Phenomenon
Planetary Conjunction
When two worlds appear to meet in the sky.
A line-of-sight alignment of two or more planets (or a planet and the Moon). Conjunctions are a perspective effect — the bodies remain millions of km apart.
See also
Solar Eclipse
When the Moon stands between Earth and Sun.
Lunar Eclipse
When Earth's shadow falls on the Moon.
Meteor Showers
Cosmic dust burning in our atmosphere.
Aurora (Northern/Southern Lights)
Solar wind painting the upper atmosphere.
Supernova
The catastrophic death of a massive star.
Transit
A small body crossing in front of a larger one.