Mohawk Valley Astronomical Society

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Midnight Twilight

by Perry Pezzolanella

It is a beautiful late June day and you have flown out of Syracuse on an early flight landing in Anchorage, Alaska, late in the afternoon. You look forward to some great astrophotos in the dark Alaskan skies. The Sun finally sets at 10:30 P.M. and the sky slowly darkens. A stubborn twilight lingers, and lingers. By 1 A.M. the twilight is deep, but only the brightest stars are visible. Slowly you notice the sky begins to brighten as dawn approaches! The Sun rises by 3:30 A.M., ending a very strange night. Suddenly, you realize the reason.

The big mistake began in science class when a globe of the Earth was held up to a light in a darkened room. This clearly illustrated day and night, and if the globe was tilted, the effect of the midnight Sun, or no Sun at all, could be noted at the poles. We failed to take one major factor into account. Nowhere on Earth do we suddenly go from bright sunlight to complete darkness. The atmosphere always provides a twilight that separates day from night.

Only areas within the Arctic and Antarctic Circles experience periods when the Sun remains either above or below the horizon twenty-four hours a day. These circles lie at 66.5 degrees north and south latitude. The atmosphere acts as a lens and actually lifts the Sun’s image about one-half degree higher when, in reality, it may be just below the horizon. Therefore, a midnight Sun can be seen further south at 66-degrees north latitude. To keep it simple, we will consider only the Northern Hemisphere where we live.

The period of the midnight Sun lasts only a few days near the Arctic Circle but increases to a full six months of sunshine at the North Pole. On June 20, all areas from the North Pole to latitude 66 degrees N experience a midnight Sun. The Sun is lowest at midnight (1 A.M. daylight savings time) and is due north. Areas immediately south of the Arctic Circle have a bright twilight at midnight. The farther south from the circle one travels, the shorter and darker the twilight will be.

The period known as civil twilight lasts from sunset to the point when the Sun is six degrees below the horizon. This is the twilight that people still find bright enough for outdoor activities (except astronomy). Most people agree that night has begun when the Sun is twelve degrees below the horizon and the sky appears dark from horizon to horizon.

With this in mind, one would find a bright civil twilight at midnight as far south as 60 degrees N on June 20. There would be no true night until at least 54 degrees N and it would be quite short. At Anchorage (around 61 degrees N) there can never be a midnight Sun, but one would instead experience a bright twilight at the deepest part of night at 1 A.M. As summer fades away, the twilights shorten and deepen, and true nights begin to set in.

When travelling northward, astronomers should check the sunset-sunrise schedule for their destination before packing. It would be foolish to lug all that equipment thousands of miles only to find that there is no night!

Meanwhile, here at home (43 degrees N) we do not live anywhere near the land of midnight twilight. During June, the Sun sets around 8:45 P.M. EDT, the dead of night is at 1 A.M., and the Sun rises around 5:20 A.M. It is sufficiently dark for observing by 10:15 P.M. until the first hint of twilight arrives at 4 A.M. The Sun is below the horizon for barely nine hours, but that is enough time to do some pleasant exploring of the summer sky through a telescope.