The planets revolve around the Sun at different distances and speeds. Viewed from the Earth, they move constantly across the sky. If we look at two planets at any given moment, they form an angle of which the Earth is the apex. If we see these two planets in alignment, the angle formed with the Earth is 0°. When this happens, they are in conjunction. When one planet sets and the other rises, it is known as an opposition (180°). When the position of a planet creates an angle of 60°, 90° or 120°, it is in aspect.
The most important aspects are : conjunctions (0°), sextiles (60°), squares (90°), trines (120°), oppositions (180°).
In this illustration the Moon is in conjunction with Mercury, sextile with Venus and Trine with Mars.
In this illustration the Moon is square with Mars and opposed to Mercury.