Orwell Astronomical Society (Ipswich)
Transit of Mercury, 07 May 2003
A transit of Mercury occurred on 07 May 2003. This was the first such event visible from the UK since 10 November 1973, so it aroused much interest amongst amateur astronomers. The majority of members of OASI who observed the transit did so from Orwell Park Observatory; only three observers provided observing reports from other locations. The observing reports are summarised below.
A transit of Mercury can be seen when the planet is at inferior conjunction (in line between the Earth and the Sun). However, a transit does not occur at every inferior conjunction because the orbit of Mercury is tilted at 7.0° to the ecliptic (the plane of the Earth’s orbit around the Sun); a transit can only be seen when inferior conjunction coincides with the planet being near one of the nodes of its orbit. The nodes are the points where the orbit crosses the ecliptic as follows:
Mercury’s orbit is notably eccentric (in fact it is the most eccentric of all the planets). Perihelion occurs on 09 November and aphelion on 18 May. As a result, when a transit occurs close to Mercury’s ascending node, the planet is close to its nearest point to the Sun so appears relatively small; conversely, when a transit occurs close to Mercury’s descending node, the planet is at its furthest from the Sun so appears relatively large. The average apparent diameters of the planetary disk are 10.0 and 12.0 arcsec respectively. The effect of perspective means that transits of Mercury in November (near perihelion) are visible over a wider range of dates than those in May; this results in there being 1.8 times as many transits in November as in May, in the long run. (Note that the above considerations are valid for epochs close to 2003. In the long term, secular variations in the orbit of Mercury mean that the nodes, aphelion and perihelion of the orbit of the planet gradually evolve, slowly shifting the key dates and relative frequencies.)
James Appleton, Bill Barton, Garry Coleman and Martin Cook were first to arrive at Orwell Park Observatory, assembling in the car park and opening the observatory at 04:45 UT. Upon their arrival and for some time thereafter, weather conditions were very poor with unbroken thick cloud completely blotting out the Sun – see figure 1. While waiting to see if the cloud would clear, the would-be observers' conversation turned to the observing report published by John Isaac Plummer, Colonel Tomline’s astronomer, of the transit of Mercury of 06 May 1876 which he had observed at Orwell Park Observatory. Plummer reported his observations in the May 1878 MNRAS  as follows:
My wife [Marion Plummer], who assisted me at the observation, first detected the planet against the solar corona, though I had previously looked for it in vain.
On taking my station at the telescope a minute later I also detected it, dimly visible, but sufficiently so to direct my attention to the exact point where contact was to take place, and to lead me to suppose that, but for the clouds through which the observation was made, it would have been fairly conspicuous.
The four thwarted observers of 1st contact at Orwell Park Observatory some 127 years later were incredulous of Plummer's report, given that the solar corona is generally invisible through being completely drowned out by the solar photosphere. They noted this as an area for potential future research...
First contact (the disk of Mercury first touches the solar disk) on 07 May 2003 occurred at 05:11 UT and was not visible due to the thick cloud. As the Sun continued to rise in the sky, the cloud cover gradually diminished and it became possible to obtain images intermittently during breaks in the cloud. Figure 2, a photograph of an image projected by the Tomline Refractor onto a sheet of white card, was taken at 05:15 UT during a break in the cloud. It shows Mercury shortly after ingress onto the solar disk together with a large sunspot group.
By 06:00 UT the cloud cover had much reduced, and the remaining haze rapidly disappeared. As sky conditions improved, the difference between Mercury and the sunspot became increasingly obvious. Mercury appeared as single, round spot that was positively black, much darker than the umbra of the sunspot; in addition, the sunspot had a distinctly elongated shape with a clearly differentiated umbra and penumbra. By approximately 06:15 UT, cloud cover had diminished sufficiently to enable the taking of photographs at the eyepiece of the Tomline Refractor, stopped down to 180 mm diameter and with a Baader filter over the object glass. Figure 3, taken by this method, shows learly the difference between Mercury and the sunspot. Figure 4 (obtained by projection) shows a close-up of the sunspot and figure 5 (taken by afocal method) shows a close-up of Mercury.
By 06:30 UT a steady stream of members of OASI began arriving at Orwell Park Observatory, bringing a variety of digital and traditional cameras and video recorders. The assembled observers used afocal and projection imaging and a video eyepiece displaying the transit on a TV screen. Figure 6, taken at this time, shows Martin Cook struggling to attach a digital stills camera to a bracket mounted on the Tomline Refractor for afocal imaging. Figure 7 shows an observer using a shroud to cover his face and head at the eyepiece end of the telescope in order to improve the contrast of the image. Some 17 members of OASI and 40 schoolchildren and teachers observed the transit throughout the morning and, in order to impress upon the pupils the danger of observing the Sun through a telescope without a certified solar filter, David Payne focussed the Sun’s rays onto a piece of newspaper held behind the eyepiece, causing it to smoulder and burst into flames within a few seconds – see figure 8.
Figure 9, captured by the afocal method, shows Mercury approaching the egress phase of the transit. As the time of egress approached, the observers decided, based on experience of the event thus far, that the best method to observe would be to project the image using a 50 mm eyepiece onto a sheet of white card. The observers positioned a radio controlled digital clock, synchronised to the UK Rugby time signal, close to the projected image to facilitate estimates of the times of 3rd and 4th contact. (See below for estimates of contact times.) Figure 10 shows Mercury close to egress and the digital clock.
During most of the event, Bill Barton observed using his Hα telescope on one of the balconies at Orwell Park Observatory. The equipment gave a fine view of a solar prominence in addition to the transit. Figures 11 and 12 show observers using Bill's telescope.
The first hour or so of the transit was not visible as the Sun was obscured by a neighbouring house and the sky was too hazy. However, once the Sun became visible from my location, every so often I recorded a movie clip using a Meade ETX90 and a Philips ToUcam and then later stacked the best frames using Registax to make a still. I then mosaiced these using Photoshop: see figure 12.
Yes I saw the transit of Mercury! At least some of it. I projected the Sun with a 90 mm instrument when I got up at 07.30 UT - rather later than some keener observers. With a crack in the curtain of a bedroom window I projected onto the ceiling. I was tricked momentarily by the large sunspot: Wow, Mercury looks bigger than I expected! Then I woke up fully and looked for and found the small dot that was the real Mercury.
Then I then went into work. At about 09.45 UT I went into the car park and tried projection with 10x50 binoculars. I decided not to drag any colleagues out to squint at a tiny dot that they might or might not see. It was only just visible on the 140 mm disk I projected but it was there - so, I had a more exciting tea break than usual!
Dave observed using an equatorially mounted Meade ETX 70 refractor fitted with full aperture solar filter. He took pictures with a Nikon FM camera body attached via a tele-extender - see figure 13. Dave recorded the times of 3rd and 4th contact using a digital stopwatch synchronised to an Oregon radio controlled clock.
Having been woken by the alarm at 04:30 UT I crawled out of bed at this unaccustomed hour to see if the Sun were visible. There was some low-lying hazy cloud, with the Sun rising behind it though not itself visible other than as a bright glare. However, it looked like things would improve as the Sun rose so I got dressed, set my watch to within approximately one second of the time on Teletext and went out to the garden. It became immediately apparent that I could not obtain an unobstructed view of the Sun due to surrounding houses. I hastily set up a ladder to provide access to the flat roof of my garage and then rather precariously ascended it carrying first my telescope tripod and mount then the telescope itself. The instrument is a 114 mm F8 Orion Newtonian with RA motor drive and Baader solar filter - see figure 14.
At first I was unable to locate the Sun in the eyepiece, as it was too dim to find through the combined effect of solar filter and low lying hazy cloud. Eventually, the Sun came into a slightly clearer patch of sky and I was able to find it and set the RA drive running to track it. The Sun appeared rather like Jupiter, with parallel bands of cloud but, of course, it was Earth-bound clouds in this case! Due to the various confusing effects of the orientation of the telescope and eyepiece, telescopic image inversion and clouds, I had no idea at which edge of the visible solar disk Mercury would first appear, so it was not until 05:18:02 UT that I first spotted it, already past 2nd contact (i.e. completely within the solar disk). At 05:22:40 UT I judged Mercury to be about two apparent planetary diameters within the solar disk. At this stage I realised a further disadvantage of being on the garage roof – the image in the telescope bounced every time I moved! So after some breakfast I moved the telescope down to the back lawn, for more comfortable viewing - see figure 15.
I took a couple of photos by pointing my Pentax ESPIO lens into the eyepiece of the telescope, with a 10 mm eyepiece, and one of these came out reasonably well - see figure 16 (using 400 ASA Fujichrome slide film with slides scanned at 800 dpi on an Epson 1660 scanner). It shows a single large sunspot, with a well defined penumbra, and also the smaller but perfectly round silhouette of Mercury above it. On the solar limb you can see a suggestion of some more complex but less intense sunspots, which were much better seen by eye.
At about 09:30 UT I joined the crowd of people in Orwell Park Observatory to view the end of the transit. Simple eyepiece projection onto a white card offered the best view for those assembled. Viewing the projected image (jostling for position to do so!), I timed 3rd contact at 10:28:02 UT and 4th contact at 10:32:23 UT – though it was very subjective as to when these events actually occurred, and others present recorded times differing from mine by up to about plus/minus one minute.
It was fascinating to see such a rare event and inspiring to think of astronomers past using transits of Mercury (and Venus) to estimate the scale of the Solar System. My main impression was how very small Mercury appeared compared to the Sun. Lesson learnt for the forthcoming transit of Venus: work out in advance the effects of telescope and eyepiece configuration to determine the position on the solar limb at which 1st contact will occur!
J. J. Plummer, "The Transit of Mercury, May 6, 1878", MNRAS, vol. 38, pp. 413-414 (1878).