April sky at night — click on the image below to see a larger version

As the lengthening days of spring are extinguishing evening viewing of the winter sky, so Mars too is vanishing from view. It spends the month low in the west after sunset, crossing the constellation of Gemini. An obvious target only a month or two ago, by the end of the month it will have faded to magnitude 1.2, slightly fainter than the brightest star in the constellation, Pollux, which it approaches and then moves past as the month wears on.

If you are not familiar with this region of sky, simply look due west just after sunset and search for the three brightest stars forming a right-angled triangle (at the start of the month) or a nearly straight line (at the end).

Mars is now more than 124 million miles from Earth, and the view through all but the largest telescopes will be disappointing as a result. In fact, the best way to get a detailed view of the planet will be online; the flotilla of spacecraft currently studying the Red Planet will be paying particular attention before the landing of Nasa’s Phoenix probe next month.

Returning to the view from Earth, Mars is accompanied in the evening sky by Saturn, in the constellation of Leo. Saturn is visible next to and brighter than Regulus, the brightest star in Leo. If you have a small or medium-sized telescope you really should take this opportunity to catch Saturn’s rings. The relative movement of the Earth and Saturn is steadily closing them from our perspective and they will appear edge-on in 2009.

However, the position of the planets is such that the rings open up to 10 degrees in April, providing the last really good opportunity to see them in all their glory. A good test of sky conditions (and eyesight) when the rings are at this angle is to try to see the main gap in the rings, the Cassini division. These changes from year to year and month to month completely confused Galileo, their discoverer, who couldn’t work out why the ansae, or “handles”, that he had recorded around the planet were sometimes visible and at other times absent.

The third and final planet visible on April evenings is Mercury. The innermost planet reaches superior conjunction on the 16th and will be invisible for a week or so either side of that date. By April 24, though, it will be visible in both the dawn and evening sky, although because of the angle that its orbit makes with the equator the latter is by far and away the better chance to see it. Look west immediately after sunset and Mercury will be the brightest star close to the horizon, just south and east of Aldebaran in Taurus.

In the morning sky, Jupiter is unmissable at brighter than magnitude -2, more than three times brighter than the brightest star in the sky. In the constellation of Sagittarius, it remains frustratingly low for observers in Britain, but does at least provide the best photographic target when the Moon passes to the south on April 27.

The lighter evenings of April are a time of transition for the constellations. While the winter sky is dominated by the familiar form of Orion, the great hunter of the sky, and warm summer nights are marked by the three bright stars of the summer triangle, the spring sky lacks familiar landmarks. However, with the most familiar signpost of all, the Plough, overhead, it is reasonably easy to orientate yourself.

Think of the Plough as a saucepan, and the two stars farthest from the handle point the way to the pole star. Take the other two stars of the “pan”, Phecda and Megrez, and follow their line south until you come to a pair of bright stars. The one on the left is Saturn, and the one on the right Regulus, the foot of the sickle that marks the head of Leo the lion.

Regulus is translated from the Latin as either “prince” or “little king” (depending on how literal one is), but I’ve always preferred the Arabic Qalb al-Asad — the lion’s heart. The rest of the constellation stretches out to the East; the second brightest star, Denebola, marks the fairly prominent triangle of the lion’s feet.

Denebola is a young star, with an age of only 400 million years compared with the Sun’s 4.5 billion. Like many teenage stars, it is unstable, and varies in brightness on a timescale of only a few hours. The fluctuations are small in Denebola’s case, though, and it will soon settle down to a stable middle age.

Return to the Plough, and this time follow the curve of the saucepan’s handle away from the bowl. The first bright star you reach is Arcturus at the base of the Y shape of Boötes, and the second is Spica in Virgo. Virgo is the neighbouring zodiacal constellation to Leo, a sprawling set of stars that includes the largest nearby cluster of galaxies to the Milky Way. To explore the riches of the Virgo cluster properly you’ll need a telescope, but binoculars will show the brightest.

Imagine Spica as the bottom left star of a parallelogram. The top right star is delta Virginis, and to the north of this star you find Epsilon Virginis, or Vindematrix. The heart of the Virgo cluster of galaxies lies midway between this star and Denebola in Leo.

From a dark sky, binoculars larger than 10x50 will show a few of the brighter members — M84 and M86 in the same field, and M60. They will appear as faint, fuzzy patches, but it’s an awe-inspiring thought that the light reaching one’s eyes has travelled for more than 50 million years.