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There's a little black spot on the sun today...

© Gregg Pasterick

On June 8, Venus will pass in front of the Sun as seen from Earth. It might not sound like much, but the last time this happened was in 1882. In fact, Venus transits the Sun only 13 or 14 times every 1,000 years. It does, however tend to do it in pairs, separated by a period of 8 years. This transit of the sun by Venus will be repeated in 2012.

The fun gets started at about 5h13m Universal Time (U.T.), and lasts until about 11h25m U.T. Of course not all of us will get to see it. Folks in the extreme northwestern corner of New Zealand, for example, will watch the Sun set just as Venus's transit begins. At the opposite end of it all, the transit ends as the Sun rises for folks in the central U.S. But in between, folks will get to see some or all of it, depending upon where they are. The best places to be are most of Europe and Asia, most of Africa, and the ... bbbrrrr ... Arctic.

Venus passes between the Earth and the Sun once every 584 days; this is called Inferior Conjunction. Usually it passes above or below the Sun, as we see it from our vantage point, but sometimes it drifts across a portion of the sun, or transits it. These transits only occur in early June and early December. This is because, for it to be seen against the Sun, it must be near one of its two nodes in its orbit when it is at Inferior Conjunction. These are the points where the orbit of Venus crosses the orbital plane of the Earth. These are situated in our early-June and early-December areas of our orbit. If we happen to be in the same general longitude along our orbit when Venus passes one of its nodes, we see a transit. This happens - these things all fall into place - on average, every 120 years.

It all boils down to the sizes of our orbits, and the rates at which Venus and the Earth travel in these orbits. Venus orbits the Sun 13 times, while the Earth has done so a little shy of 8 times. (Even the Ancient Peoples of Yore knew this.) What this means is that Venus's movements among the constellations, as seen from Earth, repeats every 8 years; what it did in 1996, it is doing now, and what it is doing now it will repeat in 2012. But all the pieces have to situated just so in order to see a transit of the Sun by Venus; Venus must be at a node when at Inferior Conjunction, and we must be at the just the right spot in our orbit. We will be at the just right spot on the 8th, and again in 2012, but then another 105 years will go by before we see another transit of the Sun by Venus. (Ah, the time scale astronomical events occur on. It boggles the mind.)

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