Summer’s ghosts
We offer top tips for stargazers as the season for spotting noctilucent clouds returns
Yes, you read that correctly: Sky at Night Magazine is encouraging you to look at clouds. But aren’t clouds the mortal enemies of sky-watchers? Aren’t they the Voldemort to our Harry Potter? Not these clouds, no. Although it has to be said some astronomers and sky-watchers don’t consider noctilucent clouds (NLCs) to be astronomical and dismiss them as being “just clouds” or worse, “the poor man’s aurora”, many amateur astronomers enjoy observing and photographing them just as much as others enjoy peering through their scopes at misty spiral galaxies and glittering star clusters.
However, unlike those much-loved deep-sky objects, NLCs are not visible all the time; there is an NLC season which runs between the end of May and the start of August every year. This is part of their appeal for many sky-watchers during those long, bright summer nights: a strong display of NLCs which fills the sky gives them something to look at until the sky darkens enough for galaxies and nebulae to be visible again. Indeed, a strong NLC display can be such a stunning sight that it makes one forget all about the Andromeda Galaxy’s dust lanes or the Orion Nebula’s star-speckled heart.
If you’re now intrigued by NLCs and wish you’d seen them before, the chances are that you already have without even knowing it.
Easy on the eye
NLCs are clouds of icy dust that form at a very high altitude on the edge of space, around 76– 85km high, when temperatures and pressures in the upper atmosphere are just right. These conditions only occur during the summer months and even then not every night. NLCs only form when everything comes together. Because they are so high up NLCs are illuminated by the Sun long after it has set for us at ground level, and we see them as blue-white swirls, curls and tendrils shining in the sky. That’s what their name means – ‘nocti’ (night) ‘lucent’ (shining).
Observing NLCs is easy and, best of all, completely free. You don’t need any expensive telescopes, binoculars or cameras, just a pair of eyes will be fine. Having said that, a pair of binoculars will allow you to see fascinating detail and structure within an NLC display, which is invisible to the eye. You don’t need to be under a pitch black sky to see them, as a good display will be so bright that it will be visible from your back garden or even your bedroom window, as long as you’re facing the right direction – north.
Just as there are two recognised times of the year when conditions are better for observing the aurora, there is an ‘NLC season’ between the end of May and start of August. NLCs typically begin to show themselves around midnight, when the summer sky is as dark as it’s going to get. However, unlike the aurora, which can now be predicted with some accuracy many days in advance thanks to the work of Sun-monitoring satellites and observatories, an NLC display can’t be
predicted more than a few hours in advance, because the conditions that lead to their formation are so specific and they only exist for brief periods.
In times past, NLC-watchers had no choice but to head out on every clear summer’s night in the hope that NLCs would just appear in the northern sky. More often than not they didn’t, which was very frustrating. Now NLC watchers monitor satellite images and data to see if conditions in the upper atmosphere might be suitable for producing NLCs that evening. As sunset approaches we can monitor north-facing webcams in countries further to the east of us, where the Sun has already set, to give us an alert for any displays that are already in progress, which we might see later. And, of course, we use social media to communicate with fellow NLC hunters who will clang a virtual bell in cyberspace’s town square if a display is happening.
Plan ahead
So, if you want to see these mysterious clouds for yourself, how do you go about hunting NLCs? First of all you need to stay awake. There’s no point going out until around 11.30pm because the sky just won’t be dark enough for NLCs to be visible, even if they’re there. Then, if the sky is clear, wrap up warmly, grab your binoculars and camera if you have them and head out. If you have a clear view to the north from your garden you won’t need to go any further than that, but as is the case with observing the aurora, the lower, flatter and less obscured your northern horizon is, the better your chances will be of seeing something.
And if you’re not sure which direction is north, here are two tips for you: look to the right of where the sky is still quite bright from the late sunset or look for the bright star Capella shining just above the horizon. If you find Capella you’re looking in the right direction.
And then? Then you wait. The NLCs will either appear, or they won’t.
Lucky streaks
If NLCs do appear, the first you’ll see of them will probably be what look like a couple of streaks of gold-white cloud low in the northern sky, like distant vapour trails. Then it’s time to cross your fingers.
If you’re unlucky that will be all you’ll see and they will drop beneath the horizon out of sight. You’ll need to decide then if you’re going to stay out and see if they come back, or head home to bed. As is the case with watching a meteor shower, stay out as long as you can; it’s not uncommon for a display to fade away to nothing and then return and be bright and beautiful.
But if you’re lucky, as the sky darkens those streaks will get brighter and higher, and will become more bluewhite than gold. You might see some feathery patches of blue-white. If you can see those, you’re seeing NLCs.
Then what have you to look forward to? Those lonely few strands of golden white will bloom into an arch or swirls of electric blue, extending slowly to the east and west. As the display increases in height and brightness it will develop other structures and forms. You’ll see loops, curls and twirls of NLCs across the northern sky. In places you’ll also see flat, feathered plates of NLCs marked with a distinctive cross-hatch pattern, like mashed potato decorated with a fork.
You’ll also see movement too, but you won’t see the NLCs fluttering and swaying like the aurora. Instead
they move slowly over a timescale of minutes instead of seconds. Curls will uncurl or curl tighter; tendrils and streamers will grow longer, or ravel up again; and crosshatching will spread out in one place and grow more vivid in another. You’ll notice these changes with the naked eye, but they’ll be more obvious with binoculars.
Sometimes an NLC display develops into a storm that covers half the sky. If you find yourself out under one of those you are in for a very special night indeed – and a long one, because such a display can still be
going strong and filling the sky as the eastern horizon begins to brighten with the approach of dawn.
During an NLC storm all bets are off. All you can do is stand there, open-mouthed, entranced by what you’re seeing as the sky from east to west, and overhead, is painted with beautiful streamers, billows, whirls and swirls of electric blue light, bright enough to cast shadows. You simply won’t know which way to look, in case you miss something happening in another direction. Scanning the display with your binoculars you’ll shake your head in wonder at what you’re seeing – elegant twirls of lavender and blue wrapping around each other like snakes; spirals and curls of silver and white glowing like the filaments of light bulbs; veils of
gold spreading out from one side of the sky to the other. There’s simply nothing else like it in astronomy.
Eventually though you will have to go home, either because the sky is so bright that the NLCs are being washed from it or because you can’t keep your eyes open any longer – but what memories you’ll have.
Perhaps NLC aren’t an astronomical phenomenon, strictly speaking, but they are a welcome sight in the summer sky for many astronomers and sky-watchers starved of views of the stars, galaxies, nebulae and clusters they look at during the other seasons of the year. And they are gorgeous.
Turn to page 43 for more on observing NLCs