Your Last Chance to See Venus Pass in Front of the Sun
Every century or so, something truly special happens in the sky, and it happens twice: Venus passes in between the sun and earth. The transit of Venus, as it’s called, comes in pairs spaced exactly 8 years apart, with each pair separated by gaps more than 100 years long. As a result, only 8 transits have occurred since the invention of the telescope.
The most recent one was in 2004, and the second half of the pair is next week, during sunset on June 5th for North American observers, and during sunrise on June 6th for many in Europe and Asia. After this, the next one isn’t until 2117.
Why does it happen so rarely? Two events need to occur at the exact same time for us to see a transit of Venus. First, Venus needs to pass between us and the sun, so that to an observer looking down at the solar system, all three bodies would be in a straight line. This happens every 584 days, as shown in the bottom part of the diagram below.
Transits of Venus are so rare because the planet must pass between earth and the sun while lining up vertically, as well
However, Venus also needs to line up vertically so that it appears somewhere in front of the face of the sun from our vantage point. Because Venus and the earth don’t orbit the sun on the exact same plane—Venus’ orbit is tipped 3.4 degrees relative to ours—most of the time it’s too high or too low, as shown in the top part of the diagram. It only lines up in all 3 dimensions and traverses across the sun four times during an unusual 243 year cycle, with the transits coming in pairs separated by alternating periods of 121.5 and 105.5 years