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The Earth and the Sun
In the previous section the tilt of the Earth on its axis of
rotation was shown to affect the height of the Sun in the sky
which in turn determines how directly the Sun’s rays heat the
Earth and how long the Sun remains above the horizon. In the
summer months the Sun reaches its maximum height producing
the warmest temperatures and the longest days, while in the
wintertime the Sun rises to its minimum height all year leading
to colder temperatures and shorter days. Knowing this one
should expect the Sun’s position to transition between its
maximum and minimum heights as the seasons change from summer
to autumn to winter and then again from winter to spring to
summer. These transition days are recognized as “the first
day of summer”, “the first day of winter”, etc., but what
really determines the changing of the seasons, and how are
they defined by the Sun’s position in the sky?
Answering these questions requires a closer look at the path
that the Sun follows in the sky over the course of a day and
through the four seasons of the year.
The Sun's Path in the Sky
The figure below shows the Earth titled on its axis of rotation,
this time with an arrow indicating that the Earth rotates
counterclockwise as viewed from above the North Pole. This
counterclockwise rotation allows Tokyo (Japan is known as
“The Land of the Rising Sun”) to see the sun rise before London,
London before Washington DC, and Washington DC before Honolulu, with the
positions of the cities and their respective longitudes indicated on the globes.
Hence the counterclockwise rotation of the Earth explains why
the Sun appears to rise in the East and set in the West.
| Counterclockwise Rotation Means Sun Rises in the East |
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Globe images courtesy of
GraphicMaps.com |
But what path does the Sun follow through the course of the
day as it moves from East to West? How is this path affected
by the seasons, and why does the Sun not set at the North Pole
in the summer, or rise there in the winter?
U.S. Naval Observatory (USNO) Sun Path Website
To answer questions about the Sun’s position during the day
we consult an astronomy website maintained by the U.S.
Naval Observatory (USNO). The USNO is the preeminent source
for celestial position information in the United States, and
its Astronomical Applications Department maintains a website
tool called the Sun or Moon Altitude/Azimuth Table that
tabulates the Sun’s position as viewed from any latitude
and longitude on Earth. The Sun’s altitude or height in
the sky is measured in degrees above the horizon. (Recall that
this information was used the previous discussion to compare the
height of the Sun in Washington DC and Bahia Blanca, Argentina.) The Sun’s
azimuth is measured in degrees along the horizon with 0
degrees corresponding to North, 90 degrees to East, 180
degrees to South and 270 degrees to West as shown in the
figure below.
| Azimuthal Direction as Measured Along the Horizon |
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The tables below indicate the Sun's height and direction for Washington
DC during four different times of the year as obtained from the USNO
website. Pay particular attention to the direction the Sun's path follows
during the course of the day, as well as the length of time between
sunrise and sunset on each day.
| Sun Altitude and Azimuth from USNO Website |
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These data indicate that in the Northern Hemisphere that
the Sun rises in the East and then follows a southernly
path in the sky until it sets in the West. The Sun's path in
the sky is illustrated below for the four days tabulated.
Notice that that as the Sun's height increases or decreases so does
its "span in azimuth" across the horizon. This increased "span" along
the horizon translates into longer times above the horizon and therefore
longer hours of daylight. A decreased span indicates shorter amounts
of time above the horizon and therefore fewer hours of daylight. Notice
also that the Sun rises and sets exactly in the East and West, respectively,
on the March and September dates, with almost exactly 12 hours of daylight.
| Northern Hemisphere: Sun's Path in Washington, DC |
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The Sun's Path in the Southern Hemisphere
The figure below shows similar Sun path information for the Southern Hemisphere
city of Bahia Blanca, Argentina for the same four dates. In addition to the
reversal of paths for June and December notice now that the Sun moves through
the sky from East to West in a northernly direction! Pay particular
attention to the way in which the direction along the horizon is delineated:
North now sits at 0 degrees which is equivalent to 360 degrees,
Northeast to Southeast run from 0 degrees to 135 degrees, and Northwest to
Southwest run from 360 degrees to 225 degrees.
| Southern Hemisphere: Sun's Path in Bahia Blanca, Argentina |
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In the next section the discussion continues with a closer examination of
the Sun's path during the seasonal "markers" known as the
solstices and the equinoxes.
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Basics of Celestial Motion. Copyright 2006
S. E. Scruggs
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