Why do transits of Venus across the disk of the Sun happen in pairs?

You asked why transits of Venus across the Sun's disk happen in pairs.

Venus' orbit is tilted with respect to Earth's by about 3.4 degrees.
This means that from our point of view on Earth, Venus normally passes
above or below the Sun's disk, and never crosses directly in front of
or behind it.

But along Venus' orbit there are two places where Earth-Venus-Sun can
make an exact straight line. Every hundred years or so, Earth is in
the right place in its orbit at the same time Venus is in the right
place in its orbit, and we see a transit of Venus in front of the Sun.
Afterwards, Earth and Venus gradually drift out of phase with one
another. But eight Earth years later, Venus has completed almost
exactly 13 of its orbits, and there is another near-alignment. Venus
passes in front of the Sun again, but across a different part of the
Sun's disk.

Another eight Earth years later, we've drifted further out of synch
and Venus misses the Sun completely for about another hundred years.

To find the exact times of the transits at your location, you could
use a personal computer with a planetarium program such as Starry
Night (www.seds.org/billa/StarryNight/sn.html) or some of the many
others advertised in Sky & Telescope magazine (www.skypub.com).

Best wishes,
-Neal.

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