Astronomical/meteorological Seasons
The astronomical definition uses the dates of equinoxes and solstices to mark the beginning and end of the seasons:
Spring begins on the spring equinox;
Summer begins on the summer solstice;
Autumn begins on the autumn equinox; and
Winter begins on the winter solstice.
For six months of the year, the North Pole is tilted toward the Sun, and the Sun lies somewhere above the Northern Hemisphere. For the other six months, the South Pole is tilted toward the Sun, and the Sun lies somewhere overhead in the Southern Hemisphere.
There are only two moments each year when neither pole points toward the Sun, and the centre of the Sun lies directly above Earth’s equator. These moments are the equinoxes.
Many of us think that an even balance of day to night happens during an equinox, given that the word translates as “equal night.” However, an equinox is either of the two occasions in the year, the spring equinox and the autumn equinox, where day and night are approximately equal in length.
Twice a year, day and night reach a perfect balance of 12 hours each, creating a little-known event called the equilux.
There’s a subtle time difference between an equinox and an equilux.
The date of the equilux will vary depending on where on Earth you are, whereas the equinox is a fixed instant in time.
Because the timings of the equinoxes and solstices change each year, the length of astronomical seasons within a year and between years also vary.
Equinoxes and solstices dates for here in the UK are as follows:
Spring 2024 : March Equinox : 20 March
Summer 2024 : Solstice 20 June
Autumn 2024: September Equinox: 22 September
Winter 2024: Solstice : 21 December
Meteorological Seasons:
According to the meteorological definition, the seasons begin on the first day of the months that include the equinoxes and solstices. In the Northern Hemisphere, for example,
Spring runs from March 1 to May 31;
Summer runs from June 1 to August 31;
Autumn runs from September 1 to November 30; and
Winter runs from December 1 to February 28 (February 29 in a leap year).
© 2023. Mrs WH