The Longest Shadow

Just after one o’clock in the afternoon on Monday 2 August 2027, the daylight over Luxor will begin to fail. For a minute the light turns thin and metallic and shadows sharpen. Then the last bead of Sun slips behind the Moon, and where it was hangs a black disc ringed by the corona, the Sun’s outer atmosphere, which no one ever sees at any other time. Mercury and Jupiter come out beside it. Over Luxor it stays that way for 6 minutes 21 seconds.

Totality is never long, and most total eclipses give a few minutes at best. This one reaches 6 minutes 23 seconds in the desert west of the Nile near Sohag, and nothing longer happens anywhere on Earth until 3 June 2114, when an eclipse over Saudi Arabia lasts 6 minutes 32 seconds. The last time the Sun stayed hidden longer was 22 July 2009.

A shadow 260 km wide

The Moon’s full shadow is a narrow thing, never more than about 260 km across. It touches down in the Atlantic at sunrise, 08:24 UTC, and in less than three and a half hours it crosses the Strait of Gibraltar, runs along the coast of North Africa, follows the Nile, crosses the Red Sea and Arabia, passes the Horn of Africa and lifts off the Earth at sunset over the Indian Ocean, at 11:48 UTC. Everyone inside that band sees the total eclipse.

Everyone outside it, and that includes Seville, Tunis and Cairo, sees the Sun shrink to a crescent and grow back. In Cairo 94.8 per cent of the Sun is covered at 12:56 local time. It sounds close. It is not: the thinnest sliver of Sun keeps the sky blue and hides the corona completely.

Ninety-nine per cent is a dim afternoon. One hundred is night.

Why this one lasts so long

A total eclipse lasts as long as the Moon’s disc can keep the Sun’s covered as the shadow slides over you. Three things favour 2 August 2027.

1. The Sun looks small. On 5 July 2027 the Earth is at its farthest from the Sun for the year, 152.1 million km away, and four weeks later the Sun’s disc is still close to its smallest.
2. The Moon looks big. At 06:28 UTC that same morning, under four hours before the eclipse, the Moon makes its closest approach of the month: 357,362 km. It looks 7.9 per cent wider than the Sun, so it has room to spare.
3. The ground keeps up. Near the tropics at midday, the turning Earth carries you east at about 1,500 km an hour, the same way the shadow is going. Over Egypt the shadow’s track across the ground slows to about 2,400 km an hour. Where it first touches down over the Atlantic, it sweeps across the curve of the Earth at more than 10,000.

A family of long eclipses

Eclipses come back. After 18 years, 11 days and 8 hours, the Sun, Moon and Earth return to almost the same arrangement and the same eclipse happens again. Those extra 8 hours of the Earth’s turning move each return a third of the way round the world to the west. Astronomers call each chain a saros, and this eclipse belongs to Saros 136, a family of unusually long ones. Its longest, on 20 June 1955, lasted 7 minutes 8 seconds. Each return since has been a little shorter, and the family is fading.

Where to stand

The closer you are to the centre line, the longer you get. Near the edges, totality shrinks fast: Cádiz, close to the northern limit, gets 2 minutes 47 seconds, while Tarifa gets 4 minutes 38 seconds, a short ferry ride from Tangier’s 4 minutes 50 seconds. Málaga sits so close to the edge that its totality, under two minutes, depends on the mountains along the Moon’s rim. The Moon’s edge is mountainous, so the edges of the path can move by a kilometre or two from any prediction. If you are within a few kilometres of a limit, move further in.

What the sky holds

From Luxor at mid-eclipse the Sun is 82 degrees up, almost overhead. Venus sits just under 3 degrees from it, inside the glow of the corona. Mercury is 10 degrees away and Jupiter 22, both bright enough to see in the false dusk. Look around the horizon, too: beyond the shadow’s edge, more than 100 km away in every direction, the sky is still lit, and you stand in a ring of sunset colours.

Watching it safely

Look at the Sun only through eclipse glasses or a solar viewer made to the ISO 12312-2 standard, through every partial phase. Ordinary sunglasses are not safe, and nor is a camera, binoculars or a telescope without a proper solar filter on the front. Only when the Moon covers the Sun completely can the glasses come off, and they go back on at the first flash of sunlight at the Moon’s edge. Outside the path there is no safe moment without them.

The path, every 20 minutes

For anyone planning by map: the centre line and both limits, worked out the way NASA lists them.