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The Fall Equinox
was last Saturday. We can't let a big event like
that go unnoticed.
The figure above shows the earth
orbiting the
sun.
On
or
around Dec. 21st, the winter solstice, the north pole is tilted away
from the sun. Note that a small portion of the earth near the N.
Pole (north of the Arctic Circle) spends 24 hours in darkness.
Days are less than 12 hours long in the northern
hemisphere and the sun is low in the sky. Both factors reduce the
amount of sunlight energy reaching the ground. That's why it's
cold and wintry.
On June 21st, the
summer solstice, the north pole is tilted toward the sun. Now
there are 24 hours of sunlight north of the Arctic Circle. Days
are more than 12 hours long in the northern hemisphere and the sun is
high in the sky at noon. A lot more sunlight energy reaches the
ground; that's why it is summer.
The equinoxes are a time of transition. On the equinoxes,
the N. Pole still tilted just not toward or away from the sun.
The line
separating day and night passes through the pole and the days
and nights are each about 12 hours long everywhere on earth (except
perhaps
at
the poles).
The drawing below shows you what you would see at sunrise (about
6:30 am) on the Spring Equinox here in Tucson (the same would happen on
the Fall Equinox). The sun rises exactly in the east
on the equinoxes. The rest of the year it is a little to the
north or south of east.
At noon you
would need to look south to see the sun.
The sun reaches its
highest point in the sky at noon. On the equinoxes in Tucson
that's almost 60 degrees. The sun is lower in the sky (34.5
degrees
above the horizon) on the winter solstice. That together with the
fact that the days are shorter means much less sunlight energy reaches
the ground. In the summer the days are longer and the sun gets
much higher
in the sky at noon (81.5
degrees
above
the
horizon,
nearly
overhead). Much more sunlight energy reaches the ground and it is
much warmer.
The sun passes directly overhead at the equator at noon on the
equinoxes.
The
sun
sets exactly in the west on the equinoxes at about 6:30 pm in
Tucson.
This is the 2 pm class.
Most of you are more likely
(perhaps) to see the sun set than see the
sun
rise. The figure below shows you about what you would see if you
looked west on Speedway (from Treat Ave.) at sunset. In the
winter the sun will set south of west, in the summer north of west
(probably further south and north than shown here). On the
equinoxes the sun sets exactly in the west. This is something you
should check out for yourself this week before the sun moves noticeably
to the north of due west.
Several years ago I
positioned myself in the median near the
intersecton of Treat and Speedway and pointed my camera west. I
took a multiple exposure photograph of the sun over a 2 or 3 hour
period
that ended at sunset. I'll bring the slide photograph to
class one of these days.
Something else to note in this figure. Note
how
the sun is changing
color. It changes from a bright yellow white to almost red by the
time it sets.. This is due to scattering of sunlight by
air. The shorter wavelengths (violet, blue, green) are scattered
more readily than the longer wavelengths. At sunset the rays of
sunlight take a much longer slanted path through the atmosphere and
most of the shorter wavelengths are scattered and removed from the beam
of sunlight. All that's left in the beam of light that reaches
your eyes are the longer wavelengths: yellow, orange, and red.
If you aren't
careful, you can get yourself seriously
injured,
even
killed,
on
or around the equinoxes. Here's
an article that appeared in the Arizona Daily Star at the time of the
equinox last fall (Thu., Sep. 22).
December 21, the
summer solstice, is the shortest day
of the
year (about 10 hours of daylight in Tucson). The days have slowly
been getting longer since then. The rate of change is greatest at the
time of the equinox.
This will continue up until June 21,
the summer solstice, when there will be about 14 hours of
daylight. After that the days will start to shorten again as we
make
our way back to
the winter solstice.
There was
a very interesting coicidence last fall. We were
covering some of this same material in class on Friday Sep. 23.
There
were a few parents in class because it was Parent's
Weekend. I showed these same pictures on that
afternoon. One of the parents came up to the front
after class and mentioned having seeing the sun right at the end of
77th St.
in New York City around this time of year. That got me thinking
that a picture of sunset at the end of one of the long streets with all
the tall buildings might be spectacular.
When I started looking however I found that the major streets in
Manhattan aren't oriented EW and NS. You can see this on a Google
map
of
Manhattan. 77th St. is oriented in more of a NW-SE
direction. So the sun doesn't shine straight down 77th St.
at sunrise and sunset on the equinoxes. I was pretty disappointed
but then I stumbled on the this
Manhattanhenge
map which shows the direction of sunset (the left, west,
side of the map) and sunrise (the right, east, side of the map) at
various times of the year.
If you remember that as you move past the Spring Equinox toward
summer
sunrise move north of east and sunset is north of west. On May 31
the sun has moved far enough north that it does set right at the west
end of 77th St. Sunset continues to move north up until the
summer solstice on June 21. Then the sunset starts to move back
south. You can again see the sunset at the west end of 77th St.
on July 12 and 13. An
article with several Manhattanhenge
photographs from the May 31 event appeared in a story on
the Business Insider webpage. That would certainly
make a worthwhile field trip in Atmo 170A1 if the semester went that
long. The "henge" part of the
name comes from Stonehenge
where the rising and setting sun aligns with
stones on the solstices.
Manhattanhenge
is
a little confusing and hard to figure out.
But do look at the photographs above with the idea that you can see
something
similar here in Tucson on or around the equinoxes (minus all the tall
buildings).
You can also see the sunrise at the east end of 77th St. But
sunrise has to be in the southeast. This takes place on Dec. 5
and Jan. 8, just before and just after the winter solstice.