# How does one calculate the length of a planet's solar year?

I have a planet with a specified sidereal day and sidereal year, and a solar day. But how can I determine the length of its solar year from this?

Sidereal day: 1d 1h 52m 11s

Solar day: 1d 1h 54m 36.45402s

Sidereal year: 379d 5h 54m

Semi-major axis: 0.9675 au (144,735,939.90225 km)

Orbital eccentricity: 0.272

Periapsis: 0.704 au (105,316,900.9728 km)

Apoapsis: 1.231 au (184,154,978.8317 km)

Axial tilt: 9.87°

• How is this about worldbuilding? This looks like you have a math homework question or something and you're posting it here because you don't want to solve it. – Aify May 20 '18 at 22:57
• Are you actually Egoraptor? I don't feel like Arin has ever shown an interest in this sort of thing at all. – Southpaw Hare May 21 '18 at 18:57
• @Aify This: "questions regarding real world stuff are on-topic." Been there done that with energy release of two colliding planets question. That was determined to be on-topic. Plugging in numbers here is no different. – elemtilas May 27 '18 at 13:04

I'm assuming that by "solar year" you mean "tropical year".

If so, you can't calculate the length of the solar year, because the thing that makes the solar year and the sidereal year different is precession, and you don't know that.

The sidereal year is the length of time it takes a planet to return to the same point in its orbit relative to the distant stars. The solar year is the time it takes for a planet to return to the same point in the seasons. If the planet's axis is tilted (and your is) and it is precessing (most likely it is) then during the revolution around its sun, the planet's axis will have shifted minutely in space and, depending on the relative directions of revolution and precession, the same seasonal point will return a bit sooner or a bit later than the end of the sidereal year.

For Earth, the tropical year is about 20 minutes shorter than the sidereal year.

Wikipedia has good short pages on each: https://en.wikipedia.org/wiki/Sidereal_year and https://en.wikipedia.org/wiki/Tropical_year.