The first day of spring in the UK astronomically occurs on the vernal equinox, 20 March 2024. But did you know there are actually three main definitions for the season coming from astronomy, meteorology and phenology? What are the seasons? The seasons are divisions of the year which relate to the annual changes in the weather. There are four seasons; spring, summer, autumn and winter. The seasons relate to weather patterns in parts of the world such as Europe and North America where the onset of growth in plants and the reawakening of nature (spring) can be separated from the hot season when crops ripen (summer) or when trees lose their leaves (autumn or fall) and a final cold period (winter). Image The spring months: meteorological spring For most of the northern hemisphere, the spring months are usually March, April and May, and so by this definition spring starts on 1 March. This is because meteorologists tend to divide seasons into periods of three whole months based on average monthly temperatures, with summer as the warmest and winter as the coldest. Image Equinoxes and solstices: astronomical spring Astronomically, the arrangement of the planes of the orbit of the Earth and its equator are such that the planes intersect at two times, the Equinoxes, when the length of the day and night are equal. Mid-way between these are the solstices, when the Sun is at its highest and lowest in the sky at mid-day. These times can be determined very accurately and, as they occur near the times when the seasons are changing, have been used to indicate the start of each season. Thus for us in Europe: Spring is deemed to start at the Vernal Equinox (near 20 March) Summer at the Summer Solstice (near 21 June) Autumn at the Autumn Equinox (near 23 September) Winter at the Winter Solstice (near 22 December). In the southern hemisphere the cycle is displaced by half a year. Seasons in the UK and elsewhere In temperate latitudes such as the UK, the cycle of four seasons per year is a good representation of the weather pattern but in other parts of the world this is not so. In the polar regions, there are effectively only two seasons, winter when the Sun never rises and summer when it never sets. In the tropics, there can be two wet and two dry seasons, both hot, each year. In the far-east the weather is dominated by the monsoons and there are three seasons, cool-dry, hot-dry and hot-wet. The cause of the seasons is that the plane of the Earth's orbit around the Sun is inclined to the plane of the Equator. This means that the direction of the Sun's rays relative to the ground and the number of daylight hours change during the year and hence the amount of solar energy received at different latitudes changes during the year. The official start of spring In reality there are no hard and fast criteria to determine the start of each season; the onset of spring, for instance, could be the date on which the first daffodil flowers or the first birds make their nests. The dates of these are not only extremely difficult to determine but also vary quite dramatically through the United Kingdom, let alone the rest of the world. Because they are well-defined dates, the equinoxes and solstices are probably as good as any other criterion, although just as many people prefer the simple use of three calendar months for each season, with spring being March, April and May. Got a spring in your step? Visit the Royal Observatory, the historic home of UK astronomy Royal Observatory Visit the home of Greenwich Mean Time (GMT), the Prime Meridian of the world and London’s only Planetarium Plan your visit Plan your visit to the Royal Observatory Top things to do From historic clocks and telescopes to cutting edge astronomy, find all there is to see at the home of Greenwich Mean Time Shop Royal Observatory Greenwich John Harrison's H4-Inspired Chrome Pocket Watch £175.00 A stunningly crafted, chrome-plated half double hunter pocket watch inspired by John Harrison’s world-changing chronometer... Buy Now Shop On the Line: The Story of the Greenwich Meridian £8.99 The Royal Observatory at Greenwich is the home of time and space, the Prime Meridian and Greenwich Mean Time (GMT). But have you ever stopped to ask what is the Prime Meridian and why it is at Greenwich? Why are all time zones across the world based on GMT? What is longitude and why is the meridian moving? Written by Dr Louise Devoy, Senior Curator of the Royal Observatory at Royal Museums Greenwich... Buy Now Shop Greenwich Shepherd Gate 45cm Wall Clock, 24-Hour Analogue Dial £150.00 Own a Shepherd Gate 45cm clock, exclusively available from the home of Greenwich Mean Time. A stylish 24-hour analogue clock that will make a statement on any wall, with its bold monochrome face, and matte black metal casing... Buy Now