It might be autumn here but it’s spring on Uranus
Autumn’s officially here: we passed the equinox on 23 September. But on Uranus it’s the beginning of spring. The great greenish “ice giant” is closest to Earth this month on 12 October – and is just visible to the unaided eye in the constellation of Pisces (but binoculars are a huge help to locate this distant world).
The seventh planet was discovered in 1781 by William Herschel, a professional musician and amateur astronomer in Bath, who delighted in constructing ever bigger telescopes to survey the cosmos. He first thought he’d found a comet, but other astronomers checked the orbit of the mystery object and found it was a planet. King George III was known to be interested in science, so Herschel wanted the planet to be named “George” in his honour’. But convention prevailed and the new world was called Uranus (the father of Saturn in Greek mythology).
Despite not being the furthest planet from the Sun, Uranus is the coldest world in the solar system. Its average temperature is less than -220C. It also has the distinction of circling the Sun on its side, no doubt as a result of a whopping collision in the violent early history of our neighbouring worlds.
The space probe Voyager 2 sped past the planet in 1986, and took images of a seemingly bland, gaseous world some 15 times heavier than Earth. Like the other three giant planets furthest from the Sun, Uranus has a set of rings: 13 in this case, and as dark as coal. And it’s surrounded by a system of at least 27 moons. The most sensational is tiny Miranda – just 500 kilometres across – which is crumpled up and full of grooves. It was probably smashed apart by a huge impact, but managed to re-assemble itself.
But as spring approaches, things are starting to wake up on the planet that Voyager 2 encountered nearly 30 years ago. Uranus takes 84 years to orbit the Sun, and “spring” lasts nearly 20 years. Already, the first little fluffy clouds in its atmosphere are beginning to appear – although a “little” cloud on this giant world can be almost as large as Earth.
And maybe Uranus is not the conventional planet we once assumed it was. Like its near-twin, Neptune, the world harbours a diamond sea in its interior, with diamond hailstones raining into it. Expect more surprises from “Planet George” in the future!
WHAT’S UP?
This month is Saturn’s curtain call. At the start of October you’ll find the ring-world low in the south-west after sunset, but it’s rapidly sinking into the twilight glow, to leave barely visible Uranus as the main planet of the evening skies. For early birds, though, we have a true planetfest in store! You can’t mistake brilliant Venus in the east before sunrise: the Morning Star outshines everything in the sky, bar the Moon. Lower down, you’ll find the second-brightest planet, Jupiter. Between them lie Mars and the bluish star Regulus.
In mid-October, scan the horizon well to the lower left of Jupiter, about 7am, to spot the elusive Mercury, putting in its best morning appearance of the year – binoculars will help. Mars skims closely over Jupiter early on 18 October; while Venus flies under the giant planet on the morning of 26 October – a fantastic sight, as the two brilliant planets lie only two Moon-widths apart.
This month’s sky sights also include a display of shooting stars on the night of 21/22 October. The Orionids seem to streak from the constellation Orion, and you’ll catch them in the early hours of the morning, when the Moon has set. On the evening of 29 October, the Moon takes centre-stage when it moves in front of the Aldebaran, the red star marking the eye of Taurus (the Bull). The occultation starts between 9.40pm and 10.00pm – depending on your location – and the Moon re-emerges about an hour later.