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.

Equinoxes by Greg Smye-RumsbyAstronomically, 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.

Daffodils in NMM groundsThe 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.