When do the seasons start?

Why the date changes every year

Astronomical seasons start when the Sun reaches precise points of the orbit (equinoxes and solstices): since the solar year isn't a whole number of days, the instant drifts by about 6 hours per year, and leap years pull it back. That's why spring can start on March 19, 20 or 21.

Astronomical vs meteorological

Meteorological seasons, used in climate statistics, always start on the 1st of the month (March 1, June 1, September 1, December 1). Astronomical ones, these, follow the Sun, with minute-accurate times (Meeus' algorithms).

Why 21 March almost never happens

The idea that spring starts on 21 March comes from school textbooks, not from astronomy: in the twenty-first century the equinox nearly always falls on 20 March, and in 2044 it will even reach the 19th. The date is not fixed because the solar year lasts 365 days and almost six hours.

Those extra six hours make the equinox slide forward by about six hours each year, until a leap year drags it back by twenty-four. It is a sawtooth that repeats every four years, and it is why the same season starts on different days.

Astronomical and meteorological do not match

Meteorological seasons start on the first of the month (1 March, 1 June, 1 September, 1 December) and exist for climate statistics, which need whole and always identical quarters to compare one year with another.

Astronomical ones follow the Earth's position relative to the Sun, so they fall when they fall. The two definitions are about three weeks apart, which is why in early December the news already talks about winter while the calendar still says autumn.

The seasons are not the same length

The Earth's orbit is an ellipse, not a circle: in January the Earth is closer to the Sun and moves faster, in July it is further away and slows down. The result is that in the northern hemisphere summer lasts almost five days longer than winter.

In the southern hemisphere the exact opposite happens, and the seasons are reversed as well: the December solstice, which opens winter here, opens summer in Australia and Argentina. That is why a date alone is not enough to say which season is being talked about.

Nearby tools

How much daylight there really is on a given date is shown by Sunrise and sunset. The clock change sits in Daylight saving change, and the Moon's phases in Moon phases.