HOME » Society Archive » The Transit of Venus

The Transit of Venus - 8th June 2004

By Kelvin Stephens

UPDATE: Images from the transit taken by WAS members

This coming June will be a month to remember for any person alive today who has an interest in Astronomy, for this month sees a very rare astronomical event. The planet Venus will pass directly across the disc of the Sun as viewed from Earth - more commonly known as a Transit of Venus. This event is so rare for the last occurrence of this nature was some 122 years ago in the year 1882. The date for this special event being - the 8th June 2004.

The first Transit of Venus was predicted way back in 1629 by Johannas Kepler, who calculated it to occur on the 7th December 1631, (New calendar). However, it is known that no person in Europe observed this particular transit as it was only visible from the "frontier lands" of the New World. The first observed transit of Venus for any European observer was the following transit of this pair - on the 4th December 1639, by non other than Jermiah Horrocks who lived in Hoole, near Liverpool.

I mention the first European, as from my travels I have discovered that Venus in transit on the Solar disc was actually observed and recorded by the Arabic astronomers back in the 8th Century. Taking into consideration of the piercing light from the Sun in the desert lands which prohibits observations directly at the Sun with the naked eye, the observation would have probably been at sunrise, or shortly after, or before sun set when the glare of the Sun is not so intense . Also, the Arabic lands are 4 hours ahead of UT. As a result of checking the "bible" for dates and timings of Transits by Jean Meeus, the date of this observation is likely to have been - 24th May 789.

We are at a time where the Transits of Venus occur in pairs of 8 years apart, (the second of this pair will be in the year 2012). However, this will not hold out forever, eventually they will fall back to a single transit. At this period in time, on average transits will return roughly some 122 years after the second transit of the previous pair - 1631-1639, 1761-1769, 1874-1882, 2004-2012.

If the planet Venus was in the same plane as the Earth, each time Venus passed between the Earth and the Sun, (which is roughly every 584 days), we would observe a transit. However, as the planet deviates a little from this plane, we only get a transit when both the Earth and Venus cross the point (known as the Nodes) together. Any other time that Venus passes the Earth, the planet as observed from the Earth will either be above, or below the Solar disc and we do not get a transit. At this moment in time this occurrence of both planets crossing the nodes together occurs in either the month of June, or December. However, this will not always be the case. As the Centuries pass so this date gets a little later, until a time will come that they occur in July and January.

In fact, this is similar to the Moon and the Sun, where upon when the Moon passes directly in front of the Earth, we witness a Total Eclipse of the Sun. As we all know, the Moon goes around the Earth every month, but we do not observe a Total Eclipse of the Sun every month. This is due to the Moon be just under 6 degrees above, or below the plane of the Earth to the Sun.

This year, 2004, we are fortunate in that the entire transit from beginning (ingress) to end (egress) will be observable from the UK. Our only problem will be the weather. However, if you intend to observe this event you will need to get up early on the morning of the 8th June.

OBSERVING THE SUN IS DANGEROUS AND CAN DAMAGE YOUR EYESIGHT. YOU MUST ENSURE THAT YOU HAVE ADEQUATE PROTECTION.

FOR THOSE WHO HAVE ACCESS TO A TELESCOPE, OBERVE THIS EVENT BY THE PROJECTION METHOD ONLY - YOU DO NOT LOOK DOWN THE EYEPIECE. UNLESS YOU HAVE THE REQUIRED FILTERING - THOUSAND OAKS OPTICAL FILTER (TYPE 2+).

THE SAME WARNING APPLIES FOR BINOCULARS - BY PROJECTION ONLY.

If you still have your Solar Eclipse glasses from the 1999 Total Eclipse, you can use the for the early part of the event. The planet Venus should be observable as a black disc on the Sun without the use of a telescope, as it will be 1/32nd the size of the Sun. (Compared to Mercury being 1/195th the size of the Sun).

Unlike a Total Eclipse of the Sun, where the Moon impinges the Solar disc from the right hand side (west) and travels across to the left hand side (east). An inferior planet, (Mercury or Venus) will travel in the opposite direction - from east to west, or the left hand side of the Sun across to the right hand side of the Sun.

Venus will enter (ingress) the Solar disc at 116 degrees and exit (egress) the Solar disc at 216 degrees. The path will follow roughly a line from 8.00 to 4.30 on a clock face.

The times to note down in order to look out for this event are as follows:

06.13 BST - known as t1 - this is when the edge of Venus will touch the edge of the Solar disc. You will not see anything at this point, but very soon afterwards you will observe the black disc of Venus impinge on the Solar disc.

By 06.32 BST - known as t2 - the whole of the disc of Venus should be on the Solar disc. Keep an eye especially as the planet leaves the Solar edge as it progresses across the Sun. You should observe an effect which is termed as the "Black Drop" - where the edge of the planet at the Solar edge seems to get distorted, much like a water droplet as it is about to fall off the surface that it is hanging from.

The point where Venus will be at its closest to the centre of the Sun along its transit path is known as tm. This is due at 09.20 BST.

Venus continuing its path across the Solar disc will reach the opposite edge of the Sun at the point known as t3. This is at 12.06 BST. Just before the planet touches the right hand edge of the Sun, you should again notice the droplet effect, as the disc of the planet seems to get distorted. This droplet effect will be on the right hand side of the planet - the side that is due to go off the Solar disc first.

By 12.26 BST - Venus will have reached the position known as t4, by this time the transit will be finished. However, keep an eye on the planet as its makes its way off the Solar disc.

The duration of the transit from start to finish will be 6 hours 13 minutes.

I mentioned earlier that transits of Venus come along in pairs at this period in time. The second transit of this pair will be on the 6th June 2012. However, you will not be able to observe this transit from the UK In order to observe this particular transit, you will need to get yourself to the Pacific islands. After this date - the next pair of transits of Venus will not occur until the years 2117 and 2125.

DO NOT FORGET TO TAKE ADEQUATE PRECAUTIONS WHEN YOU ARE OBSERVING THE SUN AS FAILURE CAN RESULT IN DAMAGE TO YOUR EYESIGHT.

Kelvin Stephens is a long standing member of Wycombe Astronomical Society and is well known in the Society and the UK amateur scene. He is travelling to the Middle-East to witness the Venus transit following a previously successful trip to watch the Mercury transit in May 2003. He will be giving a lecture at Wycombe Astronomical Society's meeting on July 21st.

Content © 2008 Wycombe Astronomical Society. No part of this site may be reproduced in any way without prior written permission. Read our Privacy policy.
Wycombe Astronomical Society is a UK Registered Charity No. 297024. Connectivity supplied by Internet Services.