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Systematically observing the sunrise, people discovered that it occurs between two extreme locations at the horizon and eventually noted the midpoint between the two. Later it was realized that this happens on a day when the duration of the day and the night are practically equal and the word "equinox" comes from Latin aequus, meaning "equal", and nox, meaning "night".
In astronomy and celestial navigation, the hour angle is the dihedral angle between the meridian plane (containing Earth's axis and the zenith) and the hour circle (containing Earth's axis and a given point of interest). [1] It may be given in degrees, time, or rotations depending on the application.
Any units of angular measure can be used for declination, but it is customarily measured in the degrees (°), minutes (′), and seconds (″) of sexagesimal measure, with 90° equivalent to a quarter circle. Declinations with magnitudes greater than 90° do not occur, because the poles are the northernmost and southernmost points of the ...
However, at latitudes closer than 8°35' (between 81°25’ and 90°) to either Pole, the Sun cannot rise above the horizon nor sink more than 18° below it on the same day on any date, so this example of twilight cannot occur because the angular difference between solar noon and solar midnight is less than 17°10’.
At 49° north/south latitude (e.g., in Karlsruhe), it varies between 16 equinoctal hours in summer and 8 equinoctial hours in winter. Due to the continuous change of the duration of daylight over the course of the year, the duration of the day division, i.e. the temporal day hours and the temporal night hours, also changes over the year.
The angular diameter, angular size, apparent diameter, or apparent size is an angular separation (in units of angle) describing how large a sphere or circle appears from a given point of view. In the vision sciences , it is called the visual angle , and in optics , it is the angular aperture (of a lens ).
The single standard watch of 4 hours (two double hours) was divided into 60 time-degrees (ush). One double hour had 30, and one complete stellar day, 360 (12 times 30). [ 19 ] This assignment was the creation of the 360-degree circle, as the degree went from being a time division to an angular distance of rotation.
At civil twilight, the center of the Sun's disk appears to be between 1/4° and 6° below the horizon. At nautical twilight, the Sun's altitude is between –6° and –12°. At astronomical twilight, the Sun is between –12° and –18°. When the Sun's depth is more than 18°, the sky generally attains its maximum darkness.