Season is a term used to describe the division of a year. Most commonly, it refers to the calendar seasons: winter, spring, summer, autumn (fall). In tropical regions, there are usually only two seasons: the wet/monsoon season and the dry season.

Each season lasts approximately 3 months. There are various definitions of when the calendar seasons begin and end, dependent on a number of factors, including where you live. The two main ways that seasons are usually defined are the astronomical definition and the meteorological definition.

Meteorological definition:

The meterological definition of the seasons assigns fixed dates based on temperature, with summer being the hottest quarter of the year and winter being the coldest. The seasons in the Northern hemisphere of Earth are the opposite of those in the Southern hemisphere at the same time of year:

Northern hemisphere Southern hemisphere Start date End date
Winter Summer 1 Dec 28 Feb
Spring Autumn 1 Mar 31 May
Summer Winter 1 Jun 31 Aug
Autumn Spring 1 Sep 30 Nov

Astronomical definition:

The astronomical definition of the seasons is based on the dates of equinoxes and solstices, which mark the beginnings and the ends of seasons. Equinoxes occur when the sun travels over the Equator, and solstices occur when the sun travels over the Tropic of Cancer (June) and Tropic of Capricorn (December).

Unlike meteorological seasons, the timing of astronomical seasons varies from year to year because the sun does not travel over these points at exactly the same times each year. Because of this, seasons in the astronomical definition also are not the same length, both within the same year, as well as between years.

Below is a table showing the astronomical seasons from 2016 - 2024:

  Spring equinox Summer solstice Fall equinox Winter solstice
  March June September December
Year day time day time day time day time
2016 20 04:30 20 22:34 22 14:21 21 10:44
2017 20 10:28 21 04:24 22 20:02 21 16:28
2018 20 16:15 21 10:07 23 01:54 21 22:23
2019 20 21:58 21 15:54 23 07:50 22 04:19
2020 20 03:50 20 21:44 22 13:31 21 10:02
2021 20 09:37 21 03:32 22 19:21 21 15:59
2022 20 15:33 21 09:14 23 01:04 21 21:48
2023 20 21:24 21 14:58 23 06:50 22 03:27
2024 20 03:07 20 20:51 22 12:44 21 15:03

Why do we have seasons?

We have seasons because of the Earth's tilt relative to what is known as the ecliptic plane as it rotates around the Sun. Essentially, this means that the plane on which the Earth rotates about its axis is not level with the plane around which it orbits the Sun; its axis of rotation is not orthogonal to its orbital plane. Refer to the figures below for clarification:

The figure above represents the Earth's orbit around the Sun, with the ellipse being the path the Earth travels around the Sun. Below is a side-view such that we are looking at the plane of the Earth's orbit around the Sun.

The dotted line represents the Earth's axis of rotation, which goes through the center of the Earth. The solid white line with arrows through the Earth represents how it rotates about its axis. Picture this as a circle going around the full sphere that represents the Earth. From the figure, we can see that the plane that this circle lies on is tilted relative to Earth's orbital plane, specifically at an angle of 23°.

Thus, differences in the Earth's seasons around the globe are due to this tilt. The Northern Hemisphere has more sunlight during the months of May, June, and July because this is the period during which the Northern Hemisphere faces the Sun. The same is true of the Southern Hemisphere during the months of November, December, and January.

The reason that certain parts of the Earth, namely the tropical and subtropical regions, don't really experience the four seasons (winter, spring, summer, autumn), is because of their location close to the Equator. Regions around the Equator see little variation in sunlight throughout the year, so average temperature does not really change as they do in the Northern and Southern hemispheres of Earth. Instead, these regions around the Equator experience wet and dry seasons where the amount of precipitation significantly varies.