Keep Those Solar Eclipse Glasses

Hopefully everyone has kept their solar eclipse glasses from this past weekend’s solar eclipse, because in two weeks time on June 5/6 you can use them again to witness an extremely rare celestial event: the transit of Venus.

Like clockwork Venus transits in front of the Sun in a repeating pattern of 8 years followed by 121.5 years then 8 years again and finally 105.5 years before starting all over again. If, like me, you missed the 2004 transit the next one won’t be for another 105.5 years and I’m afraid I likely won’t be around in December of 2117.

So what makes the transit so rare? The orbits of the planets around the Sun are not all perfectly aligned with the Earth’s orbital plane around the sun (called the ecliptic). When observed from Earth, every planet’s orbit is inclined slightly off the ecliptic by a few degrees (Venus is inclined 3.4°). During a transit, the orbits of the Earth and Venus align in just a way that Venus crosses the photosphere of the sun when observed from Earth.

The orbits of Earth and Venus and their intersecting nodes in December and June (Image Source: Popular Science Monthly Volume 6)

It will take Venus approximately six hours to cross the surface of the Sun and even though Venus is almost as large as the Earth, it is miniscule when juxtaposed with the Sun. Venus will appear as little more than a small black dot that will just be visible without magnification. Viewing it through a properly filtered telescope or filtered binoculars will, admittedly, give you a much better view.

Not only is the transit a rare event, but it also answered the questions of just how big our solar system is. Before using the transit of Venus as a measuring stick, we knew the relative positioning of the planets to one another thanks to Kepler’s laws of planetary motion. However, we could only assigning the distance from the Sun to the Earth the arbitrary value of 1 Astronomical Unit (A.U.) and we could then calculate the distance of all other bodies in the solar system in A.U. Unfortunately, we simply had no idea exactly how long in kilometres (or miles at the time) one A.U. was.

Using parallax, a way of measuring distance by observing an object from two know locations and noting its apparent change in position on a fixed background, astronomers in the 18th century positioned themselves at different points on the globe and observed the transit. They had to of course know their latitude, which was easy enough to calculate, and their longitude, which was no easy task even in the 18th century.

These pioneering astronomers did manage to find their positions on Earth within relative accuracy and also time the transit to the second. Then, with some relatively simple math they had finally put a value to the Astronomical Unit! It did of course have fairly large margin of error, but for the first time in our history we knew the approximate size of our solar system.

Six hours of watching a black dot moving silently in front of a star may not sound like the most riveting spectacle to watch in the world, but coupling the rarity of the transit with its historic importance should make this once in a lifetime event (twice if you caught the 2004 transit I confess) a bucket list must for anyone who has gazed at the heavens and wondered how all the celestial pieces move in their elegant dance.