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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