Transits of Mercury and Venus

The occasions when amateur astronomers suddenly wish they had some expertise in solar imaging are those when Mercury and Venus transit the face of the Sun. Transits of Mercury are relatively common compared to those of Venus, although, with the kind of malicious cloud cover we get here in the U.K., an observer could conceivably miss every transit in his or her lifetime! When the transit of Venus occurred on June 8, 2004, it was the first Venus transit since December 6, 1882. The next transit will occur on June 6, 2012, but there is not another one until December 11, 2117. Amateur astronomers do not accumulate much experience in imaging Venus transits! Mercury transits occur at a rate of 13 or 14 per century. However, you do (obviously) need to be on the right hemisphere of the Earth, i.e., the daylight hemisphere, with the Sun above the horizon, to witness one. Mercury transits

JUL.Y 171K >004. 16:03 UTC Vi.T BOmm FL APO û F39 Evrndtr Filltf.

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Figure 16.2. A medium-sized sunspot on the lunar limb. July 17, 2004, 16:02 UT. 80-mm Vixen apochromat at f/29. ATiK 1HS webcam and filters. Baader solar film. Image: Damian Peach.

JUL.Y 171K >004. 16:03 UTC Vi.T BOmm FL APO û F39 Evrndtr Filltf.

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Figure 16.2. A medium-sized sunspot on the lunar limb. July 17, 2004, 16:02 UT. 80-mm Vixen apochromat at f/29. ATiK 1HS webcam and filters. Baader solar film. Image: Damian Peach.

Figure 16.3. A large sunspot on July 19, 2004, 07:03 UT. 80-mm Vixen apochromat at f/29. ATiK 1HS webcam and filters. Baader solar film. Image: Damian Peach.

can only occur in November or May. In my time as an amateur astronomer there have been five Mercury transits, but, due to geographic longitude and cloud I have only seen one: that of May 7, 2003. Amazingly, 13 months later, I saw the much rarer Venus transit, in a totally clear sky, from astronomer Patrick Moore's garden at Selsey in Sussex!

Needless to say, in the 122 years separating the 1882 and 2004 Venus transits, technology has moved on quite a bit. The 2003 Mercury and 2004 Venus events were the first to benefit from the application of webcam technology to the horrendous seeing conditions that result when the Sun is well above the horizon. Prior to this technological leap much had been made of the so-called "Black Drop" event seen at the 1882 Venus transit. Indeed, look at many astronomy textbooks of the 20th century and they will tell you that the black line joining Venus' limb to the solar edge at the 1882 transit was due to the Venusian atmosphere. In fact, webcam images of the 2004 event proved, categorically, that the Black Drop had nothing whatsoever to do with Venus, but plenty to do with poor seeing, poor instrumentation, and relatively long exposures. With perfectly collimated amateur telescopes and webcam exposures of 1/1,000th of a second (or less) the true nature of the Black Drop appears to have been resolved. In poor daytime seeing the disc of Venus seems to "breathe," i.e., to shrink and grow with the seeing. But when seeing is momentarily sharp, there was a clear gap between the black disc of Venus and the solar limb, even when only an arc-second of limb was left! In fact, the webcam results from the 2003 Mercury transit gave a clue that this would happen. Mercury too showed a Black Drop effect on frames exposed in moments of very poor seeing, despite that planet having no atmosphere. After the 2004 Venus transit images were analyzed it became obvious that the webcam had made a major scientific contribution. It had dispelled the 122-year-old myth of the Black Drop being caused by the Venusian atmosphere. A superb image of Venus transiting the Sun is shown in Figure 16.4.

Many excellent GIF animations were produced from webcam frames of the 2003 Mercury and 2004 Venus transits. With Venus' disc, almost an arc-minute across, taking 19 minutes just to cross the solar limb, these events are a leisurely affair, somewhat akin to watching paint dry. But at least there is time to correct technical problems, should they arrive. The 2003 and 2004 transits occurred during days when the Sun was virtually spotless, so the progress of the inky black dots across the solar disc was particularly dull. In the case of tiny Mercury, the disc size can vary considerably depending on whether the transit occurs in May or (three times as common) November. In May transits, Mercury's black silhouette can be 13 arc-seconds across, whereas in November transits it is a mere 10 arc-seconds. The next four Mercury transits (mid-transit times are given) are: November 8, 2006, 21:42 GMT; May 9, 2016, 14:59 GMT; November 11, 2019, 15:21 GMT; November 13, 2032, 08:55 GMT.

Figure 16.4. Venus leaving the solar disc after the rare transit of June 8, 2004 R+IR band (550-1000 nm). B&W Vesta Pro webcam. Astrophysics AP 130 f/6 apochromat + 2x Barlow and Herschel wedge. Image: Paolo Lazzarotti.

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