Year Length on Planets Calculator
Convert a number of sidereal orbital years on any of the eight planets to Earth days and fixed 365.25-day Julian years.
Convert a chosen number of planetary years into a terrestrial duration. This calculator uses a sidereal year: one orbit around the Sun relative to the stars. It covers the eight planets and displays both Earth days and fixed-length Julian years, with a reference table that makes the adopted periods explicit.
How to use this tool
- Select the planet whose orbital year you want to use.
- Enter a nonnegative number of those years, including fractions if needed.
- Read the resulting Earth-day duration and its equivalent in Julian years.
- Compare the reference table when moving between planets or checking a published figure.
Formula
Earth days = planetary years × sidereal orbit period; Julian years = Earth days ÷ 365.25
Each Earth day is 24 hours. Orbital periods come from the individual NASA factsheets, not the rounded overview table’s tropical-period column. The source values are approximate reference periods; extra displayed decimal places do not imply extra astronomical accuracy.
Example
Two Venus sidereal years are 2 × 224.701 = 449.402 Earth days. One Jupiter sidereal year is 4,332.589 Earth days. Earth’s own sidereal period in this table is 365.256 days, slightly longer than the 365.25-day Julian duration unit.
Frequently asked questions
Why does one Earth year not return exactly one Julian year?
They are different definitions. A sidereal year describes an orbit relative to the stars, while a Julian year is a fixed unit of exactly 365.25 days. A civil calendar year is another convention.
Can this predict the date of a planetary return?
No. Precise future positions require an ephemeris, an initial time and a reference frame. Multiplying a fixed mean period is a duration comparison, not an orbital event prediction.
Why are some published year lengths different?
Sources may quote tropical, sidereal or synodic periods and may use different rounding. Compare definitions before comparing numbers; the Sun-relative viewing cycle from Earth is not the same as a planet’s orbital year.
Sources
Last updated: 2026-10-30