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.

NB launch actually took place on Thursday 11 February, 10:23 EST (15:23 GMT).

SDO is a powerful solar observatory which will look deep into the Sun and record images at 10 times higher resolution than HDTV. It is the first mission to be launched for NASA's Living With a Star (LWS) Program, designed to understand the causes of solar variability and its impacts on Earth. It is hoped that it will reveal how solar storms erupt, help us understand the Sun's influence on Earth and Near-Earth space and also help scientists build effective models for space weather forecasting.

The SDO spacecraft will orbit the Earth at a distance of about 22,300 miles, relaying its readings instantly to a ground station in New Mexico.

Image: Artist's impression of SDO (NASA)

Come and see the Sun in a whole new light with new exhibition Solar Story, planetarium show Secrets of the Sun and a programme of talks, tours and workshops. Speakers include solar physicist Dr Lucie Green and ROG Curator for the History of Science and Technology Dr Rebekah Higgitt.

This is the second extension to the mission which was originally launched in October 1997. Dubbed the Cassini Solstice Mission, this seven-year extension will allow scientists to study seasonal and other long-term weather changes to the ringed planet and its moons as it moves from winter to summer. It also will allow continued observations of Saturn's rings and magnetosphere.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency (ESA) and the Italian Space Agency. JPL manages the project for NASA's Science Mission Directorate in Washington. The Cassini orbiter was designed, developed and assembled at JPL.

Read the full story on NASA's JPL website. Further Cassini information is available at www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.

Image: Looking down on Saturn's northern hemisphere, Cassini uses false-colour imaging to reveal different layers of cloud. PIA09212. © NASA/JPL/University of Arizona. View the ROG's online gallery of Cassini-Huygens images, Visions of Saturn.

Image: NASA / HST

Read the ROG's Pluto fact file

The truth is of course that nobody knows for sure, but there are some more and less convincing theories.

Only Matthew's gospel mentions the star and the Magi or wise men, in the following passage:

After Jesus was born in Bethlehem in Judea, during the time of King Herod, Magi from the east came to Jerusalem and asked, "Where is the one who has been born king of the Jews? We saw his star in the east [or at its rising] and have come to worship him."... Then Herod called the Magi secretly and found out from them the exact time the star had appeared... After they had heard the king, they went on their way, and the star they had seen in the east [or at its rising] went ahead of them until it stopped over the place where the child was. When they saw the star, they were overjoyed. (Matthew chapter 2, verses 1-2, 7, 9-10, New International Version)

Astronomically, it's been suggested that the star may have been a nova or supernova explosion; a comet; a triple conjunction of Jupiter and Saturn; a close grouping of the three planets Jupiter, Saturn and Mars; a stationary point of Jupiter; or a variable star (one whose brightness changes over time).

Chinese records mention a possible nova or comet in 5BC - an unusually bright star which appeared in the eastern sky for 70 days, and which may have been a nova outburst from the variable star DO Aquilae. This occurred at about the same time as a triple conjunction of Jupiter and Saturn in the constellation of Pisces. The rare combination of these two events may well have been seen by the Magi as a religious sign.

It's likely that the Magi studied astrology, so the star's astrological aspects are probably at least as important as its astronomical explanation. Rutgers astronomer Michael Molnar has recently suggested that a double occultation of Jupiter by the Moon in Aries in 6BC could have astrologically signified the birth of a divine 'king of the Jews'.

Others of couse think that the writer of Matthew's gospel simply invented the star, perhaps to fulfil the Old Testament prophecy that 'A star will come out of Jacob; a sceptre will rise out of Israel' (Numbers chapter 24, verse 17). More likely is that Matthew's star is simply an example of 'Midrash' - an established Judaic tradition of theological writing in which non-factual elements can be used to bring out the religious meaning of the factual account. So whether or not there actually was a star is less important than the spiritual message Matthew is trying to convey.

We can't know for sure whether, what or when the star was. But perhaps the answer is not either/or out of the alternative strands of explanation - astronomical, astrological, theological, supernatural - but both/and. It's plausible that the Star of Bethlehem was a genuine astronomical event - perhaps a nova associated with a variable star - that had astrological significance to the Magi and theological significance to Matthew.

Whatever the truth is, we wish you a very happy Christmas!

Find out more about solstices and equinoxes in our fact file.

This weekend the annual Geminids meteor shower reaches its peak, on the night of 13 December and the morning of 14 December.

The Earth will be ploughing through a stream of debris left behind by asteroid 3200 Phaethon, and we see these fragments burn up as they hit the Earth's atmosphere, causing the shooting stars. Some of the fragments can reach the size of big fireballs.

The Geminids shower seems to be intensifying each year, and an average of 100 meteors per hour are expected to radiate from near the bright star Castor. The peak date this year also occurs two days before the new Moon which should make for ideal viewing conditions.

Greg Smye-Rumsby explains how to see the meteors, courtesy of Astronomy Now:

Details of all the major annual meteor showers visible from the UK are available on the NMM website.

Christmas Sky Watch

Explore the wonders of the winter skies, and gain a useful introduction to the use of binoculars and small telescopes. The session will include a planetarium talk, a short seminar and a practical session.

Date: 11 December 2009
Time: 19.00-22.00
Cost: £15/£11

Christmas stargazing

Part of our programme of observing evenings, these sessions offer you an opportunity to use our historic 28" diameter refracting telescope, the seventh largest in the world. With a chance to look at star clusters and double stars, the programme provides an unusual Christmas experience that's out of this world. Places limited.

Please note: If adverse weather conditions prevent viewing through the telescopes, an alternative programme will be offered. We are not unable to offer refunds under such circumstances.

To book a place

Image: NGC 2264 including the Christmas Tree Cluster (ESO)

The Leonids have very brief periods when hundreds or even thousands of meteors can be seen. This seems to come to a peak every 33 years, the period of Comet Tempel-Tuttle. These 'storms' of shooting stars were seen in 1799, 1833, 1866, 1966 and 1999-2001 (although the expected 1899 and 1933 storms were very disappointing). The 1833 storm was particularly spectacular, with estimates of 100,000 meteors per hour. The 1999-2001 storms produced about 3000 per hour.

Image: Leonid shower; photo by Thomas Paulech and Juraj Toth, Bratislava, Slovakia

The first spacecraft, the 2200kg Centaur rocket stage, will hit the Moon at about 4800 km/h or twice the speed of a bullet, throwing up debris plumes to an expected height of 10km above the lunar surface, visible from Earth-based telescopes 10-12" and larger.

The smaller 'shepherding spacecraft' will follow, descending through the debris plume to impact 4 minutes later. As it travels through the plume, onboard spectrometers will monitor the chemical components of the debris, looking for water, hydroxyl compunds, salts, clays and organic molecules, and relay this information back to Earth before impact.

Projected first impact is currently 12:31 BST.

You can watch the impact live online on NASA TV.

Image: Artist's rendering of the LCROSS spacecraft and Centaur separation (NASA).