Two planets are at their best in our evening sky at present while a third, Mars, is brightening sharply low down in our southern predawn sky as it heads for opposition, directly opposite the Sun in the sky, in May.
Jupiter reached opposition a month ago and is unmistakable in the middle of our southern evening sky. More of a challenge is Mercury which, because it never strays far from the Sun’s glare, has never been seen by most people, or perhaps by most astronomers.
Currently, though, Mercury is better placed for visibility from our latitudes than at any other time this year. Our best plan may be to find an aspect with a clear western horizon, note where the Sun sets in the WNW, then wait 40 minutes.
Binoculars should then enable us to spot Mercury in the twilight around 10° above the sunset position. Indeed, it should become a naked eye object as it sinks towards the horizon and the twilight dims. We may follow this procedure until the 24th or later, but Mercury is fading from magnitude -0.7 on the 11th to 1.3 on the 24th, so catch it sooner rather than later.
Turning to an entirely different subject, our chart depicts a window high in Britain’s northern sky in the late-evening at present. The seven stars that make up the Plough are wheeling overhead as they rotate counter clockwise around the northern celestial pole near the Pole Star, Polaris.
The Plough’s resemblance to a ladle or spoon has led to its more usual title of the Big Dipper in the US. In fact, it is not a constellation in its own right, but only part of the extensive constellation of Ursa Major, the Great Bear. The Lesser Bear, Ursa Minor, includes Polaris and has its own smaller and fainter Little Dipper asterism.
We dub the two leading stars of the Plough, Merak and Dubhe, the Pointers since the line through them points conveniently towards Polaris. It is a coincidence that the latter is only 0.7° away from the celestial pole so that it remains virtually stationary in our sky and due N as the Earth turns on its axis once each day. Incidentally, reversing the line through the Pointers takes us to Jupiter at present.
Polaris lies some 434 light years from us and, far from being conspicuous as some people imagine, it shines near the second magnitude and is comparable to several stars in the Plough and even Kochab in its own constellation. Polaris belongs to the Cepheid class of stars, pulsating very slightly in size and brightness although the amplitude and period of its fluctuation shows some unusual changes.
Polaris’s status as our pole star is not permanent. The wobbling of the Earth’s axis called the precession of the equinoxes means that it will be closest to the celestial pole, within 0.5°, as this century ends, but will then drift away. Indeed, for centuries around 2800BC the star Thuban in Draco, midway between Kochab and the Plough’s handle, was the closest bright(ish) star to the pole – even closer than Polaris is now. It was still the pole star when the Egyptians were aligning their pyramids.
Before the returning moonlight robs us of the chance, telescope-users have two well-known and beautifully photogenic galaxies to admire near the end of the Plough’s handle. The famous Whirlpool galaxy, M51, lies 3.6° SW of Alkaid and is an eighth magnitude spiral galaxy that is interacting gravitationally with a smaller neighbouring galaxy.
Although M51 was discovered by Charles Messier in 1773, its spiral nature, the first to be recognised in any galaxy, was found by the Earl of Rosse in Ireland in 1845. It lies at a range of 23m light years, around 2m light years further away than the larger and brighter face-on spiral galaxy that is the Pinwheel, M101, 5.6° to the NNE of Alkaid. Decent binoculars show both M51 and M101 as hazy smudges under ideal dark skies.
