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Night and day

I was driving to the Wassaic train station early one morning as a red seam of dawn was just starting to spread on the eastern horizon. My companion and I started talking about the upcoming winter solstice on Dec. 22, when there will be just nine hours and 10 minutes between sunrise and sunset. This lead to a discussion of the equinoxes, when as the name suggests the entire planet experiences an equal amount of daylight and darkness. I soon found myself in difficulty trying to explain how there could be an equal amount of daylight at the poles as well as at the equator and at every latitude in between at the same time. As is often the case, the answer depends on how you define the question. We experience seasons in our latitude thanks to the 23.5° axial tilt of the Earth. On the winter solstice, the sun reaches its lowest maximum angle above the horizon in our latitude and we experience our shortest day. The season variation of sunrise and sunset increases with latitude. The sun appears directly overhead in the southern hemisphere along the Tropic of Capricorn during our winter solstice, and by the summer solstice is right above the Tropic of Cancer. On the equinoxes it is directly above the equator and at a right angle to the rotational axis of the Earth. On these two days, both hemispheres experience days and nights of equal length.But wait a moment. Days and nights are always the same length at the equator. Exactly at the poles, they last six months, with polar days reckoned by the disc of the sun making one full revolution above the horizon during this period. So how can there be the same amount of daylight, everywhere, on the equinoxes? There can only be 12 hours of daylight everywhere on Earth if this is true of the poles as well. There are only two days when this happens, during the shift between polar day and night. This occurs on the equinoxes when the sun passes very slowly through the horizon over the course of 12 hours.Sunrise or sunset is usually defined as the moment when the top of the solar disc crosses the horizon. There are slight deviations based on refraction of light by the atmosphere that can change the apparent size of the solar disc, and whether or not the ground is level from the viewer’s vantage point all the way to the horizon. In order for days and nights to be truly symmetric on the equinoxes, one has to ignore the effects of atmosphere and accept a slightly different definition of day and night as the point where the middle of the solar disc passes the horizon. The difference is minimal, a few minutes at most except at the poles, but the end result is a 12-hour day.All this is months away. On these long dark nights if the skies are clear, the heavens are available to us in all their glory. Between Dec. 14 to 27, early risers may be treated to the sight of Mercury in the southeast about an hour and a half before sunrise. Between Dec. 22 and 23 it will be just to the left of the sliver of the waning moon. In fact, over the course of a full night during this period it is possible to see all five of the planets in our solar system that are visible to the naked eye. Venus and Jupiter are prominent after sunset, and Mars and Saturn are best viewed along with Mercury in the hours before dawn. Tim Abbott is program director of Housatonic Valley Association’s Litchfield Hills Greenprint. His blog is at greensleeves.typepad.com.

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