Mercury Transit on May 7, 2003


Questions and answers related to the May 7, 2003 Mercury transit.

See the images from the Mercury Transit here!

Here is a collection of Q&A's from the event, as received at ESO and answered by the astronomers there.

I know that Venus transits were/are very valuable for the determination of the size of the solar system. As far as I know the path of Venus on the Sun's surface is of different length, depending on from where on Earth you see it. In this way you can calculate Venus' distance from Sun. Is this also possible with a Mercury transit?

Thomas Gleissner, Germany

Answer from Jean-Eudes Arlot at the IMCCE (Paris) : In principle, it is possible to calculate the distance from the Earth to the Sun (1 AU) from a transit of Mercury. However, Mercury is too far from the Earth and the "parallax" (shift in position of the planet's disk caused by the different locations of the observers on the Earth) to be measured is too small. Since the measurements have, at least, the same error for both planets, the errors on the calculations will be larger for Mercury. So, in principle, it should also be possible to measure the length of 1 AU expressed in km through the parallax of the Sun itself, but this measurement never succeeded because of the even larger Earth-Sun distance.

What would the sun actually look like from the surface of Mercury (the side facing the Sun, of course)? Would it almost fill the horizon?

Kate Ray, UK

The Sun will appear much larger in the sky on Mercury. On our special page on Mercury's rotation (and "day" and "night"), we write "At the point closest to the Sun, Mercury is about 46 million km away from the central star; in the remotest orbital point, this distance is nearly 70 million km. The corresponding sizes of the solar disk in the Mercurian sky are 1.73 degrees and 1.14 degrees, respectively, that is 3.2 and 2.1 the size of the Sun as seen from Earth."

I have been observing the transit from 15:30 (Indian time). I noticed that Mercury sudenly fades away and in the next updated picture it darkens again. Why is that?


This effect is most likely due to turbulence in the Earth atmosphere or - if you have been looking through the telescope for a while - eye fatigue. In both cases, it is not a change in Mercury and/or the Sun.

I want to know the significance of this transit of the planet Mercury and also why it is happening today 7/5/03. Thank you.

Oyeneye Oluranti

The Mercury transit happens today because Mercury is located exactly on the line-of-sight from the Earth towards the Sun at this very moment. It has no particular significance except that it is a relatively rare event. You can read about this at our website at:

and the links from there.

Is it possible to have a simultaneous Mercury and Venus transit? If so, when will it happen next time (and how accurately can it be calculated) ?

Bertil Nyman

No, this is not possible at this moment, due to the different positions in space of the orbits of these two planets. However, as Mercury's orbit is slowly rotating (see the webpage on this subject at this website), it may become a possibility at a later moment. This, however, is a question for the specialists and it may take a little time to have their answer. We shall pass it on to them and let you know in due time.

Answer from Jean-Eudes Arlot and Patrice Rocher at the IMCCE (Paris): The possibility of a simultaneous transit of Mercury and Venus will occur only after 10 000 years! This is due to the relative motion of the nodes of Mercury and Venus on the ecliptic.

Je suis en train de suivre la progession de Mercure sur votre site. C'est magnifique et je vous remercie de nous fournir ces superbes images. Ce qui me frappe le plus, c'est le rapport de taille entre Mercure et le soleil. Impressionnant ! Merci et bravo !


Thanks a lot for the nice words - we are all lucky that the weather is so co-operative and you are right - Mercury is indeed a small planet!

Your coverage is very impressing and interesting. Would you tell me which one of the spot is Mercury, is the LOWER one or the UPPER one? Thanks a lot and bravo.


Mercury is the upper right one, closer to the limb of the solar disk. It appears sharper than the lower one which is a small sunspot group.

Is the configuration very different when one observes the phenomenon from different points of the Earth (from where the Sun is visible, of course) ?

Jean Michel Canaguier

No, the configuration is very much the same - there are small differences due to the slightly different lines-of-sight, but this is not easily perceptible on the available images.

The planet seems to be travelling in the opposite direction to that predicted in the information on the E.S.O. web site. It seems to be coming in at 2 oclock and travelling towards 8 oclock or sth west. Is this right!

Geoff Richardson

You are absolutely right! This effect is due to the way the image is obtained by the camera - the solar image is reversed by the telescope optics. So Mercury does indeed move as predicted. We have put a note about this on the webpage.

If we had a really good telescope/photometer and were on the order of a few light years away, would we be able to detect the Mercury transit by the light curve? What is the percentage drop in light output?

Tim Dean

The drop in luminosity would be about 1:20000, so it would be beyond the possibilities of modern observational technology.

I cannnot understand those pics on net. The dark spot is Mercury? And it seems not as fantastic as expected.

As we have stated in many places, Mercury's disk is indeed very small - but it is well visible anyhow! You will find the Venus transit next year more impressive!

I live in New York and I am wondering at what time in the day I will be able to see the transit.

Brian Linder

You may be able to see the last phase of the transit, just after sunrise Wednesday morning. However, you must have an unobstructed view of the Sun as it is near the horizon and the sky must be very clear and transparent for you to succeed.

Could you please tell me what the effects are for the planet Mercury (temperature of the surface, deformation, etc) ? Are there new things which scientists learn from these events ? Are there already questions which scientists want to be answer with this event ? Which questions are these ?

Patrick van den Berg, Netherlands

There are no particular effects on Mercury at the time of the transit today - this is only a geometrical alignment where the Earth, Mercury and the Sun happen to be on the same straight line. The solar physicists are happy to be able to observe a dark disk on the surface because it gives them the opportunity to "calibrate" their measurements of the various features on the Sun's surface, e.g. sunspots. Otherwise, the exact timings of the event will serve to verify the accuracy of the theory of planetary motions (which is already very good). If Mercury had an atmosphere like the Earth, it would have been possible to see the refraction of solar light at the border of Mercury's dark disk. However, although an atmosphere is indeed present at Mercury, it is extremely thin and it will therefore not be observable.

We are members of the local astronomical association, and we want to observe tomorrows Mercury transit. However, there are some discrepancies amounting to minutes (!) in the provided transit times. Could you maybe provide me algorithms that give me the possibility to calculate exact data with my HP 48SX programmable calculator, given our coordinates, ( 52° 14' N, 5° 16' E ) and the geocentric times ? (I hope this message will arrive in time)

Jan Boekschoten

Yes, you are right, slightly different times have been published in different places. However, the timing can in theory be predicted with very high accuracy, better than 1 second. This is because of our very good knowledge of the motions of Mercury and the Earth, relative to the Sun.

We suggest that you look at the special page at the IMCCE in Paris at:

Here, you will find the exact times for many cities - and also for some in your vicinity.

Sorry, but we cannot provide you with the software mentioned, it is not available here.

I am based in N.Scotland (57*49'N; 4*04'W) and working with a small refracting telescope and projection set up. Where on the sun's disc can I expect to see Mercury first appear?

David, UK

The position where Mercury will first appear depends only very slightly on the location of the observer. Mercury will be at about 10-11 o'clock if you assume the sun is a clock with 12 o'clock upwards. Please have a look at our web page :

What chance do I have to see the transit from Claryville NY 12725? I was hoping to catch at least the last 1/2 hr. Thanks in advance,

Catherine, USA

You should indeed be able to see the last 1/2 hr. The sun rises at 9:49 UT (5:49 a.m. local time) in Claryville, NY and the last contact of Mercury on the Sun is at 10:34 UT (6:34 a.m. LT). At that time, the Sun will be 7.35 degrees above the horizon. You can of course also see the full event directly from our web site. Please note that it is very dangerous to observe directly the sun. For direct views, a safe solar filter must cover the entire entrance aperture of the telescope. Another way is to project an image of the sun on a white surface (for instance, a piece of paper or cardboard).

At what time does Mercury move in front of the Sun ?

Davide, Italy

The exact time depends on the location. In Rome, the first contact, that is, when the closest point of the planet first touches the solar disk, takes place at 5h11m53s UT (Universal Time; this corresponds to 7h11m53s CEST), and the last contact is at 10h32m25s UT. In other locations, the exact time will be different by a few seconds up to a few minutes.

How long does the transit last ?

Mercury transits do not all last equally long. The time it takes Mercury to move across the solar disk varies - it depends on the momentary velocity of the planet in its orbit and, in particular, on the exact path across the Sun's face. A Mercury transit may last up to 9 hours. The transit of May 7, 2003 will last a little bit above 5 hours and 20 minutes.

How often does a Mercury transit occur ?

On the average, there are 13 Mercury transits each century, i.e. about one every 7 1/2 years. However, they do not occur at regular intervals, but in succession at intervals of 13, 7, 10 and 3 years. Each year, the Earth passes Mercury's line of nodes around May 7 and November 9. Mercury transits can therefore only happen around these dates. The most recent Mercury transit happened in 1999 but it is was not visible from Europe. This year, a transit takes place on May 7, 2003, and the next events occur on November 8, 2006 and May 9, 2016.