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Glossary ahargan .a the number of accumulated days since the beginning of some designated epoch. altitude one of the pair of orthogonal coordinates in the horizon coordinate system (the other is the azimuth). It is the angular distance from the horizon to a given point on the celestial sphere. analemma a diagram that represents the celestial sphere projected onto the twodimensional plane and captures the relations between great circles, points, angles, and arcs. aṅgula the unit of linear measure equivalent to a digit. anomaly ( α ) the angular distance between the apogee and the position of the mean celestial body. apogee (A) the point at which a celestial body is farthest from the earth on its orbit. ascendant (H) the point of the ecliptic which is rising on the eastern horizon. astrolabe an instrument used by astronomers that captures the celestial sphere as projected stereographically onto the plane of the equator. Its moving parts allow it to act as a quantitative working model of the heavens. It can be used to observe celestial phenomena and then compute various astronomical details, such as rising times, altitudes, positions, intervals, and times of various celestial phenomena. ayana the Indian term used to refer to a half-year interval—the northern ayana is when the sun travels north (from the winter solstice to the summer solstice) and the southern ayana when the sun travels south (between the summer solstice to the winter solstice). azimuth one of the pair of coordinates in the horizon coordinate system (the other is the altitude). It is the angular distance along the horizon from the north point to any given point on the horizon. bı̄ja a correction that converts the parameters of one astronomical system to another. celestial sphere a giant sphere enclosing the observer, who is positioned at its center, on which all celestial bodies are imagined to be projected. celestial equator the great circle on the celestial sphere which falls in the plane of the terrestrial equator. chord a line segment that joins two points on the circumference of a circle. 377 GLOSSARY clime a geographical region created as a result of dividing up the terrestrial sphere into distinct regions, usually using latitude circles parallel to the terrestrial equator. concentric can refer to a circle or a sphere whose center is located at the same place as the earth. conjunction the instant at which two celestial bodies, usually the sun and the moon, have the same longitude and hence are at the same (or very nearly the same) position in the sky. cosmography a description or a representation of the structure of the universe. cosmology an accounting for the structure of the universe. day circle a small circle on the celestial sphere parallel to the celestial equator, which represents the apparent course of a celestial body (usually the sun) during one daily rotation of the celestial sphere. declination (δ) one of the pair of orthogonal coordinates in the equatorial coordinate system (the other is the right ascension). It is the angular distance from a given point on the celestial sphere to the celestial equator. deferent a circle whose center coincides with the center of the earth. A smaller circle, or epicycle, moves on its circumference. eccentric a circle whose center is distinct from the center of the earth. The distance between these two centers is called the eccentricity (e). eclipse, annular a type of solar eclipse in which the apparent size of the moon is smaller than the apparent size of the sun, which appears to cast a ring or annulus around the disk of the moon, outlining its shape. eclipse, lunar a celestial alignment that occurs when the moon passes through the shadow created when the earth blocks light from the sun. Because this alignment occurs only when the sun, the earth, and the moon are on the same line (or very nearly so) and in that order, lunar eclipses can only occur at opposition. eclipse, solar a celestial alignment that occurs when the moon passes across the face of the sun, hence depriving certain locations on the earth wholly or partially of its light. Because this alignment occurs only when the sun, the moon, and the earth are on the same line (or very nearly so) and in that order, solar eclipses can occur only at conjunction and their appearance is highly dependent on the location of the observer on the earth. ecliptic the great circle which delimits the apparent path of the sun in a...

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