Lunar Occultation of Venus,
21 May 2004
While most interest in Venus around mid-2004 was focussed on the transit on 08 June, just three weeks earlier, on Friday 21 May, there occurred a lunar occultation of the planet, which also held interest for keen observers. Bill Barton, Garry Coleman and I decided to attempt an observation.
The occultation was forecast to take place at 11:09 UT, with the bodies only 22° from the Sun (the Moon being only two days old). Both the Moon and Venus can be observed during the day, the former generally easily, the latter if you employ a telescope and know where to look. However, we anticipated difficulty in finding the planet on 21 May due to the Sun's glare; the observation would be difficult from one's back garden and we therefore intended to observe the phenomenon using the Tomline
Some preparation was clearly required. Therefore, on the evening of Wednesday 19 May, at Orwell Park Observatory, we practised locating Venus in proximity to the Sun. Bill used data from the BAA Handbook to calculate the RA and dec of both the Sun and Venus. This enabled us to centre the Tomline Refractor on the Sun (via projection of the solar image on a piece of white card held behind the eyepiece) and then move the instrument by the appropriate offset in RA and declination to find Venus. This approach worked satisfactorily and we intended to use it again on 21 May.
At 10:00 UT on 21 May, I joined Bill and Garry in the dome at Orwell Park. Bill had again calculated the RA and dec of the Sun and Venus. The sky was briefly clear before 11:00 UT enabling us to project the Sun's image onto white card, read the RA from the telescope's setting circle (the brass drive wheel at the base of the instrument), and move the instrument in RA and dec by the necessary amounts (1h 46m in RA and 6° N in dec) to centre it on Venus. Alas, by the time we had re-positioned the telescope, the sky had clouded over, and it remained cloudy for the entire period of the occultation. We didn't see Venus.
However, Bill, being very determined, insisted that we keep the telescope drive runnning for another half hour after the occultation ended at 12:19 UT, because "a clear patch of sky was coming". And he was right: the sky cleared and we saw Venus in the eyepiece with the Moon a large fraction of a degree beyond it! So, we had missed the occultation, but our technique for positioning the telescope on the planet had been accurate.
It was a difficult and interesting observing project, and quite different from the forthcoming transit on 08 June 2004.