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 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 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.

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 equinoxes.

The sun sets exactly in the west on the equinoxes at about 6:30 pm in Tucson.

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.

You 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 great):

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 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.

There 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 from Stonehenge where the rising and setting sun aligns with stones on the solstices.  An article in Wikipedia reports similar events in Baltimore, Chicago, Toronto and Montreal.  I tried Parishenge and sure enough found this image of sunrise.

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).