The evening sky in April is in transition. Orion and his retinue, the familiar constellations of winter in the northern hemisphere, are gone, while Cygnus and Lyra, bearers of the summer’s Milky Way, skirt the northeastern horizon. In between, key to the spring stars is the most familiar asterism of all, the Plough, easily spotted overhead at about midnight.
Think of the Plough as a saucepan, and the two stars furthest from the handle are the pointers. Draw a line between them and follow it down in the direction of the northern horizon to find Polaris, the pole star, around which the sky turns. Take Megrez and Phecda, the two stars of the bowl closest to the handle, instead. The line between them, extending to the southwest, reaches Regulus, the brightest star in Leo and recognisable at the base of the sickle asterism, which is the celestial lion’s most recognisable feature.
The handle of the Plough has a distinct curve to it. Follow the line of its stars across the sky, and you pass Arcturus in Boötes, the herdsman, before finding Spica, the brightest star in Virgo, labelled on this month’s map. One of the 20 brightest stars in the sky, Spica owes its prominence to its proximity, lying just 260 light-years away from Earth. It is, in fact, a binary, consisting of a pair of stars each much more massive than the Sun, orbiting closely enough around their common centre of mass that each must be distorted into a teardrop shape. They are, in fact, not much further apart than the Earth is from the Moon.
No telescope yet built has resolved the two stars. Instead, we infer their presence from the changing brightness as the stars orbit each other, and from the so-called Doppler shift that moves the spectrum of the stars toward the blue and then the red as they first approach then recede from us. As one of the nearest examples of such a massive binary, Spica has been much studied in the last century, but its importance is actually millenniums old.
At the time that agriculture was becoming important in the civilisations of the Nile Valley in Egypt, about seven or eight thousand years ago, Spica’s heliacal rising would have heralded the start of the harvest season. As a result, several prominent temples seem to have been aligned in the direction on the horizon when Spica would have been first seen each year. Hipparchus, the great Greek astronomer whose models and calculations of the movements of the Earth, Moon and Sun were the first accurate astronomical tables, is reputed to have visited a 2,000-year-old temple in Thebes and noted that this alignment was no longer true.
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He explained this drift by the precession of the Earth’s equinoxes, publishing in 127BC a text that compared his observations of Spica and other bright stars to those of earlier observers. We now know that this precession is due to the wobbling of the Earth’s axis, the same phenomenon that causes the location of the celestial pole to change over time. Polaris will not be our pole star for ever.
The planetary sky in April is marked by an unusually bright evening apparition of Mercury, after a superior conjunction on the 2nd at which time it will be hidden behind the Sun. Emerging rapidly into twilight, it will be bright and low in the west-north-west after sunset by the 12th, setting an hour after the Sun. Though it fades over the course of the month, it remains above the horizon for longer as the month wears on. Perhaps the easiest way to find it is to look on the evening of the 29th, when the innermost planet is just a touch more than a degree below the easily recognised Pleiades star cluster.
Venus is brilliant in the morning sky, but is low, and Jupiter, Mars and Saturn never get more than ten degrees above the horizon during the hours of darkness. The four planets have a series of reasonably close conjunctions throughout the month, with Mars and Saturn close on April 5 and Venus approaching Jupiter from the 27th onwards.
A final solar system highlight will be the year’s first moderately promising meteor shower, with the Lyrids, whose peak arrives on the night of the 22nd. With the Moon at last quarter and low in the sky, and the peak expected in the early evening of the 22nd, there is a good chance to see a few bright meteors an hour. While not spectacular, the presence of such a reliable shower will add interest to the often overlooked spring skies of April.
