How long is a year?
About 365 days and six hours, but the last few minutes depend on what you measure from: the seasons, the stars, or the point where Earth comes closest to the Sun.
The year of the seasons
The tropical year is the year of the seasons: about 365 d 5 h 48 min 45 s, averaged around the whole orbit. It is the year our calendar follows and the seasons keep. But measured from one particular point, an equinox or a solstice, it comes out slightly different, and our data shows it plainly:
| From one … to the next | Average, 1900 to 2100 | Short of a full circuit |
|---|---|---|
| March Equinox | 365 d 5 h 49 min 2 s 365.24238 d | 20.1 min |
| June Solstice | 365 d 5 h 47 min 58 s 365.24165 d | 21.2 min |
| September Equinox | 365 d 5 h 48 min 31 s 365.24202 d | 20.7 min |
| December Solstice | 365 d 5 h 49 min 33 s 365.24275 d | 19.6 min |
Why aren’t they equal? Picture Earth as a runner on a track, where one full lap is the year measured against the stars. Each lap, the finish line (an equinox or a solstice) is moved one small step back toward the oncoming runner, by the slow wobble of Earth’s axis. So every lap ends a step short of a full circuit, and the time that step saves depends on how fast the runner is going there.
Earth runs fastest in early January, near perihelion, and slowest in early July. The December Solstice sits on the fast stretch: the step takes almost no time, so little is saved and that year is the longest. The June Solstice sits on the slow stretch: the same step takes longer to run, so more is saved and that year is the shortest. The step is the same size every time, about 50 arcseconds of orbit; only the speed changes. Longest and shortest differ by about 95 seconds.
The year against the stars
The sidereal year is one full circuit measured against the distant stars: about 365 d 6 h 9 min 10 s, 20 minutes longer than the tropical year. The difference is precession: Earth’s axis wobbles like a slowing top, once every 26,000 years or so, dragging the equinoxes backward around the orbit. It is why a calendar tied to the stars would slowly lose the seasons.
The year from perihelion
The anomalistic year runs from one perihelion to the next: about 365 d 6 h 13 min 53 s, 25 minutes longer than the tropical year, because the orbit itself slowly turns. Single years vary far more than that: between 1900 and2100 they run from 363.0 to 368.0 days, because the Moon tugs Earth back and forth around their shared center of mass and moves the exact moment of closest approach by more than a day.
The calendar’s year
The Gregorian calendar averages 365.2425 days: a leap day every four years, except in century years not divisible by 400, so 1900 and 2100 have none. That is only about 27 seconds longer than the tropical year, so the calendar drifts from the seasons by one day in roughly 3,200 years. Over shorter spans, dates wobble by about six hours a year through each leap cycle, which is why a solstice can land on the 20th one year and the 21st the next. The older Julian calendar used exactly 365.25 days and drifted a day every 128 years, which is why the switch in 1582 had to skip ten days.
Other years
- Draconic year, 346.62 days
- From one passage of the Sun through a node of the Moon’s orbit to the next: the rhythm of eclipse seasons.
- Lunar year, 354.37 days
- Twelve cycles of the Moon’s phases, the year of purely lunar calendars.
The averages above are measured from Solmanac’s own data. The sidereal and anomalistic figures are the standard values at epoch 2000. See also the time between any two moments.