Inverness drops below twelve hours of daylight on Friday

The September equinox fell at 1.05am BST this morning, Wednesday 23 September. Day and night in Inverness are not equal today, though. The sun rose at 7.03am and sets at 7.13pm, which is twelve hours, nine minutes and forty-two seconds of daylight. The first day this autumn with less than twelve hours is Friday the 25th, at 11 h 59 m 57 s. From then until March, night is the longer half.

Why the equinox is not the equal day

The equinox is measured from the centre of the sun, as the moment the point directly beneath it crosses the equator. Sunrise and sunset are measured from its edge: the first sliver of disc appearing and the last one going. On top of that, the atmosphere bends the light, so the sun looks as if it’s above the horizon when strictly it’s just below. Both add minutes at each end of the day. timeanddate calls the real crossover the equilux and puts it at about 25 September at both 55° and 60° north. Inverness sits between them, and its own table lands on the same day.

Skye has the same arithmetic on a later clock. Portree is further west, so the sun rises and sets about eight minutes later there, at 7.11am and 7.21pm today. The length of the day is within two seconds of Inverness’s, and it drops below twelve hours on Friday too.

Nearly five minutes a day, and this is as fast as it goes

Right now Inverness loses four minutes and fifty-two seconds of daylight every day. The rate has been at that figure since the middle of September and stays there into the first week of October. For comparison, it was four minutes nine on 1 August. After early October it slows: four minutes thirty-seven by the end of October, two minutes forty-nine on 1 December, and seven seconds on 21 December, when the day bottoms out at six hours thirty-five minutes. Sunrise that day is 8.57am and sunset 3.32pm.

A month from today, on 23 October, the day is nine hours and forty-four minutes. That’s two hours and twenty-five minutes gone in thirty days, which is why late September and late October feel like different seasons rather than the same one a bit colder.

Then the clocks go back at 2am on Sunday 25 October. That doesn’t change the length of the day. It moves an hour of it from the evening to the morning: sunset is 5.50pm on the Saturday and 4.47pm on the Sunday, and sunrise goes from 8.10am to 7.13am. It’s the sunset people notice.

What still fits inside the light

None of our days starts at sunrise, so “fits” here only means the day is no longer than the daylight. It isn’t a timetable. But it’s the honest test of whether the last stretch is driven in the light, and the tables give a clean answer for each length of day we run.

• Twelve hours, meaning Skye and Wester Ross. It stops fitting this week. Both pages already say so. The Skye day comes home the long way by Loch Carron and Achnasheen, and its page says that if the light has gone we come back the way we came. The Wester Ross page says that out of season the loop is better cut short than driven in the dark. From here that stops being an edge case and becomes the ordinary decision in the car.
• Eleven hours, Applecross and the Bealach na Bà. The last day with eleven hours of light is 7 October, at eleven hours one minute.
• Ten hours, the Glen Coe, Jacobite and Road to the Isles days. The last day with ten hours of daylight is 19 October, at ten hours three minutes. On the 20th it’s nine hours fifty-eight.
• Nine hours, Glen Affric and Loch Ness. Nine hours of light lasts until 1 November, at nine hours two minutes. The Glen Affric page says that a nine-hour day wants about ten hours of daylight, which Inverness has until October. The table puts the end of that at 19 October, so the page and the almanac agree.
• Eight hours, the east shore. Eight hours of light lasts until 15 November, at eight hours one minute. By the solstice there are six and a half hours, and even the shortest day we run is longer than the daylight.

What the tables do not know

Every figure above comes from the almanac, which assumes a level horizon and says nothing about cloud. Neither holds here. In a steep glen the sun leaves the road well before the official sunset, by an amount that depends on the hill and that nobody publishes. And a low November overcast makes three in the afternoon look like five, which nobody can know until the morning. Both are the argument for a day in the dark months having its best stretch in the middle rather than at the end.

Civil twilight gives some back. Today it runs from 6.25am to 7.52pm, which is close to forty minutes at each end when the sky is still light after sunset. It isn’t enough to walk the Fairy Pools by, but it’s enough for the last hour of a drive to still show you something.