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ASTRONOMICAL TERMS
sunrise The time at which the first part of the sun
appears above the horizon in the morning.
sunset The time at this the last part of the sun
disappears below the horizon in the evening.
dawn or civil dawn This is the time at which the sun is 6 degrees
below the horizon in the morning. Civil dawn is defined as that time at which
there is enough light for objects to be distiguishable and that outdoor
activities can commence.
nautical dawn This is the time at which the sun is 12 degrees below
the horizon in the morning. Nautical dawn is defined as that time at which there is
just enough sunlight for objects to be distiguishable.
astronomical dawn This is the time at which the sun is 18 degrees below the horizon in the
morning. Astronomical dawn is that point in time at which the sun starts lightening the sky. Prior to
this time, the sky is completely dark
dusk or civil dusk This is the time at which the sun is 6 degrees below the horizon in the evening.
At this time objects are distinguishable but there is no longer enough light to perform any outdoor activities.
nautical dusk This is the time at which the sun is 12 degrees below the horizon in the
evening. At this time, objects are no longer distinguishable.
astronomical dusk This is the time at which the sun is 18 degrees below the horizon in the
evening. At this time the sun no longer illuminates the sky.
solar noon This is the time at which the sun is the highest in the sky. This time varies
through the year due to the change in speed of the earth's orbit around the sun.
daylength The total time the sun is above the horizon
twilight The average time of civil twilight which is the time between civil dawn and
sunrise and sunset and civil dusk.
elevation The angle that the sun is above the horizon (assuming no atmosphere)
elevation (r) The angle that the sun appears to be above the horizon assuming the refraction
of the earth's atmosphere.
azimuth The direction that the sun appears to be over with 0 degrees being north,
90 being east, 180 being south and 270 being west.
zenith angle The angle between the sun and directly overhead, the zenith
declination The latitude that the sun is directly over which is ~23N in the summer, ~23S
in the winter and 0 at the equinoxes.
right ascension The celestial longitude of the sun. This value is 0 at the vernal equinox,
90 at the summer solstice, 180 at the autumnal equinox and 270 at the winter solstice.
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National Weather Service Forecast Office 4 Falcon Drive Peachtree City, Georgia 30269 Phone: 770.486.1133 Page last modified: December 11, 2003 |
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