And a Fourth

The planets show again and again all the phenomena which God desired to be seen from the Earth.

Georg Joachim Rheticus (1514-1576)

Because Mercury and Venus are sunward of the Earth they, like the Moon during a solar eclipse, may pass across the face of the Sun. Such events do not occur frequently. Eclipses of the third kind are called occultations; the present subject, eclipses of the fourth kind, are termed transits.

Earlier we noted that, if the Moon orbited us in the same plane as the Earth itself orbits the Sun (the ecliptic), it would be inevitable that eclipses (both solar and lunar) would happen every month. Because the lunar orbit is tilted at five degrees to this plane, however, they occur with a lesser frequency.

The same reasoning applies to Mercury and Venus: likewise they do not orbit in the same plane as the Earth. Mercury's orbit is tilted by just over 7 degrees, and that of Venus by 3.4 degrees. Using those angles one can go through the same rigmarole as for the Moon, to derive ecliptic limits on the nodal longitudes producing a transit. We won't trouble to step through those calculations though. Let's simply note that, due to regularities in their orbital motion, transits of both Mercury and Venus occur in distinct cycles. Those cycles will be discussed later. Historically, the transits of Venus comprise the most significant and rare phenomenon, and so we will first discuss those, and then look at Mercury.

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