March is a transitional month in the evening sky, with the winter constellations led by Orion sinking in the west after sunset, giving way to the parade of recognisable constellations that characterises early spring and summer. As the star map shows, it is also a month in which the most famous constellations of the northern sky are at their best.
Start with Ursa Major, with the familiar asterism of the Plough or Big Dipper at its heart. While most constellations are illusions, with stars at different distances merely appearing to lie next to each other on the two-dimensional globe of the sky, all but two of the stars that comprise the Plough really are associated, moving together through space. It is likely that they formed together in the same nebula, and may even still be gravitationally bound.
The two exceptions, Dubhe and Alkaid, are currently located at opposite ends of the asterism, but their movements will, over the next few tens of thousands of years, reshape the Plough. In the meantime, we can still use it as a guide to the rest of the winter sky. Follow the curve of its “handle” down toward the horizon to find brilliant Arcturus, in Boötes, and then, a little later in the evening, down towards Spica, the brightest star in Virgo. The latter marks the end of a glorious parade of the most distinctive constellations of the zodiac in the evening sky at this time of year, from Taurus and Gemini in the East, through Cancer and Leo before ending up in galaxy-speckled Virgo.
Next, take the “Pointers”, the two stars furthest from the handle, and travel five times as far as the gap between them up, away from the saucepan’s bowl, to find Polaris, the Pole Star, around which the sky turns. The brightest star in the faint constellation of Ursa Minor, its reputation as a constant and steadfast friend to navigators is somewhat undermined by its status as a variable. It is, in fact, a Cepheid, a star whose slow pulsations, driven by an instability in the reactions which power it, cause it to change brightness.
Such stars can be used to measure the expansion of the universe, and so Polaris’s status as the nearest example has made it a subject of close scrutiny. As a crucial rung of the cosmic distance ladder, by which we measure the universe, it is somewhat disturbing that its behaviour has been somewhat erratic. It now varies by rather less in brightness than it did at the start of the 20th century, and the period, which is about four days from maximum to maximum, is increasing, all adding up to a sign that we do not understand quite what is happening in these important stars.
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Much better understood is the notion that Polaris’s status as Pole Star is temporary. The Earth acts as a gyroscope, precessing on its axis in a cycle that takes just over 25,000 years to complete. Five thousand years ago, the star nearest the northern celestial pole was Thuban, in Draco, which is marked on this month’s map. It can be found by following the line made by Phecda and Megrez, the two stars of the Plough’s bowl closest to its handle.
The rest of Draco snakes around Ursa Minor, ending in a circlet of stars next to Lyra with brilliant Vega, itself a former and future Pole Star. The brightest, and most significant, of the circlet stars is Gamma Draconis, observed with care by the third astronomer royal, James Bradley. Bradley’s careful measurements of the its aberration, a tiny change in position due to the velocity of the Earth as it speeds around the Sun, allowed an early measurement of the speed of light. Despite the extreme difficulty of the measurement (and the fact he was trying to measure something else entirely), Bradley got the answer right to within a couple of per cent, and provided direct proof that the Earth was in motion around the Sun as a bonus.
Back in the Solar System, the only real prospect for planetary observation this March is Jupiter, which remains unmistakably bright in the evening sky. There is also a penumbral lunar eclipse in the early morning of March 25, when the full Moon will pass through the edge of the Earth’s shadow. Viewed from the lunar surface, this would be a partial eclipse of the Sun by the Earth, but as light still reaches every part of the Moon from down here it is a much less spectacular show. Though the Moon’s surface will darken measurably before it sets just after 7 that morning, the change will not be easily noticed with the naked eye.
