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

The next few months bring with them the chance to enjoy Saturn’s rings at their best attitude for 2009. As the giant planet orbits the Sun, the inclination with which we see the rings changes; twice in every Saturnian orbit we see them edge-on in an event called a “ring-plane crossing”. This changing appearance caused trouble for Galileo, who announced his discovery of two companions to Saturn in 1610 (in reality, a poorly resolved image of the rings), only to find in 1612 that these companions had vanished. “Has Saturn devoured his own children?” he asked, plaintively, in a letter to a friend.

Observers with a small telescope wanting to check on the welfare of Saturn’s offspring should be able to catch the rings this month. With Saturn positioned in the evening sky, and the rings widening to almost 4 degrees, this is the time to seek it out.

The planet is easily visible to the naked eye as a star of magnitude 0.2 in among the stars of Leo. To find Leo, start from the Plough which is itself almost overhead before midnight during most of April. If you think of the familiar asterism as a saucepan, take the two stars that make up the side of the pan nearest the handle. These are Phad and Megrez and following the line they make downwards toward the southwestern horizon brings you to Regulus, the brightest star in Leo.

Returning to the Plough provides a signpost to April’s other prominent zodiacal constellation, Virgo. This time follow the curve of the bear’s tail (or plough’s handle, if you prefer) round, through Arcturus in Boötes to reach Spica, alpha Virginis. Spica, the sixteenth brightest star in the nighttime sky, is named after the ear of wheat that Virgo holds in her hand. Virgo herself carries a whole sheaf of mythological associations; while she is often identified with the goddess of Justice, Astraea, my favourite is the strand of thought that places her as the mother goddess Cybele, who travels the sky in a carriage drawn by lions.

The carriage has an extra load this month, as Saturn sits on a line joining Regulus and Spica. As the month wears on the planet will be lower down as the sky darkens, but conditions remain favourable throughout April. The ringed planet is joined by a nearly full moon on the 6th, one of a series of conjunctions that take place between our large satellite and the planets. Venus is the brightest object in the morning sky. Even on April 1, when it is extremely low down at sunrise, it should be visible to those with a clear horizon. In fact, it remains close to the horizon for the entire month, but should nonetheless be easy to spot as the crescent moon swings by on April 22. Mars is there too, almost 5 degrees below Venus, but it will probably be lost in the horizon’s murk for most observers.

Later in the day, the gap between Venus and the Moon will reduce, until by 16.15 BST they will be only half a degree apart. Despite the sunlight, they will be visible in binoculars in the west-southwest, but if you go looking, be very careful. Even a glimpse of the Sun through binoculars could do serious damage to your eyes, so stand in shadow when sweeping the sky.

Mercury also emerges from the gloom at the end of the month, having reached superior conjunction with the Sun on March 31. The fastest-moving planet becomes visible on or around the 12th, with greatest eastern elongation occuring on April 26. Mercury will be fainter by this point, but that is more than compensated for by the thin crescent Moon that sits just 2 degrees away, just below the Pleiades star cluster in Taurus.