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Tropical Year

Also called: solar year, equinox year

Definition: The tropical year is the time between successive spring equinoxes, about 365.2422 days (about 31,556,930 seconds). It is the natural year the Gregorian calendar approximates.

Updated November 4, 2026 · How we check facts

The tropical year measures the cycle of the seasons. The U.S. Naval Observatory gives it as approximately 365.2422 days, and NIST lists it as 3.155693 × 10⁷ s (not an exact value). Seasons follow this year, not the 365-day calendar year, which is why calendars add leap days.

Formula

days = 3.155693 × 10⁷ s ÷ 86,400 s/day = 365.2422

Example

The Gregorian calendar averages 365.2425 days, so it runs 365.2425 − 365.2422 = 0.0003 day longer than the tropical year, about 26 seconds (0.0003 × 86,400). The gap is under half a minute per year.

Calculate it

Free toolDate Duration CalculatorDays between two dates, or a date plus N days.Open →

Guides that explain it

Time Conversions: Seconds, Hours and Weeks in a Year

Related terms

Gregorian CalendarThe Gregorian calendar is the civil calendar in worldwide use. Its 400-year cycle has 146,097 days, an average year of 365.2425 days, kept by the leap-year rule.Leap YearA leap year has 366 days instead of 365. In the Gregorian calendar a year is a leap year if divisible by 4, except centurial years that are not divisible by 400.Second (s)The second is the SI base unit of time, defined from the caesium-133 transition frequency of 9,192,631,770 Hz; there are exactly 60 seconds in a minute and 86,400 in a day.Sidereal DayA sidereal day is one rotation of the Earth relative to the stars: about 86,164.09 seconds, or 23 h 56 min 4.09 s, roughly 3 min 56 s shorter than a 24-hour solar day.Hour (h)The hour is a unit of time equal to exactly 60 minutes or 3,600 seconds, and one 24th of a day. It is accepted for use with the SI but is not an SI unit itself.Leap SecondA leap second is a one-second adjustment added to Coordinated Universal Time (UTC) so it stays within 0.9 s of Earth-rotation time, giving a day of 86,401 seconds.Minute (min)The minute is a unit of time equal to exactly 60 seconds, one 60th of an hour. It is accepted for use with the SI, and a day has 1,440 of them.

Sources