November 24 - First Observation of a Transit of Venus

Posted on November 24, 2021

Key points

This is an update of my post published on November 24, 2010:

Every once in a while, one of the planets gets between us and the Sun. These planets are too far away to cast a shadow on our planet or to block much of the sunlight—in other words, they don’t eclipse the Sun like the Moon occasionally does. Instead, from our point of view, they appear as tiny black dots that travel across the disc of the Sun. This is called a transit.

Only two planets can be seen in transit across the Sun from the Earth. Do you know which ones they are?

ANSWER: Mercury and Venus are both closer to the Sun than we are here on Earth, so they are the only planets that can get between us and the Sun.

Anyway, Jeremiah Horrocks was an English astronomer. He was the only person known to have predicted the transit of Venus on this date in 1639, and he is one of only two to have observed and recorded it. (His friend William Crabtree was the other.)

You may be wondering how he—or anybody—could watch a transit across the blindingly bright Sun. Well, he focused the image of the Sun through a simple telescope onto a piece of cardboard. He then watched the not-so-bright image on the cardboard. People still make this simple kind of helioscope these days, although astronomers have of course figured out more sophisticated systems with which they study the Sun.

NOTE: Never look at the image of the Sun EVEN with a helioscope without adult supervision.

A Rare Treat!

Transits of Venus are very rare. They happen in pairs that are eight years apart—and these pairs are separated by around 100 years! The last two transits of Venus occurred in 2004 and 2012. For many of us, that will likely be the last of our lifetimes! The next transit will be in December, 2117.

Transits of Mercury are much more frequent. The last one was in 2019, and the next one will be in 2032.