Astronomical Units (AU) → unit based on the average distance between the Earth and Sun

• distance varies, because of having an elliptical orbit
• equal to 150 million kilometers $(1.50 x 10^8 km or 1.50 x 10^11 m)$
  • in 2012 the International Astronomical Union (IAU) defined one astronomical unit as 149,597,870,700 meters.

parallax → apparent displacement of an object viewed along two different lines of sight.

• observed that the parallax of an object changes with distance. The closer the object the greater the apparent displacement.
• It diminishes as the object moves farther.
• The parallax of your finger was greater when it was in front of your nose compared to it at arms-length.
• If your finger was to be moved even further, the parallax becomes smaller.

stellar parallax → apparent shift in position of stars viewed on Earth at extreme points in its orbit six months apart.

parsec → distance of a star with a parallax angle of one arcsecond.