Wednesday, January 7, 2009

Rural Delivery: Dark of Winter

In the dark days that follow the winter solstice, the last of December through the middle of January, I anxiously track the growth of daylight for reassurance that the tide has indeed turned and that winter will eventually give way to the brightening of early spring.

At this latitude of approximately 45 degrees, daylight grows ever so slowly at first, just a minute more each day until the middle of January, when it starts to grow by twos and then by threes at the month's end.

What I always find curious, and faintly disturbing, is that the day does not grow evenly. The sun sets a minute later each day for the week following the solstice, but it rises the same time day after day.

How could this be? If the earth rotates at a constant speed and tilts at an angle to the sun that's roughly the same at dawn as at sunset shouldn't the amount of daylight grow evenly, the same half-minute at sunrise as at dusk?

The answer to these questions is neither simple nor obvious. At this time of year at our latitude the Sun actually takes more than 24 hours to complete its cycle through the heavens. From mid-November to early February a solar day is actually 24 hours and about 30 seconds. Noon on our clocks is not exactly the sundial noon.

And since we do not adjust our clocks for this effect, the Sun's passage moves later and later each day, creating a phenomena known as the Equation of Time. For several weeks, our clock time moves ever so slightly ahead of solar time. Then things even out, and during the weeks bracketing the summer solstice the Equation of Time is ticking in the other direction.

The other factor affecting the times of sunrise and sunset is the Sun's declination, which determines how high the Sun rises in the sky on any given day and the length of time it stays above the horizon. Most of us know the Sun is at its "lowest point in the sky" on the first day of winter, so we expect the Sun to be above the horizon the least amount of time that day.

It is the Equation of Time and the Sun's declination that determine the times of sunrise and sunset, and neither progresses steadily. The speed of change in both varies from day to day and week to week throughout the year.

In late December, the daily rate of change in the Sun's declination is very small (actually zero on the day of the solstice), while the rate of change in the Equation of Time is at its highest. This is why, at this time of year, the time of sunset changes but the time of sunrise stays much the same.

Come mid-January, the Sun's declination is beginning to increase, growing the day at both ends while the effects of the Equation of Time become less and less apparent.

By February, daylight's growth is clearly evident. I can reshelve the almanac now, confident that winter is receding and an orderly progression of the heavens restored.

Rural Delivery

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