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
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.
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.
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.
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
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 receeding and an orderly
progression of the heavens restored.