A
friend of mine who lives near Cleveland,
Ohio, noticed that as the 'day
gets longer', i.e., as the duration of sunlight increases by about 1 minute per
day at this time of year, the increase is always due to a later-setting sun.
The time of sunrise hardly changes at
all. My friend wondered why the day doesn't just get 30 seconds longer equally at both
sunrise and sunset.
The
primary cause is the angle of the Ecliptic (the imaginary path of the Sun in
the sky) to the horizon. The angle of the ecliptic to the horizon is a
consequence of the Earth's tilt, the time of year and the observer's location.
During most of the year there is an asymmetry between the dawn and dusk
distances of the Sun to the horizon when looked at on a day-to-day basis. At
this time of year that asymmetry is pretty large. (Roughly 6 months from now,
the asymmetry shifts the other way.)
So
at this time of year at dusk, as the sun moves along the ecliptic its
distance to the horizon increases quite a bit from day to day, since at dusk
the ecliptic is at a steep angle to the horizon. Conversely, at dawn the
ecliptic is at a shallow angle to the horizon, so as the Sun moves along the
ecliptic from day to day its daily motion is roughly parallel to the horizon.
The following four diagrams show the Sun's position at dawn and at dusk on
January 1 and January 21. So you can see at dusk the distance from the
sun to the horizon increases a lot on a day-to-day basis. And conversely, at
dawn the distance of the sun to the horizon hardly changes at all over this
same 20-day period. The person's position on the Earth affects this a lot, of
course -- e.g., no sunrise at all above the Arctic circle and no sunset beyond
the Antarctic circle at this time of the year.
Diagram
1) The Sun near sunrise (8:00 AM) on Jan 01, as seen from Cleveland, Ohio.
(Assuming a clear sky.) The black band across the bottom of the diagram
represents the horizon.
2)
The Sun near sunrise (8:00 AM) on Jan 21. Compare with Diagram 1) and note how
the distance between the sun and the horizon has hardly changed in 20 days.
You
can really see how the angle of the green line to the horizon changes between
dawn in these two diagrams as compared with the angle at dusk in Diagrams 3)
and 4).
Comparing
this diagram with diagram 4), the sun itself will move about 3/8ths of a
degree, or slightly less than its diameter, along the green line between dawn
and dusk.
The
lines you see here rotate nearly 135 degrees between dawn and dusk, which
changes the angle between the horizon and the green line that the sun is on
(the ecliptic) between dawn and dusk. The 135 degree rotation is due to there
being only about 9 hours of daylight between dawn and dusk; 9/24ths of 360 =
135 degrees. This is what causes the asymmetry in the lengthening of the day.
(In a 12-hour day, the rotation is 180 degrees and the angle is the same for
dawn and dusk.)
3)
The Sun near sunset (5:02 PM) on Jan 01.
4)
The Sun near sunset (5:02 PM) on Jan 21. Compare this diagram with 3) and notice
the large increase in the distance of the sun to the horizon in 20 days.
For reference, note that Capricornus
is above the sun here, but below the sun at dawn in Diagram 2).
These
diagrams are screen shots from an Astronomy program, Starry Night Pro, that I
have on my computer. It allows me to select any location on Earth or elsewhere
in the solar system, at any date and time over a range of thousands of years.
Sunrise, Sunset - What's Going On?
Re: Sunrise, Sunset - What's going on?
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