2004 TRANSIT of VENUS

Background

The first of the two 21^st-century transits of Venus will be visible in its entirety from the British Isles. Given that the last transit of Venus took place in 1882, it is sobering to think that there is no one on the planet who has seen one these events before! As there are better methods for determining the solar parallax, the scientific significance of this transit is limited. However, it has caused great interest in the area of public understanding of science and scientific education in general.

For observers in the United Kingdom, the main consideration for observing the transit will be the weather. The statistics show that there is only a very small chance of the weather being completely clear. It is more likely that broken cloud cover will prevail. Sadly, there is a 20% chance that we may be clouded out completely. If you want greater certainty of seeing the transit, southern parts of Europe close to the Mediterranean Sea offer a significantly better chance of clear skies. However, visiting Saudi Arabia or Kuwait may provide an almost guaranteed view of the transit.

Visibility

Venus will cross the disc of the Sun on 2004 June 8^th. The map above shows the visibility of the event. The entire transit can be seen from Asia except the extreme eastern part, Africa except the western parts, Europe except the south-western tip of the Iberian Peninsula, Greenland except the southern tip and most of the Indian Ocean. The Sun will set while the transit is in progress in the grey area encompassing north-western Canada, Alaska, parts of north-east Russia, Japan and Australasia. The Sun will rise while the transit is in progress in the grey area taking in western Africa and eastern parts of the Americas. The yellow lines on the diagram show the position of the terminator, where the Sun is either rising or setting, at the key phases of the transit.

Geocentric Circumstances

The geocentric circumstances of the transit are shown in the diagram to the right. During the transit, the diameter of the Sun is 1890.8 arcseconds and that of Venus is 57.8 arcseconds. In other words, the diameter of Venus is 0.03 that of the Sun, making it look like a rapidly moving sunspot. The whole transit lasts just under six and a quarter hours.

All timings are given in Universal Time (UT).

The Transit from the United Kingdom

To convert to British Summer Time (BST), add one hour to the Universal Time.

In the United Kingdom, the transit begins between one and quarter and two hours after sunrise depending on your location. Taking London as an example, sunrise occurs at 03^h45^m (UT) in the north-eastern sky. The transit begins with exterior ingress at 05^h20^m (UT) when the disc of Venus begins to cross the limb of the Sun. Interior ingress occurs at 05^h40^m (UT), when the whole of the disc of Venus has crossed over the limb of the Sun. Venus then moves across the solar disc, reaching a minimum separation from the centre of the Sun of 641.8 arcseconds at 08^h23^m (UT). Interior egress, when Venus starts to cross the solar limb for the second time, occurs at 11^h04^m (UT), and the transit ends with exterior egress at 11^h24^m (UT).

The progress of the transit is summarized in the diagram below. The left-hand panel shows the movement of the Venus across the solar disc. The top of the diagram points to the zenith, the point directly overhead. The position of Venus is marked every UT hour. The right-hand panel shows the movement of the Sun in the sky. At the beginning of the transit, the Sun is east north east at an altitude of just under 12°. By the end of the transit the Sun is south south east at an altitude of just over 60°.

Local Circumstances

Summary plots like the one shown above and animations showing the motion of Venus relative to the Sun as seen by someone observing the transit through appropriate eye protection are available for several locations across the UK and Ireland. The summary gif files are ~18Kb and the animations are ~200Kb. To view the animations properly, it may be better to download the animations and view them locally.

If you wish to generate your own predictions for this transit, the Astronomical Applications Department of the US Naval Observatory has produced an online Transit Computer.

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HM Nautical Almanac Office UK Hydrographic Office E-mail: hmnao@nao.rl.ac.uk Last revision was made on 2007 November 5