The Fall Equinox
The fall equinox is going to take place on Sunday, Sep. 22 at
20:44 UT (1:44 pm MST) this year. That's something we
would ordinarily celebrate in ATMO 170A1. But because
class doesn't meet on Sunday (fortunately) here's a short
explanation of what to expect.
The drawing below shows you what you would see at sunrise
(about 6:30 am) on either the Fall or the Spring Equinox here
in Tucson. 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.
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
At noon you would need to look
due 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 sun sets
exactly in the west on the equinoxes at about 6:30 pm in
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 south of due west.
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.
might have noticed that the sketch seems to be
pretty carefully drawn. That's because
several years ago I positioned myself in the
median near the intersection 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.
Here's the photo I ended up with (it's a copy of a
copy so that picture quality isn't all that
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 2011 Fall equinox.
June 21, the summer
solstice, is the longest day of the year (about 14 hours of
daylight in Tucson). The days have slowly been getting
shorter since then. The rate of change is greatest at the
time of the equinox.
This will continue up until December 21, the winter
solstice, when there will be about 10 hours of
daylight. After that the days will start to lengthen
again as we make our way back to the summer solstice.
was a very interesting coincidence a few semester's ago.
We were covering some of this same material in class on
Friday Sep. 23, near the time of the Fall Equinox. 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 sunset 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 (the sun rises in the east and
sets in the west on the equinoxes). 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.
Well consider the sunset. As you move past the Spring
Equinox toward summer, sunrise will 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 even further 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. Here's a gallery
of Manhattanhenge images. That
would certainly make a worthwhile field trip in Atmo 170A1 if the
semester went that long. The "henge" part of the name comes
where the rising and setting sun aligns with stones on the
solstices. An article in Wikipedia reports similar events in
I tried Parishenge and sure enough found this image of
Manhattanhenge is a little confusing and hard to
understand. But do look at the photographs with the idea
that you can see something similar here in Tucson on the equinoxes
(minus all the tall buildings).