Sun Times
For St. Joseph, Minnesota
W 94° 19'; N45° 34'
The below graphs are in Central Standard Time,
you'll need to add an hour if Central Daylight Standard Time
is in effect. Note that because the Earth's orbit about the
Sun is not a whole number of days (e.g., 1 tropical year=365.24220 days)
these plots (made for the year 1998) are not exactly valid
for any other year. However, differences are small. In particular
the day with the latest sunset etc., will move around by a few
days (much as the solstice themselves move a bit).
Definitions
- Sunrise and Sunset
- are times when the upper edge of the disk of the Sun is on the
ideal horizon (assumed to be unobstructed). Because of atmospheric
refraction (bending) of light, this occurs when the Sun is totally
below the horizon, and depends in part on atmospheric conditions.
Under typical sea-level conditions atmospheric refraction amounts
to about 34 arcminutes; the average apparent radius of the Sun
is about 16 arcminutes. Thus a 50-arcminute geometric depression of the
Sun's center is used for these computations.
- Nautical Twilight
- is defined to end in the evening,
when the center of the sun is
geometrically 12° below the horizon. At end of nautical twilight,
under good atmospheric conditions and
in the absence of other illumination, general outlines of ground
objects may be distinguishable, but detailed outdoor operations
are not possible, and the horizon is indistinct.
- Local Noon
- is the time when the Sun is on the meridian,
i.e., "high noon" when the Sun reaches is maximum altitude.
The fact that this is not exactly at noon has to do with our
displacement from the center of the time zone, and irregular motion
of the Sun (a.k.a, "the equation of time") you might remember
from the analemma on your globe.
make your own table
The below form is designed to compute a table (valid for
St. Joseph) for a year you select. The calculations are
performed by the
U.S. Naval Observatory!
