Defining seasons: Meteorological versus astronomical seasons
Seasons are periods in a year marked by specific weather conditions, temperatures and length of day. Most calendars around the world divide the year in 4 seasons. These are: spring, summer, fall (autumn) and winter.
The dates on when these seasons begin and end can differ depending on who you ask. People in Australia and New Zealand for example, consider March 1 as the beginning of spring. The Irish on the other hand believe that spring begins on February 1, when they celebrate St Brigid's Day.
In order to be consistent and to make forecasting easier, meterologists divide the year into 4 seasons of 3 months each:
- spring - starting March 1 and ending May 31,
- summer - starting June 1 and ending August 31,
- fall (autumn) - starting September 1 and ending November 30, and
- winter - starting December 1 and ending February 28 (Februrary 29 in a Leap Year).
These seasons are known as meteorological seasons.
Astronomers and scientists on the other hand, use the dates of equinoxes and solstices to mark the beginning and end of seasons in a year.
Like meteorological seasons, there are 4 astronomical seasons in a year:
- spring - March equinox to June solstice,
- summer - June solstice to September equinox,
- fall (autum) - September equinox to December solstice, and
- winter - December solstice to March equinox.
Equinoxes and solstices
Seasons occur because of the 23.5 degree tilt of the Earth's rotational axis - around the June solstice, the Northern Hemisphere is titled towards the sun. This causes summer in the Northern Hemisphere. The Southern Hemisphere on the other hand, is tilted away from the sun and therefore, experiences winter. The opposite occurs around the December Solstice, when the Southern Hemisphere is tilted towards the sun, while the Northern Hemisphere is tilted away.
The solstices and equinoxes, on the other hand are caused by the Earth's orbit around the Sun.
March equinox: The March equinox occurs when the sun crosses the true celestial equator – or the imaginary line in the sky above the Earth’s equator – from south to north on a day between March 20 and 23. In other words, the Sun moves north of the Equator during the March equinox.
June solstice: The June solstice is also referred to as the summer solstice in the Northern Hemisphere and the winter solstice in the Southern Hemisphere. This is the time when the Sun is at its furthest point from the Equator and the Earth’s north pole tilts towards the Sun.
September equinox: The September equinox is also referred to as the autumnal, autumn or fall equinox in the Northern Hemisphere. It is known as the spring or vernal equinox in the Southern Hemisphere. It occurs on a day between September 21 and 24 when the Earth’s axis of rotation is perpendicular to the imaginary line connecting the centers of the Earth and the sun.
December solstice: The December solstice marks the beginning of winter in the Northern Hemisphere, and summer in the Southern Hemisphere. It also sometimes called the winter solstice in the Northern Hemisphere and summer solstice in the Southern Hemisphere. It occurs on a day between December 20 and 23. At this point, the sun appears directly above the Tropic of Capricorn, and the days are shortest at locations north of the Tropic of Cancer. South of the Antarctic Circle the Sun is visible 24 hours per day.
Seasons in the southern hemisphere are opposite to those in the northern hemisphere. For example, the March equinox, also popularly known as the vernal equinox in the northern hemisphere, marks the start of spring in the northern hemisphere but the start of autumn in the southern hemisphere. The same rule applies for the other three seasons.
In this Article
Solstices & equinoxes
- Spring / Vernal equinox
- Summer solstice
- Autumnal / Fall equinox
- Winter solstice
- Equal day & night?
- About seasons
Watch daylight move across the planet... More