Measuring True East
Name: Lee A.
Date: February 23, 2003
May we assume that the sun rises in the true east on
March and Sept 21 (or so)? I live in the country. There is a lone farm
silo one mile east of my house. When I watch the sunrise on March or
Sept 21 and the sun first peaks over the horizon it is on the north side
of the silo. But by the time the sun is fully emerged above the horizon,
it has moved south far enough so that the whole circle of the sun is on
the south side of the silo. So what is true east?
From your description, it is apparent that the silo is just
south of true east from where you are looking. That is why
the Sun can be seen on the north side of the silo when it is
on the horizon. As the Sun rises, it makes an arc in the sky
towards the south. So it passes south of the silo as it rises.
On March 21 and Sept. 21, the Sun is directly above the equator
of the Earth. However, you live far north of the equator and,
at noon on those two days, the Sun is at an angle to the vertical
(called the zenith) equivalent to your latitude.
So, if you live at 42 degrees north latitude, the Sun will be
42 degrees to the south of vertical at your noontime. This is
almost halfway to the horizon. The arc of the Sun through the
day at your location is thus greatly tilted to the south, and so
the Sun passes to the south of the silo by the time it is fully
above the horizon.
David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
Yes, the sun rises at true east on the equinoxes. This is rigorously true
only for the locations where the sun rises at the moment of the equinox, and
actually, the diffraction of sunlight by the atmosphere throws the direction
off just a little bit for everywhere away from the equator. But the
position of the sun at dawn on the day of the equinox is pretty close to
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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