Science in the Bluegrass —
The darkest evening of the year

Chris Graney

Robert Frosts’ poem “Stopping by Woods on a Snowy Evening” tells of him pausing to watch snow fall, far from any house, at a spot that was —

Between the woods and frozen lake
The darkest evening of the year.

The darkest evening of the year is in early December. Frost’s stop occurred during Advent.

This year, here in Kentucky, our darkest evening fell on December 6. That was the evening of our earliest sunset. The evenings have been getting brighter ever since.

Have you noticed? Tonight (December 16) the sun will set almost two minutes later than it set on the 6th. As the days pass we will keep gaining minutes of evening light.

Our shortest day of the year is the solstice, December 21. On that day the sun is at the southernmost point of its yearly journey (seen from Earth) through the stars of the zodiac. The sun’s mid-day altitude on the solstice is the lowest of the whole year. The length of the sun’s day-long arc above the horizon is therefore shortest, so the sun is up for the least time, and its rays are most slanted.

With less sunlight time and less direct illumination, the sun’s warming power is at its weakest. That’s why for us in the northern hemisphere December 21 is the “winter” solstice. In the southern hemisphere everything is reversed, and so it is the “summer” solstice and Christmas is a summer celebration.

Our latest sunrise is around January 6. That is our “darkest morning” of the year. So, from December 6 to December 21, the evenings brighten, but the days shorten, and the mornings darken. From December 21 through January 6, the evenings brighten, the days lengthen, and yet the mornings still grow darker. Only after January 6 do the mornings begin to brighten as well.

Why does this strange solar behavior occur? It has to do with how Earth’s axis of rotation is tilted with respect to its orbit. It also has to do with the fact that on January 3 Earth is closest to the sun (and moving fastest in its orbit — with the closeness doing almost nothing for warmth). These things cause the whole span of daylight to move against the clock at this time of year.

But if grasping the “why” behind the sun’s behavior is difficult, seeing it is not. Just keep your eye on the morning and evening sky, and on the clock. Watch the evenings brighten, even as the solstice approaches. Watch the mornings darken, all the way into January.

I first noticed this, not because of any study of astronomy, but because my son started taking a bus to school. The bus came at a set time in the morning. How dark or light it was when that bus came caught my eye. In particular, I expected that since the shortest day was the solstice when school resumed after New Year’s it would be lighter at bus time. It was not. It was darker. I was surprised by the “darkest morning.”

The universe holds surprises if you are watchful. The sun’s behavior around the solstice is a surprise that seems suitable for the season. At the start of Advent, things are still getting darker overall, but something is turning around — the light in the evening. But only upon the completion of Christmas, with the Epiphany and the Baptism of the Lord, is everything brightening up all around.

Chris Graney is a parishioner of St. Louis Bertrand Church and is on the staff of the Vatican’s astronomical observatory, www.vaticanobservatory.org.

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