It’s an odd fact that amateur astronomy seems filled with “once in a lifetime” events. With bright comets, unusually active meteor showers, local solar eclipses, closest approaches of planets to Earth, supernovae, and various alignments of planets, the Moon and bright stars, we seem to experience some astronomical event every couple of years that is described (sometimes incorrectly) as “never to be seen again in our lifetimes”. But coming June 5th, there will indeed be an event worth observing if you can — the final transit of the planet Venus across the Sun that will occur in the 21st century.
A transit, in astronomical terms, occurs when a smaller visual body passes in front of a larger body, blocking some of the larger body’s light, and showing the smaller object in silhouette. This differs from an eclipse only in the relative sizes of the two objects involved. A transit can only occur when there is a very precise alignment of the two objects involved and the Earth’s location.
The transits of Venus follow a complex cycle of 243 years, with the event occurring at intervals of 105.5 years, then 8 years, then 121.5 years. The transit coming up next week completes an 8-year pair with a previous transit which occurred in 2004. The rarity of transits of Venus is caused by the slight tilt (3.4 degrees) between the orbits of Venus and Earth, and relatively close distance from Earth to Venus.
As a result, when Venus lies roughly between us and the Sun, it is usually either north or south of the actual disk of the sun by several degrees, and a transit does not occur. The period of 243 years is caused by the lengths of Earth’s year, 365.26 days, and Venus’ year, 224.70 days, with 243x365.26 approximately equal to 243x224.70, so that every 243 years Earth, Venus and the Sun come back into the same relative positions in space.
The transit on June 5th will begin at 6:03 p.m. and the sun will set at 8:23 that night, hours before the transit ends. To get an idea as to whether you will be able to see the sun during the transit, the sun will be about 23 degrees above the horizon at 6:03 p.m. If you hold out your fist to arm’s length, with your thumb on top, your fist covers about 10 degrees; so the sun will be about two fists above your western horizon. The best observing places are on hilltops or rooftops.
PLEASE NOTE: You cannot safely observe the transit of Venus by looking directly at the sun! The safest technique for this observation is to project an image of the sun onto paper through a pinhole in a thin piece of cardboard. Using this approach you should be able to see the transit easily — Venus will appear as a dark spot notching edge of the sun by 6:10 p.m., and fully entering the solar disk by 6:21 p.m. Using a telescope you can obtain a much better projection — but you must absolutely not attempt to look through the telescope to locate the sun — instant blindness will be the result.
Because the transits of Venus are not easily visible to the naked eye, this is only the 7th observed transit of Venus in history following the invention of the telescope (1639, 1761/1769, 1874/1882, 2004/2012), though it is certainly possible that earlier transits may have been casually noticed near sunset or sunrise.
Observers at different locations on Earth will see the start and end of the transit occur at very slightly different times, due to the change in angle between their locations and the position of Venus and the Sun. In the transits observed in the 18th and 19th centuries, the use of precise timings of the start and end of the transit observed from widely different locations on Earth allowed the distance from the Earth to the Sun to be directly measured using parallax, to a precision not achieved by other methods of the day. The transits were also used to determine the chemical makeup of the atmosphere of Venus by carefully measuring the absorption of the sunlight as it passed through the Venusian atmosphere.
Since we can now measure the Earth-Sun distance using methods much more accurate than parallax from Earth-bound observations, and we have dropped space probes through the atmosphere of Venus, the scientific value of the upcoming transit could be thought to be minimal. However, several projects related to the search for planets around distant stars are planned. Modern astronomers use observations of transits of distant stars to infer the presence of planets — looking for the light coming from these stars to decrease by as little as 20 parts per million. By carefully studying the total received light output from the Sun during the Venus transit, additional refinements to this technique will be explored.
Perhaps the most ambitious experiment planned is to use the Hubble Space Telescope to observe the Moon during the transit. The amount of light coming from the moon will decrease very slightly during the transit (by about 1/1000th). The researchers will also observe the reflected light spectrum to look for changes in the spectrum indicating the gases present in the atmosphere of Venus. These measurements will be compared against the known composition of the atmosphere of Venus to help to develop similar methods to be used on planets of distant stars.
If you miss this year's transit due to clouds, trees or forgetfulness, I'm sure to be bringing you another "once in a lifetime" astronomical event very soon!