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| constellation | a group of stars visible from Earth that forms a distinctive pattern and has a name, often derived from Greek mythology, linked to its shape. There are 88 constellations and the groupings are historical rather than scientific. |
| zenith | the point of the celestial sphere that is directly over the observer and 90 degrees from all points on that person's horizon |
| horizon | a circle formed on the celestial sphere by a plane tangent to a point on the Earth's surface |
| north celestial pole | The north celestial pole is the imaginary point in the sky where the Earth's axis of rotation, indefinitely extended, intersects the imaginary rotating sphere of stars called the celestial sphere. The north celestial pole appear permanently directly overhead to an observer at the Earth's North Pole. |
| south celestial pole | The south celestial pole is the imaginary point in the sky where the Earth's axis of rotation, indefinitely extended, intersects the imaginary rotating sphere of stars called the celestial sphere. The South celestial pole appear permanently directly overhead to an observer at the Earth's South Pole. |
| celestial equator | The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. In other words, it is a projection of the terrestrial equator out into space.[1] As a result of the Earth's axial tilt, the celestial equator is inclined by 23.4° with respect to the ecliptic plane. |
| altitude | the angle of an astronomical object above an observer's horizon, measured from the horizon along the circle passing through the object and the point above the observer |
| azimuth | the angle measured from north, eastward along the horizon, to the point where a vertical circle through an astronomical object intersects the horizon |
| right ascension | one of the two reference points in the equatorial coordinate system for specifying the position of an astronomical object on the celestial sphere. Corresponding to longitude on the Earth, it is measured in hours, minutes, and seconds eastward from the vernal equinox, the point where the ecliptic intersects the celestial equator. |
| declination | the angular distance of an astronomical object measured in degrees from the celestial equator along the great circle passing through it and the celestial poles |
| Polaris | he brightest star of the Little Dipper in the constellation Ursa Minor, near the celestial north pole. Because it always indicates due north from an observer anywhere on the Earth, Polaris is important for navigation. |
| circumpolar constellation | circumpolar constellations are constellations that never set from the viewer's perspective. Considered a very important effect in astronomy, it is different from seasonal constellations. Circumpolar constellations stay in the sky. |
| diurnal circle | Diurnal motion is an astronomical term referring to the apparent daily motion of stars around the Earth, or more precisely around the two celestial poles. |
| rotation | A rotation is a circular movement of an object around a center (or point) of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. |
| revolution | A rotation about an external point, e.g. the Earth about the Sun, is called a revolution or orbital revolution, typically when it is produced by gravity. |
| precession | the movement of the rotational axis of an astronomical body, whereby the axis slowly traces out a cone. |
| summer solstice | The summer solstice occurs exactly when the axial tilt of a planet's semi-axis in a given hemisphere is most inclined towards the star that it orbits. |
| winter solstice | The winter solstice is the solstice that occurs in winter. It is the time at which the sun appears at noon at its lowest altitude above the horizon. |
| equinox | either of the two annual crossings of the equator by the Sun, once in each direction, when the length of day and night are approximately equal everywhere on Earth. The equinoxes occur around March 21 and September 23. |
| angular distance | the angular separation between two objects as perceived by an observer; "he recorded angular distances between the stars" |
| ecliptic | the apparent path of the Sun's annual motion relative to the stars, shown as a circle passing through the center of the imaginary sphere celestial sphere containing all the astronomical objects. Eclipses of the Sun or Moon can occur only when the Moon crosses the ecliptic. |
| second of arc | A unit of angular measure equal to 1/60 of an arc minute, or 1/3600 of a degree. The arc second is denoted ^('') (not to be confused with the symbol for inches). |
| minute of arc | A minute of arc, arcminute, or minute arc (MOA), is a unit of angular measurement equal to one sixtieth (1⁄60) of one degree (circle⁄21,600), or (π⁄10,800) radians. |
| degree of arc | A degree (in full, a degree of arc, arc degree, or arcdegree), usually denoted by ° (the degree symbol), is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians. It is not an SI unit, as the SI unit for angles is radian, but it is mentioned in the SI brochure as an accepted unit. |
| penumbra | a partial outer shadow that is lighter than the darker inner shadow umbra, e.g. the area between complete darkness and complete light in an eclipse |
| umbra | the darkest portion of the shadow cast by an astronomical object during an eclipse, especially that cast on Earth during a solar eclipse |
| lunar eclipse | an eclipse of the Moon caused by Earth passing between the Sun and the Moon and casting its shadow on the Moon |
| solar eclipse | As seen from the Earth, a solar eclipse occurs when the Moon passes between the Sun and Earth, and the Moon fully or partially blocks ("occults") the Sun. |
| moon phases | A lunar phase or phase of the moon is the appearance of the illuminated (sunlit) portion of the Moon as seen by an observer, usually on Earth. |
| synodic period | the time required for a body within the solar system, such as a planet, the Moon, or an artificial Earth satellite, to return to the same or approximately the same position relative to the Sun as seen by an observer on the Earth. |
| sidereal period | the time required for a celestial body within the solar system to complete one revolution with respect to the fixed stars—i.e., as observed from some fixed point outside the system. |
