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Noontime Twilight

by Perry Pezzolanella

This is a simple quiz. Think carefully. On the first day of winter, December 22, everything north of the Arctic Circle experiences twenty-four hours of darkness. Is this a correct statement? If you answered “yes”, then you are absolutely…wrong. Read on and learn a true astronomy lesson dealing with our Earth and Sun.

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. But we failed to take one major factor into account. Nowhere on Earth do we suddenly go from bright sunshine to pitch darkness. The atmosphere always provides a twilight that divides the day and night.

Only certain latitudes within the Arctic and Antarctic Circles experience periods when the Sun remains either above or below the horizon twenty-four hours a day. The circles lie at 66.5 north and south. The atmosphere acts as a lens and actually lifts the Sun’s image about one-half degree higher in the sky, when in reality it is on the horizon. Therefore, on December 22 a noontime Sun can be seen at 67-degrees north latitude, but one would have to be more than 67 degrees north not to see the Sun at all at noon. To keep life simple, we will ponder only the Northern Hemisphere, where we live.

The period of no Sun lasts only a few days near the Arctic Circle but grows longer toward the North Pole. At the North Pole, the period of no Sun lasts a full six months. The period of bright twilight, called civil twilight, lasts from sunset to the point where the Sun is six degrees below the horizon. This is the twilight that people still find bright enough for outdoor activities. When the Sun is twelve degrees below the horizon, most people agree that night has begun, and the sky will appear dark from horizon to horizon.

On December 22, the only place that has total darkness at noon is an area from the North Pole southward to latitude 78 degrees N. South of that there is a faint glow in the south at noon, a deep twilight. As one travels farther south at noon, there will be a bright civil twilight within six degrees of the Arctic Circle. The Sun does not rise above the horizon, but the sky is bright. In this area, where Barrow, Alaska, is located, the sky will gradually brighten from 10 A.M. until noon, when twilight will be brightest. Then by 2 P.M. it will be night again—lasting 20 hours, not 24.

As September arrives at the North Pole, the amount of sunlight changes dramatically after a summer of the midnight Sun and bright twilit nights. True night begins to set in, but soon becomes very long. The Sun sets on the September equinox and will not rise until March. Twilight grows deeper and from mid-October until mid-February it provides a continuous night. Farther south at Barrow, Alaska (around 71 degrees N) there is no period of 24-hour darkness. The Sun sets in late November, and with each passing day, the nights grow longer and the noontime twilight grows shorter and deeper. After December 22, the noontime twilight grows brighter and last longer. The Sun rises briefly in late January, and as spring approaches, it remains up longer each day to eventually become a midnight Sun by May.

Meanwhile, here at home (43 degrees N), on December 22 the Sun rises around 7:30 A.M., struggles to climb to only 23 degrees above the southern horizon at noon, and sets by 4:30 P.M. The shadows are long on this shortest day of the year - if it is not cloudy. The Sun is up barely nine hours and the long nights provide the perfect opportunity for us to do some serious observing, but only if one is up to the challenge of observing in the intense cold of an Upstate New York winter where the cold originates far away in a land where the Sun is nothing more than a noontime glow on the southern horizon.