Four months without a proper night: summer at fifty-nine north

Where the darkness goes

Astronomical twilight ends when the sun is eighteen degrees below the horizon, and at the latitude of the archipelago the sun stops reaching that depth around the twentieth of April. It does not manage it again until nearly the end of August. Between those dates there is no astronomical night whatsoever, whatever the clock says and however far you drive. The sky at one in the morning in June is a pale blue-grey with perhaps a dozen stars scattered across it, and that is not local misfortune. It is geometry, and it applies to everybody at this latitude.

It gets worse before it gets better. From roughly the middle of May until the first days of August the sun does not even reach twelve degrees below the horizon, which is the boundary of nautical twilight, so the northern sky keeps a bronze band across it all night long. At the solstice the sun is only some seven degrees down at its lowest, barely deeper than civil twilight. Vega and Arcturus hold up well, Deneb remains findable, and everything fainter than about second magnitude has been quietly erased.

What is still worth doing

The season is not wasted, but the target list shrinks to things that are bright and have structure. The moon is the obvious candidate and the most rewarding, although the low summer full moon skims the southern treeline and shows its limb through a great depth of turbulent air. A first-quarter moon in June sits far better placed in the evening sky. Bright double stars survive twilight almost unaffected: Albireo splits at low power and keeps its colours, and Mizar with its companion is a decent test of whether the air is steady tonight.

Planets depend entirely on where they happen to be, and in a northern summer that usually means low. The ecliptic sits at its flattest in the midnight sky in June, so a planet near opposition then culminates ten or fifteen degrees up and swims badly. I have looked at a summer Saturn through an inch and a half of shimmering air and seen a yellow oval with a suggestion of ears. The same planet at a winter opposition, sixty degrees above the horizon, is a different object altogether and worth waiting for.

Noctilucent cloud, the summer consolation

The compensation for losing the night is noctilucent cloud, which only exists because the sky is half-lit. These are ice clouds at around eighty kilometres, high enough to catch sunlight long after the ground has gone dark, and this latitude sits comfortably inside their best band. The season runs from about the third week of May to the first week of August. They appear low in the north, between roughly six and sixteen degrees of solar depression, which in practice means the hour or two on either side of local midnight.

They do not look like ordinary cloud, and the first time you see a real display you will not need anybody to confirm it. The colour is an electric blue-white with a faint pearl edge, and the structure is remarkably fine: herringbone bands, whirls, and long parallel streaks that hold their shape for twenty minutes at a stretch. Ordinary tropospheric cloud in the same part of the sky looks grey and flat beside them. I keep an alarm for half past eleven through June and look north from the jetty for five minutes.

• Season: roughly 20 May to 5 August at this latitude
• Look low in the north, an hour to three hours after sunset
• Blue-white, finely structured, brighter than the twilight behind it
• Binoculars help with the structure; no telescope required

Using the season for everything else

Summer is when the equipment gets looked after, because nothing else is going on. The reflector comes out onto the grass in daylight and gets collimated properly with time to spare, which is an entirely different experience from doing it at half past ten in October with cold fingers and a head torch. Eyepieces get cleaned, the focuser gets its slop taken out, and the tarpaulin is replaced before it turns into a bucket. I also repaint the marks on the tripod legs that let me set the thing up level on a sloping lawn.

The other summer job is learning the sky I cannot currently see. Autumn arrives quickly here, and the difference between a productive September and a frustrating one is whether I already know where things are. I spend a few pale evenings with a planisphere and an atlas, working out what will be well placed at midnight in October, what culminates too low to be worth chasing, and which objects I failed to find last year and should approach from a different direction. It is unglamorous, and it pays for itself every autumn.