by What connects a 1960s rock musical to the ancient Greeks, and the best way to find your way home at night (if your GPS fails)? The answer involves a slow change in the sky that accumulates to have a serious impact over the centuries.
Let’s start by getting home safely. If you want to find your way around, take a look in the sky for the familiar seven plow stars, which are part of the constellation Ursa Major (the Big Dipper). Extend the line joining the two extreme stars – Merak and Dubhe – to locate a star of roughly the same luminosity. This is Polaris, often known as North Star or North Star. The name is a giveaway: this star always lies due north, so once you’ve found the North Star, you can figure out which direction you need to head.
The Great Pyramid of Giza was built in such a way that its sides run precisely from south to north. But the ancient Egyptians did not use Polaris in their alignment. At that time – 4500 years ago – the star that was directly to the north was the dimmer Thuban in Draco (the Dragon). Polaris was then not stationary in the sky: when the Earth rotated, it revolved around Thuban.
In fact, there is an endless procession of “northern stars”, because the axis of the Earth is not fixed in space. Instead, it sweeps in a big circle, like the axis of a spinning top that’s about to fall, but in extreme slow motion. It takes almost 26,000 years for the axis of our planet to complete a circle in the sky. Currently, the northern 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 Earth’s north pole.
Residents of southern latitudes currently face such a stellar wasteland: no significant stars mark the southern pole of the sky. The nearest star languishes under a simple catalog name, Sigma Octantis, and it is barely visible to the naked eye. But wait 12,000 years, and the bright Canopus – the second-brightest star in the sky – will form a resplendent Southern Star; while simultaneously the fourth brightest star Vega will be the North Star.
Astronomers call this oscillation of the Earth’s axis “the precession of the equinoxes”. It was discovered by the ancient Greek astronomer Hipparchus in 127 BC. It was checking where various stars were, relative to the point where the Sun’s path across the sky intersects the equator – the position where the Sun is at the spring and fall equinoxes.
And Hipparchus was perplexed. The stars seemed to have changed position. When he looked closer, he saw 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 moved slowly along the path the Sun traces across the sky. We now know that this is another consequence of the oscillating axis of the Earth.
In the time of Hipparchus, the Sun crossed the celestial equator in the constellation of Aries, and astronomers still call the intersection of the path of the Sun and the equator the “first point of Aries”. But over the centuries, this celestial crossroads has shifted to Pisces. His move to the next constellation marks ‘the dawn of the Age of Aquarius’ – immortalized by the eponymous song from the 1967 rock musical Hair.
But there is a problem. Experts clash over the exact location of the border between Pisces and Aquarius. As a result, the long-awaited Age of Aquarius may dawn anytime up to the year 3597!
What’s new
The much-publicized “green comet” C/2022E3 (ZTF) is closest to Earth and brightest in early February, when it passes 42 million kilometers from our planet. Although technically only visible to the naked eye, moonlight means you’ll actually need binoculars or a telescope to spot the celestial visitor. Even then, don’t expect a verdant sight: the carbon atoms in the comet’s gases glow a bright green that shows up amazingly in photographs, but the color is too dark to register on the human retina.
The comet is easy to spot on the night of February 5, when it passes close to the bright star Capella; and February 11 as it swings to the left of Mars. But it fades throughout the month as the comet heads back into deep space toward Orion.
Venus is bright in the southwest after sunset. Grab a telescope, if you can, on February 15 to spot Neptune just to the lower right of Venus: the dimmer planet is 60,000 times dimmer than the glorious Evening Star.
Jupiter – second only in brightness to Venus – sits higher in the sky. There is a beautiful sight on February 2, when the crescent Moon joins these two luminous worlds.
Still on the trail of the planets, Mars is high in the southern sky, among the stars of Taurus. It is now fading as Earth recedes, and the red planet is now a bit brighter than the red giant star Aldebaran that marks the raging eye of the celestial bull.
The so-called Snow Moon will arrive on February 5. The name is derived from the heavy snowfall associated with the first full month of the year. “The Moon will appear full for about 3 days around this time, from Saturday morning to Tuesday morning,” NASA explained in a blog post on Tuesday.
Diary
February 3: Moon near Castor and Pollux
February 5, 6:29 p.m.: Full Moon (snow moon)
February 6: Moon near Regulus
February 10: Moon near Spica
February 13, 4:01 p.m.: Last quarter moon
February 15: Venus very close to Neptune
February 20, 7:06 a.m.: New Moon
February 22: Moon between Venus and Jupiter
February 23: Moon near Jupiter and Venus
February 26: Moon near the Pleiades and Aldebaran
February 27, 8:06 a.m.: first quarter moon near Mars and Aldebaran
Nigel Henbest’s latest book, stargazing 2023 (Philip’s £6.99) is your monthly guide to everything happening in the night sky this year.
