I BEGIN my tour of the stars this month with a closer look at the Plough. Ursa Major, or the Great Bear, is a large constellation and the Plough, which is made up of seven stars, is only part of it. A pattern of stars within the larger pattern of a constellation, like the Plough, is referred to as an 'asterism'. The Plough is extremely easy to find, not because its stars are strikingly brilliant, but because the pattern is so well known.

During spring the Plough is practically overhead. The group is often known by its American name; the Big Dipper, while a very old English name for it is ‘King Charles' wain’, meaning a cart. Most of the star names in the sky are Arabic, including those in the Plough which I have labelled on the sky map.

The star at the end of the handle of the plough is Alkaid; next in line is Mizar, with a faint companion star Alcor nearby. The Arab astronomers a thousand years ago used Alcor as a test of naked eye vision, hinting that it could only be seen if the sky was very clear. The third star in the handle is Alioth.

The four stars that make the blade of the plough form a rectangular shape connected to the handle. Staring with top left we have Megrez; bottom left is Phad; the top right hand star is Dubhe and the bottom right hand star is Merak. Dubhe and Merak are called the pointers because they point to the North Star.

Megrez is obviously fainter than the other six members of the Plough and there is a minor mystery here. Astronomers living a thousand years ago said that Megrez was as bright as the other stars in the Plough. It is believed that Megrez might be a type of star that changes brightness over a period of hundreds of years.

If you use the pointers to draw a line across the sky from Merak through Dubhe, you will come across a star all on its own. This is is the North Star, or to give its proper name, Polaris. The North Star is the end star of the tail of Ursa Minor, the Small Bear. Again there is a plough shape to the main stars in this constellation. The Americans refer to this group as the Little Dipper. Compared to Ursa Major the stars that make up Ursa Minor (with the exception of the North Star) are quite faint and not so easy to see.

In legend, a princess called Callisto was so beautiful that the queen of the gods Juno became jealous and turned her into a bear. Later Callisto’s son Arcas came across the bear while he was out hunting and drew his bow to shoot her, not recognising her as his mother. Seeing what Juno had done and what was about to happen, Jupiter turned Arcas into a bear. He then grabbed both bears by their tails and threw them in to the sky, stretching their tails in the process. Callisto and Arcas became the Great Bear and the Small Bear, complete with longer tails than earthly bears.

The sky's seasonal changeover is more or less complete. The symbol of winter, Orion, has almost disappeared, although the northern part of the constellation still does not set until midnight. Of the other winter stars, Aldebaran in Taurus and Sirius in Canis Major have now set, while only Capella in Auriga, Procyon in Canis Minor and Castor and Pollux in Gemini are easy to see.

The Plough is now directly overhead which means that the ‘W’ of Cassiopeia is at its lowest point in the north, although it is not difficult to find. Two of the stars of the summer triangle; Vega in Lyra and Deneb in Cygnus are now above the horizon and will be very easy to spot in the summer months.

The winter night sky is often described as the best, with its many bright stars making it a difficult act for the spring sky to follow. This is rather unfair because there are some spectacular sights to be seen in spring.

Beginning our tour around the spring sky with the Plough, follow the curve of the handle down and around and you will come across a bright orange star called Arcturus, the brightest star in the constellation of Bootes the Herdsman. There are no definite legends connected with this constellation, although one story says that Arcturus was the person who invented the two-oxen plough.

Arcturus is an orange giant star around 36 light years away. Its orange colour indicates that it is cooler than the Sun, its temperature being only around 4,000 degrees centigrade compared to the 5,800 degrees centigrade of the Sun. If you imagine Arcturus as the bottom of a large letter ‘Y’ in the sky, move upwards to the star Epsilon, then the right arm of the ‘Y’ leads to the star Gamma and the left arm to a star called Alphekka, which is actually the brightest star in the constellation of Corona Borealis, the Northern Crown.

Corona Borealis does resemble a crown with its stars forming a little semi-circle. It has two interesting variable stars, one of which is called ‘R Corona Borealis’, a star which can usually just about be seen with the naked eye but which occasionally fades away so it can only be viewed through a telescope. The other is ‘T Corona Borealis’, which normally can't be seen with the naked eye, but in 1866 and 1946 became visible as a bright star. We can't predict when this star might brighten again, so it is always worth having a look at Corona Borealis. 'T Corona Borealis' is referred to as ‘The Blaze Star’.

If the line drawn from the handle of the plough towards Arcturus is continued down and around, it will reach the bright white star Spica in the constellation of Virgo the Virgin. Apart from Spica the stars in this constellation are not very bright.

The southern sky is dominated by Leo, the celestial Lion, who in mythology was one of the victims of Hercules. Hercules had been ordered to perform a series of twelve labours, one of which was to kill a huge black lion. Despite losing the battle, Leo is more prominent in the sky than Hercules!

Leo is the most splendid of the spring constellations and is a distinctive reverse question mark shape. Its brightest star, Regulus, is not as bright as Arcturus. To find Regulus, return to the Plough and rather than using the pointers, use the two stars nearest the handle. Draw a line from Megrez through Phad and continue downwards until you reach Regulus, which is located at the bottom of the ‘backwards question mark’ of Leo.

The last star to mention in the spring sky is Alphard in the constellation of Hydra the Water Snake. Although it is the largest constellation, Hydra contains very few bright stars except the red star Alphard, which can be found by continuing the line from the pointers of the Plough past Regulus and down towards the horizon. As Alphard is in such a featureless part of the sky it is often referred to as ‘The Solitary One’.

The Planets in April - This is the first month for a long time when all the planets are too close to the sun to be seen easily.

Meteor Showers - This month sees the first of the major meteor showers since January. The April Lyrids will be at maximum on the night of 22 /23 when around 20 meteors per hour might be seen. The Lyrid shower is connected with comet Thatcher which was discovered in 1861. It was first recorded in 687 BCE, making it the oldest recorded meteor shower we can still see today.

The Phases of the Moon - Last quarter 2, New moon 8, First Quarter 15, Full Moon 23 The Full Moon in April is called the Egg Moon. This has nothing at all to do with Easter eggs but is due to the increasing amount of daylight which results in hens, ducks and geese laying more eggs.