It’s open season now for spotting one of the rarest and most unpredictable of sky sights: clouds that glow in the dark.
They are called noctilucent clouds, which is Latin for “night-shining”, and over the past few days, night owls all over Britain have reported rippling clouds low in the north, glowing bluish-white against the dark night sky.
Noctilucent clouds are made of tiny ice crystals, so high up that they are on the edge of space. At an altitude of 80 kilometres, they are 10 times higher than the next-highest clouds, the familiar “mare’s tails” of cirrus.
We see these clouds only during the summer months, when the Sun is not far below the horizon at night, and its rays can light up very high altitude clouds, just as we can see a high-flying plane lit up by sunshine even after sunset. And you need to be at a latitude between 50 and 70 degrees north – making the British Isles an ideal observing spot.
The clouds appear only rarely, and pretty unpredictably – though serious observers keep tabs on the view from Earth-observing satellites where they can discern signs of noctilucent clouds starting to form.
You need a special combination of circumstances to make a good display. First, the upper atmosphere must be very cold. Then enough water vapour must be wafted up to this extreme altitude, or created by chemical reactions: recent research suggests that methane gas can produce water molecules when hit by the Sun’s radiation at the top of the atmosphere.
Finally, you need tiny specks of solid matter for the ice to condense onto. But where does this dust come from? Here’s a big clue. Before 1885, there were no records of noctilucent clouds. They began to appear two years after the eruption of Krakatoa in 1883, and meteorologists speculated that the ice is coating particles of volcanic dust.
But big volcanic eruptions don’t happen every year. Astronomers weighed in with the idea that the clouds are due to water vapour freezing out on to the millions of tonnes of space dust that pours into Earth’s atmosphere each year.
In addition, we are seeding the upper atmosphere with dust every time we launch a rocket. Noctilucent clouds have appeared after past launches of the Space Shuttle, and – more recently – after one of Elon Musk’s Falcon 9 rockets was dispatched to space.
And these clouds are becoming more common. A century ago, you may have had to wait years to spot one of these ghostly wraiths; now you are likely to see several over a single summer. This trend is probably linked to global warming. First, there’s more the greenhouse gas methane around today, to create extra ice in the upper atmosphere. Also, higher temperatures at ground level actually lead to colder conditions at the top of the atmosphere, making it more likely water vapour will freeze out.
To go spotting these elusive clouds, stay up until after midnight, when the sky is really dark, and look towards the north. And keep your fingers crossed!
Two brilliant stars are the first to appear in the darkening evening sky: orange Arcturus in the south and pure white Vega high in the east are the fourth and fifth brightest stars in the entire sky. Arcturus is the leading light of the kite-shaped constellation Bootes (the Herdsman), while Vega heads up the dim and diminutive star pattern of Lyra (the Lyre).
As dusk turns to night, you’ll find a line of lesser luminaries below Arcturus. To the right, Regulus marks the heart of Leo (the Lion). Next is Spica, depicting a sheaf of corn held by Virgo (the Virgin), and then the red giant Antares in the Scorpius, the celestial Scorpion. And to the left lies Altair, in the constellation of Aquila (the Eagle).
Even at the darkest hour, though, you’ll find the other stars making up these star patterns rather hard to spot. The stars of early summer are generally rather dim, compared to the brilliant winter constellations like Orion.
When early astronomers drew up the sky patterns of summer, they created some huge and rambling constellations for this time of year. Look at Ophiuchus, for instance. He was the ancient god of healing, depicted holding a serpent – the constellation Serpens, which is attached to Ophiuchus. Or the long sinuous constellation of Hydra (the Water Snake) near the horizon. According to one theory, these long lines are stars were intentionally picked out by ancient navigators in the Mediterranean as a grid in the sky, like lines of latitude and longitude on the Earth.
In the first week of June, you may just catch Mercury very low in the northwest after sunset. Otherwise, to catch any planetary action you’ll have to wait up till around midnight, when brilliant Jupiter rises in the southeast. Twenty minutes later, its smaller and fainter sibling Saturn pops up above the horizon to the left of Jupiter.
Stay awake till 1.30am, and Mars follows on behind, about the same brilliance as Saturn and gradually brightening as the month goes on. And from the middle of June onwards, you’ll be dazzled by Venus appearing as the Morning Star, rising around 3.30am.
4 June: Moon near Antares; Mercury at greatest eastern elongation
5 June, 8.12pm: Full Moon
8 June (am): Moon near Jupiter and Saturn
9 June (am): Moon near Jupiter and Saturn
13 June, 7.23am: Last Quarter Moon, near Mars
20 June, 10.43pm: Summer Solstice
21 June, 7.41am: New Moon; annular solar eclipse (visible from central Africa, India and China)
25 June: Moon near Regulus
28 June, 9.15am: First Quarter Moon
29 June: Moon near Spica
Philip’s 2020 Stargazing (Philip’s £6.99) by Heather Couper and Nigel Henbest reveals everything that’s going on in the sky this year.
Fully illustrated, Heather and Nigel’s The Universe Explained (Firefly, £16.99) is packed with 185 of the questions that people ask about the Cosmos.
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