The vernal (spring) equinox occurred this morning at 05.14 GMT. But what actually is the vernal equinox and why does it happen?

During the course of a year the Earth completes one orbit around the Sun. From our perspective, we see this as the Sun moving through the year against the background of stars, along an imaginary line which we call the ecliptic. As the Sun moves (or appears to move) along this path, for half the year it’s seen to be above the celestial equator, which is the projection of the Earth’s equator onto the sky – this is summer in the northern hemisphere. For the other half-year it’s below this equator (northern winter).

So the Sun’s path appears to cross the celestial equator twice a year, in March and September. These times are the vernal and autumnal equinoxes – so called because at these times, day and night are of nearly equal length at all latitudes (equinox means ‘equal night’).

Find out more about equinoxes and solstices in our fact file, including why the equinoxes don’t always occur on the same day and why they don’t occur at the times when day and night are exactly equal.

See also Has spring started yet?

21 December 2010 – UK viewers will be able to catch the start of a total lunar eclipse this morning, the first for three years, with totality starting at 07.41 GMT and lasting a little over an hour. Within the UK, Scotland and Northern Ireland will get the best views.

Lunar eclipses occur when the Moon passes through the Earth’s shadow. During eclipse, the Moon may turn blood red or pink, with indirect sunlight giving the Moon a ghostly hue. Find out more in our eclipses fact file.

The last time a total lunar eclipse occurred on the winter solstice was in 1638, and the next time will be in 2094.

20 December 2011 – At 23.38 UTC (GMT) tomorrow, 21 December, the solstice will occur. In the Northern hemisphere it’s the winter solstice, in the Southern hemisphere the summer solstice.

At the solstices the Sun is at its furthest from the celestial equator (the projection of the Earth’s equator onto the sky). The world ‘solstice’ comes from the Latin solstitium meaning ‘Sun stands still’ because the apparent movement of the Sun’s path north or south stops before changing direction. At the winter solstice, the apparent position of the Sun reaches its most southerly point against the background stars.

Shortest day and sunrise/sunset times

The winter solstice also marks the shortest day (and longest night), but not the earliest sunrise or latest sunset. The earliest sunset occurred on around 12 December 2010 (15:51 in London) and the latest sunrise will occur around 30 December (08:06 in London).

The reason for this is the slight variation in the length of ‘natural’ days throughout the year, resulting from a combination of the Earth’s elliptical orbit around the Sun and the tilt of the planet’s rotation axis. For clocks to work all days need to have an equal length, which is therefore fixed at the average length of a natural day (hence the ‘mean’ in Greenwich Mean Time). This has a knock-on effect on sunrise/sunset times, and the earliest sunrise occurs several days before the longest day and the latest several days (about 9) after the shortest.

Opinion is divided over whether the solstice marks the start of winter or the middle of winter, or whether winter actually starts on 1 December (as reckoned by most meteorologists).

Marking the solstice

Under the early Julian Calendar, the winter solstice occurred on 25 December. When the more accurate Gregorian calendar was introduced in 1582, the solstice slipped to the 21st, but the Christian celebration of the birth of Christ has continued to be held on 25 December. This date is also associated with the Roman Saturnalia festival, and ‘Dies Natalis Solis Invicti’ (the birthday of the unconquered sun), as well as Nordic pagan festivals.

Solstice shorts: Yuletide stargazing

For an alternative way to celebrate the solstice, why not join us for our 25-minute Yuletide stargazing sessions, daily at 16.30 and 17.00 on 20-23 December? These offer a unique opportunity to look through our historic 28-inch diameter refracting telescope, and to view one of the more striking double stars.

Places are limited and tickets are £6 per person. You can book online or call 020 8312 6608 between 10.00-16.00. (Please note: If adverse weather conditions prevent viewing through the telescopes, an alternative programme will be offered. We are not able to offer refunds under such circumstances.)

Whichever way you choose to celebrate this time of year, we hope you will enjoy yourselves and have a very happy festive season!

Spring – perhaps the most eagerly-awaited of seasons, when days start to lengthen, temperatures to rise, weather to improve and all of nature to burst into bud and blossom as earth throws off the cold grip of winter. The vernal season has inspired music from Vivaldi to Elvis Presley (‘Spring Fever’) and poetry including Robert Burns’s hilariously bawdy ‘Ode to Spring’ (1794).

But when does spring actually start, in the northern hemisphere at least? Is it the first day of March, or the date of the vernal equinox around 20/21 March, or even sometime in early February with the equinox marking the middle of the season not its start? It all depends very much on who you ask, and on your definition of spring.

There are three main different ways of defining spring – astronomical, meteorological and phenological.

Astronomically, the four seasons centre around the equinoxes and solstices. However, there is disagreement between those who see the equinox/solstice as the start of the season and those who hold that it represents the middle of the season. (For example, the summer solstice is when the sun is at its highest and solar radiation received by the earth’s surface is greatest, so some argue that logically this must mark the mid-point of summer not its start.) The East Asian and Celtic calendars certainly see the vernal equinox as mid-spring, with the season starting in early February. However, the popular view of the 20/21 March spring equinox as the start of the season is likely to persist, at least in the UK.

By contrast, 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. On this basis, 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.

The third way of defining spring is to use what are known as phenological indicators. These cover a range of ecological/biological signs such as the appearance of the first daffodils, crocuses and hyacinths, the budding of trees, the nesting of birds and the emergence of animals from winter hibernation. These events of course are greatly influenced by weather and climate, and so changing climate could cause spring to start earlier than the standard astronomical or meteorological definitions.

So when does spring start? You can decide. Looking out of the window today, I’d be cautiously inclined to agree with the meteorologists that it’s already here.

Images: Equinoxes by Greg Smye-Rumsby; Daffodils in the grounds of the National Maritime Museum, Royal Observatory in background.