Sometimes, there are unique
occurrences when planets align themselves with the Earth and the Sun. We
actually have specific names for these occurrences: opposition and conjunction.
Opposition occurs when the Sun and
the planet are in opposite directions in the sky, i.e. 180° apart in the sky.
The Full Moon is also an example of opposition. Venus and Mercury, however, can
never be in opposition. In my post about the morning and evening star,
we know that Venus is never farther than 47.8° and Mercury is never more than
27.8° from the Sun.
Conjunctions happen to all the
planets. The best way to think of it is when a planet and the Sun are in
relatively the same direction in the sky. When the outer planets (Mars,
Jupiter, Saturn, Uranus, and Neptune) are in the same direction as the Sun, we
just refer to it as a conjunction. However, since Venus and Mercury can actually
be at conjunction at two different points in their orbits, we need to specify
their two conjunctions. When the inner planet is closest to the Earth, we call
that its inferior conjunction because it is closer to the Earth than the Sun
and when its at its furthest point, we call that the superior conjunction.
Planets can also be at conjunction
with other planets. A good example is now visible in the night sky. Right now
for a few days, Jupiter and Venus are near each other in the sky, and depending
on your telescope or binoculars, are in the same field of view.
Venus and
Jupiter in conjunction, with the full moon to give scale.
