February night sky: Key dates for moons, comets and stars this month
There’s an endless procession of ‘north stars’, because the Earth’s axis is not fixed in space, writes Nigel Henbest
What links a 1960s rock musical with the ancient Greeks, and the best way to find your way home at night (if your satnav fails)? The answer involves a slow change in the sky that builds up to have a serious impact as the centuries roll on.
Let’s start with getting home safely. If you want to find your way, check out the sky for the familiar seven stars of the Plough, part of the constellation Ursa Major (the Great Bear). Extend the line joining the two end stars – Merak and Dubhe – to locate a star of roughly the same brightness. This is Polaris, often known as the Pole Star or the North Star. The name is a giveaway: this star always lies due north, so once you’ve found the North Star you can work out which way you should be heading.
The Great Pyramid at Giza was built so that its sides ran precisely south to north. But the ancient Egyptians didn’t use Polaris in their alignment. At that time – 4500 years ago – the star that lay directly north was fainter Thuban, in Draco (the Dragon). Polaris wasn’t stationary in the sky then: as the Earth rotated, it circled around Thuban.
In fact there’s an endless procession of “north stars”, because the Earth’s axis is not fixed in space. Instead, it sweeps around in a large circle, like the axis of a spinning top that’s about to fall over, but in extreme slow motion. It takes almost 26,000 years for our planet’s axis to complete one circle in the sky. Currently, the north end of the axis points towards Polaris, in ancient Egyptian times it was Thuban. But most of the time, there has been no obvious star above the Earth’s north pole.
People at southern latitudes currently face just such a stellar desert: there’s no significant star marking the south pole of the sky. The nearest star languishes under a mere catalogue name, Sigma Octantis, and it’s hardly visible to the naked eye. But hang on 12,000 years, and brilliant Canopus – the second brightest star in the sky – will form a resplendent South Star; while simultaneously the fourth brightest star Vega will be the North Star.
Astronomers call this wobbling of the Earth’s axis “the precession of the equinoxes.” It was discovered by the ancient Greek astronomer Hipparchus in 127 BC. He was checking out where various stars lay, compared to the point where the Sun’s path in the sky crosses the equator – the position where the Sun lies at the spring and autumn equinoxes.
And Hipparchus was puzzled. The stars seemed to have shifted their positions. When he looked more closely, he found that all the stars had moved, and in the same direction. He concluded that the stars were in the same places, but instead the equinox point was slowly moving along the path that the Sun traces around the sky. We now know that this is another consequence of the Earth’s wobbling axis.
In Hipparchus’s time, the Sun crossed the celestial equator in the constellation Aries, and astronomers still call the intersection of the Sun’s path and the equator the ‘First Point of Aries.’ But as the centuries passed, this celestial crossroad moved into Pisces. Its shift into the next constellation marks the “dawning of the Age of Aquarius” – immortalised by the eponymous song in the 1967 rock musical Hair.
But there’s a problem. Experts clash over the exact location of the boundary between Pisces and Aquarius. As a result, the much anticipated Age of Aquarius may dawn at any time up to AD 3597!
What’s Up
The much publicised “green comet” C/2022E3 (ZTF) is closest to the Earth and brightest at the beginning of February, when it passes 42 million kilometres from our planet. Though it’s technically just visible to the naked eye, bright moonlight means you’ll actually need binoculars or a telescope to spot the celestial visitor. Even then, don’t expect a verdant view: carbon atoms in the comet’s gases are shining a vivid green that shows up stunningly in photographs, but the colour is too dim to register on the human retina.
The comet is easy to spot on the night of 5 February, when it passes close by the bright star Capella; and on 11 February as it swings to the left of Mars. But it’s fading throughout the month, as the comet heads back off to deep space in the direction of Orion.
Venus is brilliant in the south-west after sunset. Grab a telescope, if you can, on 15 February to spot Neptune just to the lower right of Venus: the faintest planet is 60,000 times dimmer than the glorious Evening Star.
Jupiter – second only in brightness to Venus – lies higher in the sky. There’s a lovely sight on 2 February, when the crescent Moon joins these two luminous worlds.
Still on the planet trail, Mars is high in the southern sky, among the stars of Taurus. It’s fading now as the Earth pulls away, and the Red Planet is now only a little brighter than the red giant star Aldebaran that marks the celestial bull’s raging eye.
The so called Snow Moon will arrive on Febraury 5. The name is derivesd from the heavy snow fall assoicated with the first full month of the year. “The Moon will appear full for about 3 days around this time, from early Saturday morning through early Tuesday morning,” NASA explained in a blog post on Tuesday.
Diary
3 February: Moon near Castor and Pollux
5 February, 6.29 pm: Full Moon (snow moon)
6 February: Moon near Regulus
10 February: Moon near Spica
13 February, 4.01 pm: Last Quarter Moon
15 February: Venus very near Neptune
20 February, 7.06 am: New Moon
22 February: Moon between Venus and Jupiter
23 February: Moon near Jupiter and Venus
26 February: Moon near the Pleiades and Aldebaran
27 February, 8.06 am: First Quarter Moon near Mars and Aldebaran
Nigel Henbest’s latest book, Stargazing 2023 (Philip’s £6.99) is your monthly guide to everything that’s happening in the night sky this year.
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