Venus Transit – Time for the Replay!


By Bill Pellerin

Houston Astronomical Society

GuideStar Editor

Did you see it? Did you see it?

After reading a couple of books about the history of the Venus transit I was very eager to see the one on June 5, 2012, and I did. The most recent book I read was The Transits of Venus by William Sheehan and John Westfall. This book tells the history of Venus transits since the first one known to have been observed by Jeremiah Horrocks and William Crabtree in December of 1639. It’s a remarkable story of determination by those observers who timed the next pair of transits in 1761 and 1769 on the recommendation of Edmund Halley. Halley died before either of these transits. The goal was to determine the distance between the Earth and the Sun, which they did but without the hoped-for accuracy.

I began to wonder about the mechanics of the event. How were Venus and the Sun arranged in the sky, and how were they moving during the transit? Did the movement of the Sun or of Venus contribute more to the relative movement that I saw?

We’ve learned that these events are rare because the orbital planes of the Earth and of Venus are not the same. Only when the planes cross each other while Venus is at inferior conjunction does a transit occur. An inferior conjunction is when the orbit of an ‘inferior’ (closer to the Sun) planet is between the superior planet and the Sun. The next inferior conjunction will be on January 10, 2014, but Venus will miss the Sun by about 5 degrees, and there will be no transit. The next transit is in 2117.

Let’s do a replay of the June 5, 2012 event to understand what was going on. Observationally, we saw Venus attack the north-eastern edge of the Sun and then move slowly westward across the face of the Sun. For the purposes of this article, the movements are considered from the point of view of an observer on the Earth.

When Venus is at inferior conjunction the apparent motion of the planet is to the west with some motion to the north or south. I simulated all eight transits back to May 23, 1526. For every May or June transit, Venus’ apparent motion was to the southwest, and for every December transit Venus’ apparent motion was to the northwest.

What was the Sun doing? The Sun is reliably moving east in the sky (compared to the background stars) year-around with some additional movement to the north or south because of the tilt of the Earth’s axis. You only have to know about the motion of the Sun during the four seasons to understand the motion of the Sun during a transit. In spring, the Sun is moving to the north and it continues to do so until the summer (northern hemisphere) solstice (the first day of summer – June 20, this year).

So, while the Sun was moving mostly east, it was also moving a bit north during the most recent transit and the position angle of the Sun’s motion was 82 degrees, just north of due east. Remember that north is 0 degrees, east is 90 degrees, south is 180 degrees and west is 270 degrees. The Sun moved 15’ 50.86” east and 2’ 13.6” north during the time of the transit the total motion of the Sun was 16’ 2”.

Until about May 13, 2012, Venus was moving mostly to the east in the sky. Between that date and about June 26, 2012 Venus was moving to the southwest at a position angle of about 244 degrees. After June 26, Venus continued its eastward trek in the sky. During the transit Venus moved 10’ 06” west and 4’ 56” south, the total Venus movement was 11’ 14.3”.

The movement of the Sun and Venus were in more-or-less opposite directions, but not exactly. If you do the vector math the combined movement is 26’ 55”.

So, if you want to see the replay, and you have software on your computer that simulates the positions of Venus and the Sun accurately, you can set this scenario up and watch it play out. We’ve been told that we won’t see another Venus transit in our lifetime. That’s true, but we can watch the replay of the event on our computers as many times as we want. Johannes Kepler, Edmond Halley, and their contemporaries would have been astonished by this capability.

After the 2012 transit ended, I thought about the observers who will see the next transit in 2117 and I wondered what tools they will have at that time. Surely it will be an amazing time, and some of those observers will be thinking about the observers of the previous transit in June of 2012 and wondering what the event was like for them.

Thanks to fellow Houston Astronomical Society member Bill Flanagan who contributed calculations to this article. (Any mistakes are mine.)




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