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The Eye of Ra or Eye of Re, usually depicted as sun disk or right wedjat-eye (paired with the Eye of Horus, left wedjat-eye), is an entity in ancient Egyptian mythology that functions as an extension of the sun god Ra's power, equated with the disk of the sun, but it often behaves as an independent goddess, a feminine counterpart to Ra and a ...
For example, if a star with RA = 1 h 30 m 00 s is at its meridian, then a star with RA = 20 h 00 m 00 s will be on the/at its meridian (at its apparent highest point) 18.5 sidereal hours later. Sidereal hour angle, used in celestial navigation , is similar to right ascension but increases westward rather than eastward.
In astronomy, position angle (usually abbreviated PA) is the convention for measuring angles on the sky. The International Astronomical Union defines it as the angle measured relative to the north celestial pole (NCP), turning positive into the direction of the right ascension .
The azimuth is the angle formed between a reference direction (in this example north) and a line from the observer to a point of interest projected on the same plane as the reference direction orthogonal to the zenith. An azimuth (/ ˈ æ z ə m ə θ / ⓘ; from Arabic: اَلسُّمُوت, romanized: as-sumūt, lit.
The components for proper motion in the equatorial coordinate system (of a given epoch, often J2000.0) are given in the direction of right ascension (μ α) and of declination (μ δ). Their combined value is computed as the total proper motion (μ). [2] [3] It has dimensions of angle per time, typically arcseconds per year or milliarcseconds ...
Angular distance or angular separation is the measure of the angle between the orientation of two straight lines, rays, or vectors in three-dimensional space, or the central angle subtended by the radii through two points on a sphere.
Denoted with the symbol Ω, it is the angle from a specified reference direction, called the origin of longitude, to the direction of the ascending node (☊), as measured in a specified reference plane. [1] The ascending node is the point where the orbit of the object passes through the plane of reference, as seen in the adjacent image.
The direction of sufficiently distant objects is the same for all observers, and it is convenient to specify this direction with the same coordinates for all. In contrast, in the horizontal coordinate system , a star's position differs from observer to observer based on their positions on the Earth's surface, and is continuously changing with ...