Question:
"A friend of mine (who is a local TV meteorologist,
and has previously spent more than a decade as an
"on air" meteorologist for the Weather Channel),
posed an interesting question to me. Here it is:
"Do you have a simple explanation of why the earliest sunsets precede the
latest sunrises by a few weeks and, correspondingly in the summer,
why the earliest sunrises precede the latest
sunsets by a few weeks?"
I am not quite certain as to this answer. AM I missing the obvious.
Any help in the form of an explanation
would be appreciated.
S.
One answer:
S
The answer is not obvious and is involved including both the elliptical
nature of the Earth's orbit and that fact that the Earth's axis is
tilted. I developed an answer many years
ago at Morehead (perhaps even as far back
as Abrams) in response to a similar inquiry from a meteorologist and
regularly used it when other people inquired. I have included it
below in its entirety.
L.S.
Time Confusion (Or why sunrise and
sunset do occur latest and earliest at the
solstices)
There are a couple of special points or times during the year that mark
the seasonal extremes
the solstices or the beginnings of winter and
summer. These times occur around December
22 and June 21. It would be more accurate
to refer to them as the December and June solstices or the southern
and northern solstices. The terms winter and
summer solstices, though common,
are inappropriate, because which one you pick depends upon the
hemisphere in which you reside. Northern
hemisphere residents consider December the time
of the winter solstice, while for southern hemisphere residents, it
marks the start of summer.
Those two dates also mark the extremes in the amount of night time and
daylight hours. In North Carolina, for example, the maximum of about
14.5 hours of night time and minimum of 9.5
hours of daylight occur around the December
solstice. Thus, one might also expect that around December 22
(the solstice) that one would witness the
latest time of sunrise and the earliest
time of sunset. However, if you watch the daily newspaper, you will
find that the earliest sunset occurs about
one week into December and the latest
sunrise occurs about one week into January.
The cause for both of these changes and similar changes around the June
solstice is related to the Equation of Time.
The Equation of Time relates the difference between time measured by
the actual position of the Sun (called the
apparent Sun) and time measured by
the use of an imaginary mathematical Sun that moves across the sky at
a strictly uniform rate. This mathematical Sun is
important because it is what is used to determine
the time kept on our watches and clocks with
further adjustments to account for differences in longitude, time zones, and
daylight savings time.
The Sun does not move at a uniform rate for two reasons. The
difference in time
between the apparent Sun (i.e. the real Sun) and the mathematical
Sun
is the Equation of Time. The cause of the first part of the difference
is the Earths motion around the Sun.
Since the Earth moves around the Sun in an ellipse rather than a
circle (as Kepler
realized), its speed changes moving faster when it is closer
to the Sun and slower when it is further
from the Sun. When the Earth is closest
to the Sun (in very early January) and moving fastest, it takes the
Earth a little bit longer to face the Sun
again and the day is slightly longer.
Conversely, when the Earth is furthest from the Sun in early July, the
day is slightly shorter.
The second reason for the variation of the day is the real Sun's
apparent annual motion along its path, the
ecliptic. Of course, it is the Earth that actually
does the moving, but the Sun appears to move along this path.
The mathematical Sun, instead of moving on
the ecliptic, moves on the celestial equator. The
celestial equator stretches symmetrically across the sky
from the east point to the west point, but
the ecliptic is tipped at a 23.5 deg angle to the
celestial equator. The difference in the two paths causes
the Sun's daily
motion to be non-uniform. This effect causes days to be
longer at the time of the solstices and
shorter at the time of the equinoxes. The sum of
both effects accumulates daily. At different times of the
year, the time measured by the apparent Sun
(i.e. sundial time) is ahead of time measured by
the mathematical Sun; while at other times of the year, the
apparent Sun is behind the mathematical Sun. This changing difference
is the Equation of Time. On some maps and
globes of the Earth, this relationship is depicted
graphically as the figure called the Analemma,
which resembles the number eight. Thus in mid-May the apparent Sun
reaches a maximum of nearly four minutes
ahead of the mathematical Sun, while in early
November it reaches a second maximum more than sixteen minutes ahead
of the mathematical Sun. In mid-February, the apparent Sun sinks to
being more than fourteen minutes behind;
while in late July it encounters a second
minimum when it is more than six minutes behind the mathematical Sun.
Near the time of the December solstice in the northern hemisphere,
the seasonal changes are bringing the daylight
hours to their shortest amount
and tending to produce the latest sunrise and earliest sunset.
However, at about the same time, the non-uniform motion of the Sun
(as illustrated by the Equation of
Time) is working to shift both sunrise and sunset later in the day.
For sunrise the two effects work together and the
date of latest sunrise is
delayed to early January. In the case of sunset, the two effects are
working in opposition, and so the earliest time of sunset occurs in
early December. There is a comparable conflict
near the time of the June solstice. One would
expect the time of earliest sunrise and latest sunset
to occur around June 21, yet the earliest sunrise comes about one
week early and the latest sunset comes about one
week late.
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