With spring just around the corner, it’s interesting to ask how the seasons and the spring equinox are defined. However, the answer isn’t straightforward and depends on who is asking the question.
Meteorologists and climate scientists use perhaps the simplest approach, defining each season as lasting three months, with spring lasting from March to May in the northern Hemisphere. Summer is then from June to August, Autumn from September to November, and Winter from December to February in the following year.
This means that each season begins and ends on the same date every year and has a similar duration, of about 90 days. This makes it easier to perform statistical comparisons between years, regions and countries.
In contrast, the astronomical seasons are tied to the tilt of the Earth and its elliptical orbit relative to the Sun, with spring and autumn beginning when the Sun is directly over the equator.
These are known as the equinoxes and fall in late March and late September in the northern Hemisphere. For example, in 2020 the Spring Equinox was on 20 March. The summer and winter solstices fall in between, marking the longest and shortest days of the year.
This definition for the seasons has been around for centuries and the spring equinox and summer solstice are marked by festivals and celebrations in some cultures. However, due to orbital variations, both the start date and length of each season vary from year to year. Some countries take a different approach, with China and India being notable examples.
Many people also associate the start of spring with the time that the clocks change to Daylight Saving Time. In the UK this is known as British Summer Time and begins on the last Sunday of March. The lighter evenings are a very welcome change after the winter months.
The clocks then go back seven months later, on the last Sunday of October, marking the return to Greenwich Mean Time (GMT).
However, the use of Daylight Saving Time varies widely around the world, depending on local cultures and climates.
In many places, the equinoxes mark the arrival of some of the highest tides of the year. This can be a great time to see some of the tidal bores that occur on about a hundred estuaries around the world. In the right conditions this leads to a spectacular wave or line of surf travelling upstream against the river flow.