Q&A: Transits of Venus and Mercury

Question: I saw the transit of Venus back in June 2004 at an event hosted by a local astronomy club. It was really cool seeing that black dot moving across the Sun. Later I read that there won’t be another one until 2117! Why are these transits so rare? — CL, Dayton, OH

Answer: Transits of Venus occur, on average, once every 80 years. For Mercury, the only other planet that can transit, the interval is roughly every 7.5 years. It’s less time for Mercury because its orbit is smaller and it moves more quickly around the Sun. But in both cases, the relative rarity is due to the fact that the orbits of Venus and Mercury are tilted with respect to Earth’s orbit around the Sun.

The top graphic shows the orbits of all three plotted against the disc of the Sun. The sizes of the discs of Venus and Mercury are drawn to scale. Venus is easy to spot, but you have to hunt for the tiny disc of Mercury. I’ve seen and photographed both. Check out:


The first graphic only shows a small part of the orbits of Venus and Mercury. This following graphic shows why, when at at inferior conjunction, Venus and Mercury will, more often then not, pass above or below the Sun as seen from Earth. For simplicity, only the orbit of Venus is shown. Note that the orbit of Mercury is tilted by twice as much (7° vs. 3.4°) out of Earth’s orbital plane:

If Venus and Mercury orbited the Sun in the same plane as Earth, then we’d enjoy transits every time they passed between the Earth and Sun. In fact, since speedy Mercury orbits the Sun in only 88 days, we’d be able to see it transit several times each year! In reality, the next transit of Mercury won’t happen until November 13, 2032.

The solar system is often portrayed in popular media as a flat arrangement of concentric circles, with each circle representing the orbit of a planet. So unless you study astronomy, you miss the fact that all the orbits are inclined relative to Earth’s orbit. But not by much.

Next Week in Sky Lights ⇒ Inferior and Superior Conjunctions

First Lab Measurement of Light Speed
Inferior and Superior Conjunctions