It is the average distance from the center of the Earth to the center of the Sun. One astronomical unit is equivalent to 149.6 million kilometers.

It is one of the most commonly used units in astronomy and is equal to the mass of the Sun. It is used to show or compare the mass of other objects such as stars and planets. One solar mass is about 2×10³⁰ kg.

A parsec (pc) is a unit that measures distance in astronomy. The idea comes from a change of perspective in astronomy called parallax. Here is one experiment to demonstrate it right away: Hold your index finger in front of your face, and try to look at it with one eye then the other eye. If you notice the shift of your index finger (although you didn't move your finger itself), that's the idea of the parallax angle. We use the parallax angle and trigonometry to find the distance between two objects.

1 pc equals 3.26 light years (31 trillion kilometers).