Appulse Of Mercury And Venus,
27 June 2005
Mercury is a relatively small planet, only about the size of Earth's Moon and, because it orbits close to the Sun, it is always difficult to observe. However, in summer 2005, the planet Venus, much easier to find because it is physically larger, can be nearer to Earth and is intrinsically brighter, came within 0.07° of Mercury. To put this figure in perspective, consider that the Moon has an apparent diameter of approximately 0.5°; so, Mercury and Venus were apart in the sky approximately 14% of the diameter of the Moon. Of course, this was just an effect of perspective: although the planets appeared close together, it was a line-of-sight-alignment and they in fact were situated at vastly different distances from Earth.
The epoch of closest apparent approach was 19:00 UT on 27 June 2005. Mercury was emerging from superior conjunction on 03 June and approaching greatest elongation east (26°) on 09 July; Venus had emerged from superior conjunction on 31 March and also was heading towards greatest elongation east. Within a few days of closest approach, the planets did not appear to move so very far, so I hatched a plan to attempt to observe them each evening from 22 June to 29 June inclusive, in the hope of seeing them at least once.
An effective method for finding an object in the sky that is invisible to the naked eye is to point the telescope first at something that is visible and then move it by the appropriate offsets in right ascension (RA) and declination (dec) to find the object of interest. In the case of Mercury, I was able to download from the Internet a table published by Jonathan Shanklin (Director of the BAA Comet Section), listing offsets in RA and dec between the Sun, Mercury and Venus. My plan therefore was to open the Observatory at 18:00 UT each evening, point the Tomline Refractor at the Sun, then swing it round by the tabulated offset in RA, search in dec for Venus and, once located, search for Mercury in close proximity.
Below is a summary of each evening's observations:
| Date | Observations |
| Wed 22 June | Offset the Tomline Refractor from the Sun, but unable to find Venus. Must be doing something wrong! However, we still have a few evenings to practice. Hoping for better luck in future... |
| Thu 23 June | Cloudy. Called off observations before leaving home. |
| Fri 24 June | Observations impossible due to lack of qualified personnel to open the dome at the Observatory. |
| Sat 25 June | No qualified assistant was available to help open the dome at Orwell Park so I abandoned thoughts of observing and instead travelled to the BAA Exhibition Meeting in Cambridge with OASI's Chairman and Treasurer, where we manned the stand of the Society for the History of Astronomy. |
| Sun 26 June | It being a Sunday, I didn't expect much support; however, two members of OASI expressed an interest. We duly opened the Observatory at 18:00 UT on an excellent evening with clear blue skies. By 18:20 UT we had found Venus, small and round, but where was Mercury? Four magnitudes fainter, it was never going to be easy to find. After 20 minutes of searching I found it, slightly to the east of Venus. Success! My two fellow observers also saw both planets. We used an eyepiece of 32 mm focal length, giving a magnification of approximately 120x. All set now for tomorrow evening's attempt at closest approach. |
| Mon 27 June | We opened the Observatory to plan at 18:00 UT and, using the previous day's experience, were able to locate both planets by 18:15 UT. Tonight, Mercury appeared north of Venus and the two planets were noticeably closer than on the previous evening. Five observers watched the planets until 20:10 UT when we shut the Observatory. Mike Harlow attached a camera to the Tomline Refractor and photographed the event. |
| Tue 28 June | Observations were prevented by poor weather: nothing but haze was visible through the eyepiece in the direction of Mercury and Venus. |
| Wed 29 June | This was the last night of observations and, by now, the two planets were visibly further apart and would no longer be visible together in the field of view of the Tomline Refractor. The sky was less hazy than on the previous evening and, although Venus was clearly visible in the eyepiece, the sky was just too bright to discern Mercury. |
So, in summary: eight nights planned, five trips made to Orwell Park, two successful observations. Not a bad score! I would like to thank Jonathan Shanklin for making his positional figures available - they saved a lot of calculation. The observers were (in alphabetic order): James Appleton, Garry Coleman, Nicky Gillard, Ken Goward, Mike Harlow, Gerry Pilling, Pete Richards, Ted Sampson, Mike Whybray.
Bill Barton, FRAS