A rare astronomical event, which will be viewable from Ireland, will take place early next month, but experts urge caution when watching it, writes Dick Ahlstrom
Britain's King George III did it. So did the famous explorer, Capt James Cook. Now you too cancatch sight of a very rare astronomical occurrence - the transit of Venus.
Very rarely does the planet Venus cross the face of the solar disc when passing between the earth and the sun. Rarer still is an opportunity to observe this phenomenon while seated in your own back yard.
The first transit of Venus visible from Ireland since 1882 takes place on June 8th and, weather permitting, we should all have an opportunity to observe it, says Prof Tom Ray of the Dublin Institute for Advanced Study's school of cosmic physics.
Ray is also chairman of a special National Steering Committee set up to make people aware of the transit and help them watch it safely. Safety is a key consideration, and under no circumstances should someone attempt to view the transit directly without adequate safety equipment, he warns.
A transit is nothing like as dramatic as a solar eclipse, he acknowledges, but it is still worth watching. "It is the rarity really. The last one was 122 years ago and the next won't come for another 122 years and that won't be visible in Ireland," he says.
"It is an opportunity for people to see a very rare phenomenon but it is important to emphasize the safety aspects."
A transit is a protracted event and will be visible for about six hours. It starts on the morning of June 8th at about 6.20 a.m. and continues until about 12.24 p.m. The planet will appear as a very dark dot about one 30th the diameter of the solar disc. It will first appear on the lower left-hand side of the disc and track from left to right at a slightly downward sloping angle.
Observing such a transit was a highly significant astronomical observation in days gone by as a way to measure astral distances, explains Ray. "Fundamentally you wanted to get the distance from the earth to the sun." This was achieved by making transit observations at two locations on earth as widely separated as possible and calculating angles between the viewing locations.
"In the past it was a way to measure distances in the solar system," adds Ray. "Now we use radar."
An all-Ireland approach to the transit is under way in an effort to get as many people and school students watching the transit as possible. The Armagh Observatory is involved and a special website has been opened to provide information about your nearest school for safe observation of the event.
The European Southern Observatory also has big plans to get all of Europe watching the transit and has a special website for this purpose. The Irish and ESO sites will also both transmit live images in case the weather decides not to co-operate.
The historical links related to the transit are intriguing. George III saw the 1769 transit in his observatory in Richmond, now Kew. By a quirk of good fortune, Armagh Observatory has some of the instruments he used and these will be on display there.
Capt Cook is also important in transit lore. The Admiralty dispatched Cook to the Pacific Ocean to observe the 1769 transit, which he did in Tahiti. He then sailed south and the following year discovered New Zealand and then reached the eastern seaboard of Australia claiming this also for Britain.
The Steering Committee strongly recommends against watching the transit with the glasses used to observe the solar eclipse several years ago, said Dr Ian Elliott, the committee's secretary. "We think this isn't safe. The mylar coating can form pinholes," and this could spell disaster with permanent eye damage, he warns.
It is possible to view the sun through welders' goggles, fitted with number 14 glass at least, he adds. "It would appear as a tiny dark circle. With good eyesight you should be able to see it."
Ray believes the best way to see the transit is by using binoculars or telescope to project an image of the sun onto a white board held several feet away. You stand with your back to the sun and then pivot the binoculars until you get the solar disc projected onto the board. Remember then to adjust the focus to sharpen the enlarged image of Venus.
Learn how to safely view the transit of Venus on the Irish and ESO sites: www.venustransit.ie and www.vt-2004.org. Armagh Observatory's site describing its exhibition is: http://star.arm.ac.uk/ venustransit/ exhibit/