by What links a 1960s rock musical to the ancient Greeks, and what’s the best way to find your way home at night (if sat nav fails)? The answer involves a slow change in the sky that accumulates to have a serious impact as the centuries pass.
Let’s start by getting home safe and sound. If you want to find your way, search the sky for the familiar seven Plow stars, part of the constellation Ursa Major (the Big Dipper). Extend the line joining the final two stars, Merak and Dubhe, to locate a star of approximately the same brightness. This is Polaris, often known as the Pole Star or the North Star. The name is a giveaway: this star is always to the north, so once you’ve found the north star, you can determine where you need to head.
The Great Pyramid of Giza was built so that its sides ran precisely from south to north. But the ancient Egyptians did not use Polaris in their lineup. At that time, 4,500 years ago, the star directly to the north was the fainter Thuban in Draco (the Dragon). Polaris was not stationary in the sky then: as the Earth rotated, it circled around Thuban.
In fact there is an endless procession of “northern stars”, because the Earth’s axis is not fixed in space. Instead, it moves in a large circle, like the axis of a top that is about to topple, but in extremely slow motion. The axis of our planet takes almost 26,000 years to complete a circle in the sky. Currently, the northern end of the axis points towards Polaris, in the time of ancient Egypt it was Thuban. But most of the time, there hasn’t been an obvious star over Earth’s north pole.
People in southern latitudes currently face a stellar desert: there is no significant star marking the south pole of the sky. The nearest star languishes under a mere catalog name, Sigma Octantis, and is barely visible to the naked eye. But wait 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 wobble of the Earth’s axis “the precession of the equinoxes.” It was discovered by the ancient Greek astronomer Hipparco in the year 127 a. He was checking where various stars were, compared to where the Sun’s path in the sky crosses the equator, the position where the Sun is at the vernal and autumnal equinoxes.
And Hipparchus was perplexed. The stars seemed to have changed position. When he looked closer, he discovered that all the stars had moved and in the same direction. He concluded that the stars were in the same places, but instead the point of the equinox was moving slowly along the path that the Sun traces around the sky. We now know that this is another consequence of the wobble of the Earth’s axis.
In the time of Hipparchus, the Sun was crossing 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 over the centuries, this heavenly crossroads moved to Pisces. Their move to the next constellation marks the “dawn of the Age of Aquarius,” immortalized by the song of the same name in the 1967 rock musical. Hair.
But there is a problem. Experts clash over the exact location of the boundary between Pisces and Aquarius. As a result, the long-awaited Age of Aquarius may dawn any time up to the year 3597 AD.
What’s happening
The much publicized “green comet” C/2022E3 (ZTF) is closest to Earth and brightest in early February, when it passes within 42 million kilometers of our planet. Although technically only visible to the naked eye, the bright moonlight means you’ll actually need binoculars or a telescope to spot the celestial visitor. Even then, don’t expect a greenish sight: carbon atoms in the comet’s gases glow a deep green that shows up amazingly in photographs, but the color is too faint 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 on February 11 when it rotates to the left of Mars. But it’s fading throughout the month as the comet heads back into deep space toward Orion.
Venus shines in the southwest after sunset. Grab a telescope, if you can, on February 15 to spot Neptune just to the bottom right of Venus: the faintest planet is 60,000 times dimmer than the glorious Evening Star.
Jupiter, second only to Venus in brightness, is highest in the sky. There is a beautiful sight on February 2, when the crescent Moon unites these two luminous worlds.
Still on the planet’s trail, Mars is high in the southern sky, among the stars of Taurus. It is fading now as Earth recedes, and the Red Planet is now only slightly brighter than the red giant star Aldebaran that marks the celestial bull’s eye.
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 at this time, from early Saturday morning to early 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.: Crescent Moon
February 15: Venus very close to Neptune
February 20, 7.06 am: 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 am: First Quarter Moon near Mars and Aldebaran
Nigel Henbest’s latest book, study of the stars 2023 (Philip’s £6.99) is your monthly guide to everything happening in the night sky this year.
