Did you know that today – 25 March – used to be New Year’s Day in England right up until this country’s very late adoption of the Gregorian Calendar in 1752?

25 March in the western church’s calendar is the Feast of the Annunciation, commemorating the Angel Gabriel’s announcement to Mary that she would be the mother of Jesus. It is also of course near the date of the spring equinox as well as of Easter.

As there’s no compelling scientific reason for choosing any particular day as the start of the year, it at least makes poetic and emotional sense to celebrate the New Year at the same time as we’re celebrating spring and new life than in the cold midwinter dark of January.

Other notable New Year’s Days this year include last month’s Chinese New Year, and Al Hijra (Islamic New Year’s Day) on 7 December 2010 (1 Muharram, 1432 AH).

Whichever way you reckon these things, since yesterday’s equinox it’s undeniably spring now in the northern hemisphere (see earlier post on the debate over when spring starts). Here in Greenwich at least, it even actually feels like spring today.

Then this coming Sunday residents of the UK will lose an hour’s sleep to gain an extra hour’s daylight in the evenings. Longer evenings are not of course good news for everyone – amateur astronomers will have to stay up later and later to get the darkness needed for observing.

So at 1.00 am GMT on Sunday 28 March clocks in the UK officially move forward by an hour as civil time changes from
Coordinated Universal Time (almost identical to Greenwich Mean Time) to
British Summer Time or BST.

Which means that though it’s definitely now spring, as of next week we’ll be in British Summer Time, which this year lasts until 31 October when by most counts we’ll be in mid-autumn… it’s no wonder we get confused about the seasons.

In search of a bit more clarity, why not listen in to David Rooney discovering more about British Summer Time or have a look at our Spring Forward: 100 Years of British Summer Time page?

International Space Station seen from Space Shuttle, courtesy NASA.

The International Space Station (ISS) is visible over the UK in the early evening this week. To see it, go outside at dusk and look for a bright light, much brighter than the stars, moving steadily from West to East. This evening, 15th March 2010, it will be visible for about 5 minutes, starting at 7.10pm in the west. It should rise to an altitude of about 60º above the southern horizon, before falling into the east and vanishing into the Earth’s shadow.

There will be other bright passes at dusk on Tuesday and Wednesday. The last chance to see the ISS on this set of passes will be Sunday, when it will be 15º above the horizon at dusk. A full set of times for London, with sky maps, is available from Heavens Above. Other locations in the UK can be selected from the Heavens Above home page.

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.