hen the clock is gaining on the sundial, the Sun
rises and sets later each day, and when the sundial is
gaining on the clock, the Sun rises and sets earlier each
day. If the two effects which give us the equation of time
were solely responsible for sunrise and sunset times, these
times would be late in summer and winter and early in spring
and fall. Most of us would say at once that, of course
this is not true. But it *is* true for anyone living on the
equator!
On a standard meridian at the equator one might
expect the Sun to rise at 6:00 A.M. and set at 6:00 P.M.,
but the Sun rises at 6:03 A.M. in July, a summer month, and
also rises late, at 6:11 A.M. in February, a winter month. It rises seven
minutes before 6:00 A.M. in mid-May, and 20 minutes before
6:00 A.M. at the end of October. At the equator these
effects are entirely accounted for by the equation of time.
The daily path of the Sun as seen at the equator on
the first day of spring, summer, fall, and winter is
illustrated in Figures 7 and 7a .
At the equator the Sun rises
perpendicularly from the horizon and sets perpendicularly,
regardless of the season. Also, the total path of the Sun,
day and night, is divided equally by the horizon. There are
always twelve hours of daytime and twelve hours of
night-time at the equator, except for two minor effects that
increase daytime by about eight minutes. First, since we
mark the instant of sunrise as the time the Sun's upper edge
or "limb" just touches the horizon, the actual center of the Sun is still
below the horizon by half the diameter of the Sun, 16 arc
minutes or ¼ degree.. It will take an additional minute
for the Sun's center to be on the horizon. At sunset the
same thing happens and so an additional two minutes are
gained for daytime. Second, when the Sun's limb appears at
the horizon, it is actually still 43 arc minutes below the
horizon but only appears to be at the horizon due to the
refraction or bending of the Sun's rays by the Earth's
atmosphere. This effect causes the sunrise to appear about
three minutes early and sunset late by the same amount.
Taking both effects together, the length of daytime is about
8 minutes more than 12 hours, and so, of course, night-time
will be 8 minutes less than 12 hours, resulting in daytime
being 16 minutes more than night-time at the equator, or for
that matter, anywhere during the equinoxes (March 21 and September 21).
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