorian calendar created in 1582, the summer solstice can occur on June 19th, 20th, 21th or 22th. It occurred on June 20th in 1896 and fall again on this date in 2008. It occurred on June 22th in 1975 and will fall again on this date in 2203, 2207, 2211 and 2215 then in 2302. The summer solstice will fall on June 19th in 2488 and it will be the first time since the creation of Gregorian calendar. On the day of the summer solstice, for an observer located on the tropic of Cancer, the direction of the Sun points toward the zenith at twelve o'clock. Farther to the North this phenomenon never occurs. The day of solstice is the day when for a given location of the Northern hemisphere, the duration of the day is maximum. The summer solstice in the Northern hemisphere corresponds to the winter solstice in the Southern hemisphere.

Publication of the IMCCE scientific and technical Note S093

2008-06-02

The IMCCE scientific and technical Note S093 ''Some uncompleted problems of newtonian and relativistic celestial mechanics'' is published. In that note V.A. Brumberg makes a review of some problems of Newtonian and relativistic celestial mechanics to be regarded as uncompleted problems worthy of further investigation. These problems include :

- the construction of general solution of the three-body problem by means of the series of polynomials,

- the construction of theories of motion using the fast converging elliptic function expansions,

- the representation of the rotation of the planets in the form compatible with the general planetary theory,

- the relativistic extansion of the newtonian theories of motion and rotation, - the simplification of relativistic celestial mechanics and astrometry in the post--Newtonian approximation by using the linearized metric of general relativity,

- the motion of the solar system bodies at the cosmological background.

Update of the lectures on relativity in celestial mechanics and astrometry.

2008-06-02

V.A. Brumberg has improved and updated his lectures on relativity in celestial mechanics and astrometry. The aim of these lectures is to prepare students for work in the field of relativistic celestial mechanics and aastrometry. They can be found on the website of the IMCCE to the rubric ''Courses of Astronomy''.

The sun under the Arc de Triomphe

2008-05-05

Each year in May, it is possible to photograph the Sun lying down under the arc of Triumphal arch. When we place at the good distance compared to the Triumphal Arch, the apparent diameter of the arch is equal to apparent diameter of Sun.
For sunset, two particular locations in general are chosen, the Place de la Concorde ans the Place Clémenceau. Seen since the Place de la Concorde the apparent diameter of the arch is a little too small, the Sun, too much large, is very slightly masked by the arch, but we benefit from a photograph tallied on the whole of the Champs-élysées. On the other hand to height of the Place Clémenceau, the second position gives apparent diameter of the arch quasi-identical to the apparent diameter of Sun.
It will be possible to photograph the Sun since the place de la Concorde on May 6th and 7th at the evening, since the Place Clémenceau on May 7th, 8th and 9th. Be careful before May 8th the arch contains a large three color flag in seen commemoration of May 8th.

Dates	Sunset at Paris
May 7th 2008	at 21h 14m standard time
May 8th 2008	at 21h 15m standard time
May 9th 2008	at 21h 16m standard time

Summer time

2008-02-25

In accordance with the decree of April 3rd, 2001 of the Ministry for the economy, Finances and of Industry, relating to the French standard time, the period from summer time for the year 2008 begins last Sunday from March to 2a.m. morning. Therefore, in the night on March 29th to 30th 2008, at 2a.m. it will be 3a.m.

Spring equinox

2008-02-25

To photograph the sunset on the Grand Canal in Versailles

2008-02-25

One reads sometimes in certain reviews that the axes of the Large Channel of the Castle of Versailles are directed East-West and North-South. Some in it is deduced, with good reason, that the Sun was to lie down in the axis of the large channel the days of the equinoxes. However the azimuth of the axis is not perfectly East-West, it deviates some from approximately 22° what changes the dates considerably where this phenomenon is observable. The altitude of the floor in front of the frontage of the castle of Versailles turned towards the Large Channel is of 142m, the horizon in the direction of the large channel elevated because of a hill of 128 meters altitude is located at 10650 meters. The azimuth of the axis of the Large Channel is of 111°49' 56 "is approximately 111°50 '; (approximately 292 ° northern). By holding account of the atmospheric refraction and these various parameters we calculated that the center of the Sun lies down in 2005 in the axis of the large channel at the following dates:

Sunset in the axis of the Grand Canal	Hour of sunset (TLF)*	Azimuth
The 26th April 2007	20h 54min 16s	111° 02' 39"
The 27th April2007	20h 55min 44s	111° 33' 02"
The 28th April 2007	20h 57min 11s	112° 03' 06"
* TLF = French legal time

Some precise details:
If the horizon were badly estimated these results can be shifted one day or two. The same if one goes up in altitude, for example, if one observes since the first stage of the castle, the results can also shift one day. It would be advisable to take photographs to check these forecasts. Moreover these calculations were made with a certain value of the horizontal atmospheric refraction, but the refraction varies with the atmospheric conditions (pressure, temperature...) the moments of sleeping can thus vary slightly compared to the predicted values. We gave these values to the second of time near but it is only the internal precision of calculation, the precision of the prediction counts held of uncertainties on the refraction is of about a minute of time.

If one studies the data above one notes that the azimuth of the center of the Sun to its sleeping never falls exactly into the axis from the channel, it is nearest for the median dates (on April 27 and on August 15), it is slightly in the south (towards the left) when the azimuth is lower than 111°49' 56 "and slightly in north (towards the line) when the azimuth is higher than 111°49' 56". In all the cases it is possible to take a photograph in the axis of the large channel but the center of the Sun will be more or less high compared to the horizon. One can also exploit the height of the observer while going down or while rising compared to the terrace of the frontage. If one goes up, the center of the Sun will be higher, if one goes down, the center of the Sun will be lower.

Caution:
If the Sun with its sleeping dazzles you, do not look at it directly, it is that it is still too high on the horizon. In this case prevent photographing it without filter, you are likely to damage your camera and your sight if you use an apparatus with aiming reflex.

Agenda astronomique 2008

2007-12-06

This diary is original in its kind! In addition to a traditional diary, it is the daily tool of all the astronomers amateurs or all those who have a passion for the world of astronomy. Every day we can know what are the great space projects, who is Leon Foucault or when the next Moon eclipses occur...
Richly illustrated and equipped with many star maps, it is a practical tool, learned... and which makes dreaming!
Size 15x21. 154 pages. This diary is available on library and at followind address :

" Agenda Astronomique 2008"
Éditeur : EDP Sciences
7 Avenue du Hoggar
Z.I. de Courtaboeuf
B.P. 112
F-91944 LES ULIS Cedex A
176 pages
Price: 12 euros
ISBN : 978-2-7598-0026-1

Publication of Guide de Données Astronomiques 2008 - Annuaire du Bureau des Longitudes

2007-12-06

The Guide de Données Astronomiques 2008 pour l'observation du ciel-Annuaire du Bureau des Longitudes has just appeared to the EDP Sciences editor. These annual ephemerides, elaborated by the Institute de Mécanique Céleste et de Calcul des Éphémérides, are published by the Bureau des Longitudes since 1796. They contain data on the calendars, the time scales and astronomical calculations. Tables, to the use of both the astronomers amateurs and the professional astronomers, give the Sun and Moon co-ordinates, the sunrise and sunset hours, the positions of planets, the satellites, the asteroids and comets as well as data on the eclipses and some other astronomical phenomena (size 15,5 x 24 cm. 384 pages)

Since 2003, this work contains a thematic notebook. It is this year about a text on "Les multiples étoiles doubles" by F. Arenou, Observatoire de Paris.

This work is available at the following address :

"Guide de Données Astronomiques 2008"
"Annuaire du BdL"
Éditor : EDP Sciences
7 Avenue du Hoggar
Z.I. de Courtaboeuf
B.P. 112
F-91944 LES ULIS Cedex A
384 pages
Prix: 29 euros
ISBN : 978-2-7598-0027-8

2008 Nautical Ephemerids

2007-12-06

These ephemerides are intended to the use by navigators. They are published by the Bureau des longitudes since 1889. They give the declinations and right ascensions of Venus, Mars, Jupiter and Saturne (hour per hour, to the tenth of minute). They give also the hours of risings and sets of the Sun and the Moon for latitudes ranging between 70 degrees North and 56 degrees South. Usually used by the sailors to take stock at sea, these ephemerides are compulsory for navigation of the high seas.

Éphémérides Nautiques 2008

Éditor: Edinautic
13 rue du Vieux-Colombier
F-75006 PARIS
Prix : 42 euros
Format 16x24. 542 pages.
ISBN : 2-9522092-3-5

Winter solstice

2007-12-04

The date of the winter solstice is in 2007 at 6h 7min UTC on December 22 (7h 7min in winter french time). In the northern hemisphere, the winter solstice corresponds to the moment when the apparent longitude of the Sun is equal to 270 degrees. This date is that of the beginning of the winter. Our calendar (the Gregorian calendar) is built so that the seasons always start at the same time. Since the creation of the Gregorian calendar (1582) the winter solstice falls the 20, the 21, the 22 or on December 23. It in general falls the 21 or on December 22. It fell one December 23 in 1903 and will fall again on this date into 2303, 2307, 2311 and 2315. It fell one December 20 in 1664, 1668, 1672, 1676, 1680, 1684, 1688, 1692, 1696 and 1697 and will fall again on this date in 2080, 2084, 2088, 2092, 2096, 2492 and 2496.

Earth at perihelion

2007-12-04

The orbit of the barycenter Earth-Moon around the Sun is in first estimate an ellipse. The distance Earth-sun is not thus constant and present a minimum (the perihelion) and a maximum (the aphelion). The Earth is going to pass a its perihelion on January 2nd, 2008 at 23h 51min UTC. The distance Earth-sun will be then of 0.98328 AU ( astronomical units) that is 147 096 593km. This minimum value is not the same from one year to the next because the orbit of the Earth is not a perfect ellipse but an ellipse perturbed by the gravitational effects. We shall notice that the distance Earth-sun has not enough effect on the phenomenon of the seasons which is essentially due to the slope of the Earth rotation axis on its orbit.

The Quadrantids

2007-12-04

Like each year in January, the Earth on its orbit is going to meet the Quadrantids swarm. It was observed for the first time in 1825 and is supposed to have been ejected from an asteroid called 2003 EH1. This swarm will be visible on January 2008, between the 1st and 5th. The maximum of the meteor shower is awaited for January 3rd. Its radiant has as co-ordinates 15h 20m in right ascension and 49 degrees in declination, it is located in the constellation of the Herdsman. This radiant is circumpolar and thus observable all the night under our latitudes. The relative velocity of the meteors to the Earth will be about 41 km/S, and the zenithal hourly rate can reach 120 meteors/h.

Clock change back to winter

2007-10-02

In accordance with a directive of the European Parliament and of the European Council on January 19, 2001 concerning the provisions related to the summer-time, the period of summer-time for year 2007 ends on the last Sunday of October at 3 a.m. in the morning. Thus, in the night from October 27, 2007, to October 28, 2007, at 3 a.m in the morning it is necessary to put the clocks back to 2 o'clock.

Autumnal equinox

2007-09-21

In the northern hemisphere, the autumnal equinox corresponds to the moment when the apparent longitude of the Sun is equal to 180 degrees. Our calendar (the Gregorian calendar) is built to preserve a fixed date for the beginning of the seasons. The date of the autumnal equinox is in 2007 on September 23 at 9:50 UT (on September 23 at 11h: 50 French standard time). Since the creation of the Gregorian calendar (1582) the autumnal equinox falls on the 21st, 22nd, 23rd or 24th of September. Generally it falls on the 22nd or 23rd of September. It will fall on September 21st in 2092 and it will be the first time since the creation of the Gregorian calendar. That will reproduce in 2096, and then it will be necessary to wait the year 2464 so that it falls on September 21st again. It fell one September 24th in 1803, 1807, 1903, 1907, 1911, 1915, 1919, 1923, 1927 and 1931, and will fall again on this date in 2303 and that will be the last time. The day of the equinox, if we disregard atmospheric refraction, the duration of the night is equal to the duration of the day. It is also the day when the Sun rises due east and sets due south. The autumnal equinox in the Northern hemisphere corresponds to the spring equinox in the Southern hemisphere.

The sixtieth satellite of Saturn

2007-07-25

During these last years many small satellites of the big planets were discovered. A recent circular of the International Astronomical Union (IAUC 8857) claims the discovery of the sixtieth Saturn satellite by the CASSINI Image Science team (Space Science Institute, Boulder, USA). This small object which would be of a diameter of approximately 1 km, was discovered on images acquired on 30 May, then found on previous CASSINI observations distributed between June 2004 and June 2007. Its orbit lies between that of the satellite S XXXII Methone and that of the satellite XXXIII Pallene. The semi major axis of its orbit is 197 700 km, its eccentricity is about 0.001, its inclination 0.1 degree and its orbital period is 1.03650 days. The provisional denomination is S/2007 S4.

Earth in aphelion on July 6th

2007-07-03

The orbit of the Earth-Moon barycentre around the Sun is, at first approximation, an ellipse. The Earth-Sun distance is not constant and presents a minimum (perihelion) and a maximum (aphelion). The Earth will pass to its aphelion on July 6th, 2007 at 23h 52 min UTC. The Earth-Sun distance will be then of 1.01670594387ua is exactly 152 097044,24 km. This maximum value is not the same year after year because the Earth orbit is not a perfect ellipse but an ellipse perturbed by the gravitational effects. It can appear paradoxical that this maximum distance from the Sun is reached whereas we are in summer and the weather is hot in our hemisphere. This is due to the inclination of the rotation axis of the Earth with respect to the perpendicular of its orbit plan (the ecliptic). At the summer solstice (on June 21th in 2007), a date close to the passage of the Earth at aphelion, the Sun passes to the zenith of the tropic of Cancer. In Northern hemisphere the elevation of the Sun on the horizon and the duration of its illumination are then maximum. This effect contributes more to the climate than the distance to the Sun. The seasons are reversed between Northern hemisphere and Southern hemisphere. However it is false to believe that for our hemisphere, the fact that the Earth is at aphelion in summer and at perihelion in winter (towards January the 4th) would moderate the contrast of the seasons then would amplify it in southern hemisphere. It is also erroneous of saying that the day of the summer solstice is the day when the Sun culminates with its higher elevation in the Northern hemisphere! That is true only for the places located at the North of the Cancer tropic. The date of passage of the Earth in perihelion and that of passage in aphelion advance in our calendar. In this way in approximately 9.800 years, the Earth will pass in perihelion on June 21th at the time of the summer solstice.

Summer solstice on June 21th

2007-05-24

In the northern hemisphere the summer solstice corresponds to the date when the apparent longitude of the Sun is equal to 90 degrees and it marks the beginning of this season. Our calendar (the Gregorian calendar) is built so as to remain close to a fixed date for the beginning of the seasons. The date of summer solstice in 2007 is on June 21th at 18h 6m UTC. In the Gregorian calendar created in 1582, the summer solstice can occur on June 19th, 20th, 21th or 22th. It occurred on June 20th in 1896 and will fall again on this date in 2008. It occurred on June 22th in 1975 and will fall again on this date in 2203, 2207, 2211 and 2215 then in 2302. The summer solstice will fall on June 19th in 2488 and it will be the first time since the creation of Gregorian calendar. On the day of the summer solstice, for an observer located on the tropic of Cancer, the direction of the Sun points toward the zenith at twelve o'clock. Farther to the North this phenomenon never occurs. The day of solstice is the day when for a given location of the Northern hemisphere, the duration of the day is maximum. The summer solstice in the Northern hemisphere corresponds to the winter solstice in the Southern hemisphere.

To photograph the sunset on the Grand Canal in Versailles

2007-03-20

Sunset in the axis of the Grand Canal	Hour of sunset (TLF)*	Azimuth
The 26th April 2007	20h 54min 16s	111° 02' 39"
The 27th April2007	20h 55min 44s	111° 33' 02"
The 28th April 2007	20h 57min 11s	112° 03' 06"
* TLF = French legal time

A new triple asteroid discovered in the Main Belt

2007-03-09

Researchers from IMCCE and Berkeley University have discovered a second moon orbiting the asteroid 45 Eugenia. This body was the first around which a satellite, named Petit-Prince, was observed from the ground in 1998.

This new result has been obtained by analyzing three images acquired in February 2004 from the 8 m VLT telescope "YEPUN" at the European Southern Observatory in Chile. This second moon is provisonaly named S/2004 (45) 1. Its size is about 6 km.

After the discover of the triple asteroid 87 Sylvia, 45 Eugenia is the second triple asteroid discovered between Mars and Jupiter in the Main Belt.

Spring equinox

2007-02-26

n the Northern hemisphere, the spring equinox corresponds to the moment when apparent longitude of the Sun is equal to 0 degree (the direction of the Sun is then that of the gamma point, origin of celestial longitudes). Our calendar (the Gregorian calendar) is built so as to preserve a fixed date for the seasons beginning. The date of spring equinox is, in 2006, March 21th, at 0h 7min UTC (1h7min Paris standard time).

Summer time

2007-02-26

In accordance with the decree of April 3rd, 2001 of the Ministry for the economy, Finances and of Industry, relating to the French standard time, the period from summer time for the year 2007 begins last Sunday from March to 2a.m. morning. Therefore, in the night on March 24th to 25th 2007, at 2a.m. it will be 3a.m.

Lyrides as beginnings of meteor shower

2007-02-26

After almost three months of weak activity, the ?shooting stars? are back. Lyrides constitute the first substantial meteoritic shower of spring and announce the return of the beautiful days. With usual activity of approximately 20 meteors per hour, and exceptionally of more than 100, it is one of the rare meteors shower associated to the comet at long period. It is about C/Thatcher, observed for the last time in 1861. Dominique François Jean Arago (1786-1853) then directing of the observatory of Paris, recognized maximum of Lyrides activity on April 22th of each year. We observe them after 21h (standard time) between the 16 and the 25. Their radiant is located near the circumpolaire star Vega of the Lyre constellation, from where them name. They are shooting stars enough fast, with a relative speed of 49 km/S.

New version of the ephemerides «Connaissance des Temps »

2007-01-22

IMCCE provides annually the ephemerides «Connaissance des Temps ». These ephemerides, created in 1679 by Joachim Dalancé, are published under the responsibility of the "Bureau des longitudes" since 1795. Therefore it results from a historical long line.
These data are published for the use by astronomers, professors and students. This book is divided into two parts. The first part gives the current state of knowledge on the fundamental astronomical constants, scales of time, reference systems, rotation of the Earth, coordinates changing as well as explanations necessary to the calculation of the predictions. The second part gives, for the current year, the positions of the Sun, the Moon, the planets and the main satellites. Furthermore an elaborate software is provided on CDROM and gives accurate positional ephemerides, as well as the hours of rises and sunsets and the dates of the phenomena of the Galilean satellites... Starting from 2007, the ephemerides of the Sun, the planets and the Moon are issued from the highly accurate numerical model INPOP06 adjusted on the most recent observations. Format 17,5x24. 368 pages.

Editor :: EDP Sciences
7 Avenue du Hoggar
Z.I. de Courtaboeuf
B.P. 112
F-91944 LES ULIS Cedex A
Price : 37 euros
ISBN : 978-2-86883-963-3