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Venus headed for the Sun There are now only 5 days before the great day. The weather has been so poor one can only hope that we are due for a brighter spell soon but, as we all know, the har (sea mist) is quite capable of rolling in on any June day. Venus is still a brilliant object in the north-west of an evening and can be picked out with the naked eye before sunset. Then is the best time to look with binoculars. A power as low as x6 is quite adequate to show the planet as a narrow crescent. The photograph (Fig. 1) was taken in full daylight on May 23rd. Even so, the atmosphere was too unsteady to give a clean image. However the planet is drawing rapidly in towards the Sun, setting some 7 to 8 minutes earlier each day. The Sun has been showing a number of spots recently (Fig. 2), some only slightly smaller than the disc of Venus as it will appear in transit on the 8th June (Fig. 3). I have to admit to finding it difficult to observe any of these spots with the unaided eye using filters. JV 23/05/04 Transit of Venus, 2004 June 08 The planet Venus is at inferior conjunction this year on June 8. This date is significant since it is close enough to the day on which the Sun passes the ascending node of the planet's orbit for all three objects - Sun, Venus, Earth - to be in line. From Earth, therefore, Venus will be seen to pass in front of the Sun, much as did Mercury last year though, of course, the circumstances will be different. (For general background see archives - Transit of Mercury, 2003.) A transit of Venus is a rare event compared to a transit of Mercury. As with Mercury, however, they come in pairs. In fact only five transits of Venus have ever been observed, as follows: 1639, 1761, 1769, 1874 and 1882. After 2004 the next will occur in 2012, the next "pair" occurring in 2117 and 2125. On June 6th 2012 only the final 1½ hours will be visible from Britain; those in 2117 and 2125 take place in December with the Sun below our horizon. The first observed transit is of particular interest. It pertains to the short career of a gifted young curate, Jeremiah Horrocks (sometimes spelt Horrox). Horrocks (1617? -1641) possessed a sharp mathematical brain which he brought to bear on the work of Johannes Kepler (1571 - 1630). Kepler's work had revolutionized our approach to computing the positions of the planets and to understanding solar system dynamics. Horrocks had detected some discrepancies in Kepler's analysis which he put to the test by predicting the date and time of the transit of Venus in 1639. The story of how the 22 year-old Horrocks at Hoole (now Much Hoole, 6 miles SW of Preston) managed to squeeze in his observations between clerical duties (the transit took place on a Sunday) has passed into the romance of astronomical history. Horrocks, using a small telescope, set up a projection screen in a darkened room by which he was able to see at first hand the vindication of his methods. At the same time some distance to the SE at Broughton, Manchester, an astronomer friend of Horrocks, William Crabtree (about eight years his senior), was also successful in observing the transit. Horrocks noted in respect of Crabtree: "...a little before sunset, namely at 35 m. past 3, certainly between 30 and 40 min., the sun burst forth from behind the clouds. He (Crabtree) at once began to observe, and was gratified by beholding the pleasing spectacle of Venus upon the sun's disc." (DNB) These two were the only persons to observe the event. It may be difficult for us today, with our spoonfed technologies, to imagine the sense of elation this must have given the two men. Sadly, and tragically for science, Horrocks died within two years of his momentous observation; Crabtree likewise died young at the age of 34 years - a victim of the civil war. Jeremiah Horrocks is mainly remembered for this event but his contributions to mathematical astronomy were profound and acknowledged by Isaac Newton some years later. Given the social turmoil in Britain at the time, together with Horrck's precarious personal circumstances, it is something of a miracle that so much of his work has survived on record. Sir John Herschel regarded him as "the pride and boast of British astronomy". He was essentially self-taught and worked with limited means, all of which emphasizes his achievement. And in these days of specialization it is interesting to note: "He had scholarly and poetical, as well as scientific, tastes in addition to an amiable and affectionate disposition" (DNB). Edmund Halley (second Astronomer Royal) and others explored the method of using accurate observations of a transit of Venus for the purpose of determining the Sun's parallax. As a consequence extensive preparations were put in place to observe the event from different parts of the Earth. The 1769 transit was extensively observed and from it a solar parallax of 8".57 derived. (The currently accepted value is 8".794148.) A number of methods have since been refined for determining this important constant*. *A constant, implying non-variability, for the purpose of practical and most theoretical considerations. The 2004 Transit. The circumstances of the transit will depend upon the observer's location. These notes are applicable to the latitude and longitude of Kirkwall and may be taken to apply for most purposes to observers in the north of Scotland and the Northern Isles. All times are GMT (UT) - add 1 hour for summer time - and are to the nearest minute.
The first contact (limb to limb) is called ingress exterior contact, and takes place at 05h 20m. (IE, Dia. 1) The moment at which Venus appears entirely on the Sun's disc is called ingress interior contact, and this takes place at 05h 39m. (Dia. 2) Similarly, when the advancing (leading) limb of Venus makes contact with the Sun's limb towards the end of the transit this is called egress, interior contact (11h 02m - Dia. 2); the moment at which Venus's following limb is in contact with the Sun's limb marks the end of the transit (11h 22m) and is called egress exterior contact (EE, Dia 1). (These are not, of course, physical contacts.) The entire event lasts fractionally over six hours. Observing the Transit Venus will have an apparent diameter of 57".74 compared to the Sun's 1891". This is about 3% of the Sun's apparent diameter and should be large enough to render Venus visible to the eye as a tiny, black dot using suitable filters.
The same stringent precautions must be adopted when observing the transit as for observing the Sun generally. For this refer to the archives "Solar Annular Eclipse, 2003" and "Transit of Mercury, 2003". As has been stated above, the image of Venus will be large enough to be seen without optical aid using filters such as those described for the annular solar eclipse last year. For many people this may be the most convenient way to observe the transit. However, minimal magnification will add greatly to the observation. But DO NOT observe through binoculars or any other optical aid even with eye filters. Observing the Sun optically must only be attempted by those with the appropriate objective filters and with experience in the practice. For those wishing to time the contacts, the projection method will prove the most reliable and accurate since the image of the Sun may be made to appear much larger on the screen. Moreover, both hands would be free to operate other equipment such as a stopwatch or camera. Other useful data: Sunrise 03h 03m Altitude of Sun at ingress (external) IE: 13º 20' Altitude of Sun at mid transit (08h 21m): 36º 01' Altitude of Sun at egress (external) EE: 52º 44' Solar viewers for the transit of Venus. The British Astronomical Association has kindly agreed to make available solar filters for the safe viewing of the transit of Venus on June 08. Please send you orders by post with payment by cheaque at £1 each or 75p for five or more (inclusive of postage) to:
Occultation of Venus, 2004 May 21
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