What's the difference between a sidereal year and a synodic year?

Answer 1

A sidereal year is measured based on the planet's position in space relative to the background stars. A synodic year is measured based on the planet's position relative to the star it rotates around (i.e. the sun) Here's an image that is pretty helpful in visualizing.

Answer 2

Synodic period relates to an object's position relative to the sun. So the cycle which goes from full moon to full moon is a synodic cycle. A sidereal period is sometimes called a 'star day'. It relates to an object's position relative to the distant stars. We need the distinction when we are talking about bodies in orbit. Because the earth and the moon are orbiting the sun, they have to rotate a little more than an exact 360 degrees (the sidereal period) in order to reach the same position relative to the sun (the synodic period).

Answer 3

A sidereal year is the time it takes for the sun to return to the same position in relation to the stars. A synodic year is the time it takes for a planet-sun alignment to reoccur. A synodic year is 20 minutes shorter than a sidereal year.