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best times to fish. Horizon:Wherever one is located on or near the Earth's surface, the Earth is perceived as essentially flat and, therefore, as a plane. The sky resembles onehalf of a sphere or dome centered at the observer. If there are no visual obstructions, the apparent intersection of the sky with the Earth's (plane) surface is the horizon, which appears as a circle centered at the observer. For rise/set computations, the observer's eye is considered to be on the surface of the Earth, so that the horizon is geometrically exactly 90 degrees from the observer's zenith. Rise, Set:During the course of a day the Earth rotates once on its axis
causing the phenomena of rising and setting. Celestial bodies â€“ stars and planets included â€“ seem to appear in the sky at the horizon to the East, then to cross the sky and
again disappear at the horizon to the West. Sunrise and sunset conventionally refer to the times when the upper edge of the disk of the Sun is on the horizon. Atmospheric conditions are assumed to be average, and the location is in a level region on the Earth's surface. Moonrise and moonset times are computed for exactly the same circumstances
as for sunrise and sunset. However, moonrise and moonset may occur at any
time of day and, consequently, it is often possible for the Moon to be seen
during daylight, and to have moonless nights. Transit:The transit time of a celestial body refers to the instant that its
center crosses an imaginary line in the sky  the observer's meridian 
running from north to south.
The New Moon transits at
about the same time as the Sun; Twilight:Before sunrise and again after sunset there are intervals of time
called
twilight, during which there is natural light provided by the upper
atmosphere, which does receive direct sunlight and reflects part of it
toward the Earth's surface. There are three different twilight Periods. Civil twilight:Defined to begin in the morning, and to end in the evening
when the center of the Sun is geometrically 6 degrees below the horizon. Nautical twilight:
Technical Definitions and Computational Details:Sunrise and sunset. For computational purposes, sunrise or sunset is defined to occur when the geometric zenith distance of center of the Sun is 90.8333 degrees. That is, the center of the Sun is geometrically 50 arcminutes below a horizontal plane. For an observer at sea level with a level, unobstructed horizon, under average atmospheric conditions, the upper limb of the Sun will then appear to be tangent to the horizon. The 50arcminute geometric depression of the Sun's center used for the computations is obtained by adding the average apparent radius of the Sun (16 arcminutes) to the average amount of atmospheric refraction at the horizon (34 arcminutes). Moonrise and moonset. Moonrise and moonset are defined similarly, but the situation is computationally more complex because of the nearness of the
Moon and the eccentricity of its orbit. If the computations are carried out
using coordinates of the Moon with respect to the Earth's center (the usual
method), then moonrise or moonset is defined to occur when the geometric
zenith distance of the center of the Moon is Accuracy of rise / set computations. The times of rise and set phenomena
cannot be precisely computed, because, in practice, the actual times depend
on unpredictable atmospheric conditions that affect the amount of refraction
at the horizon. Thus, even under ideal conditions (e.g., a clear sky at sea)
the times computed for rise or set may be in error by a minute or more.
Local topography (e.g., mountains on the horizon) and the height of the
observer can affect the times of rise or set even more. It is not practical
to attempt to include such effects in routine rise/set computations. Twilight. For computational purposes, civil twilight begins before sunrise and ends after sunset when the geometric zenith distance of the center of the Sun is 96 degrees  6 degrees below a horizontal plane. The corresponding solar zenith distances for nautical and astronomical twilight are 102 and 108 degrees, respectively. That is, at the dark limit of nautical twilight, the center of the Sun is geometrically 12 degrees below a horizontal plane; and at the dark limit of astronomical twilight, the center of the Sun is geometrically 18 degrees below a horizontal plane.
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