Tilting Time

The sun is setting. Through gaps in frosted tree tops, I can see bands of pink colour lingering on the blue-gray clouds.

According to SunriseSunset.com, we are within a day or two of our earliest sunset of the year, more than ten minutes before the hour of 5 p.m. Their calendar lists sunrise and sunset times to the nearest minute, so I can't tell exactly which day marks the turning point.

By the time the winter solstice rolls around, our sunset will be moving later again. In those last few days before the solstice, the shortening of daylight will be only on the morning side, with sunrise still moving later, right through until early January.

Why?

From a New Scientist article, Early Days, I have a shaky idea of how our days shift through this dark time of the year. The U.S. Naval Observatory offers what sounds like a more systematic explanation in The Dark Days of Winter, but my understanding still feels a bit unsteady.

The impression that stays with me is this. Solar days pulse slightly over the course of a year, making the time from one noon to the next longer and shorter. Time as measured by the sun does not march exactly to the beat of atomic vibrations. We don't correct our clocks for this pulsation. Although those 24 hours on the dial suggest that we are keeping track of solar days, we are really approximating them, and clock time wobbles around solar time without quite matching it.

Twice in the last month I've seen an "atomic clock" offered for sale. It's really just an electric clock that automatically resets itself when it detects a radio signal from an atomic clock in Boulder, Colorado. You can get one at Lee Valley; you can also get a sundial. With the atomic clock, you can make detailed observations of sunrise and sunset times and then try to get your head around their movements. With the sundial, you can observe the solar day directly, and not worry about how many wobbles of an atom fit into it.