Animated Virtual Planetarium Glossary

ALTITUDE: Part of a local coordinate system (with "azimuth") for astronomical objects. Altitude is the distance, measured in degrees, above the horizon. It is a function of the observer's location.

AZIMUTH: Part of a local coordinate system (with "altitude") for astronomical objects. Azimuth is the distance, measured in degrees, from due South. West has an azimuth of 90 degrees, North is 180 degrees, and East is at -90 (or 270) degrees. Azimuth is a function of the observer's location.

CELESTIAL EQUATOR: The celestial equator (CE) is the intersection of the plane containing the Earth's equator with the "celestial sphere." Or to put it another way, it is a line through the sky directly above the Earth's equator. If you were standing on the Earth's equator, the CE would run from due East to due West, passing directly overhead. "Seen" from northern mid-latitudes, it still runs from East to West, but at its highest (which will be due South) it only reaches a point about halfway between the horizon and directly overhead. To an observer at the North or South Pole, the CE runs along the horizon. (See Figure 1)

CELESTIAL SPHERE: From ancient times, the supposed spherical shell surrounding the Earth and containing all the stars, planets, the Sun, and the Moon.

DECLINATION: Part of a coordinate system (with "right ascention") for astronomical objects. Declination is the distance, in degrees, away from the celestial equator. Positive values indicate the object is north of the celestial equator, negative values indicate the object is south of the celestial equator. Declination is independent of the observer's location. (See Figure 1)

ECLIPSE: An eclipse occurs when the Earth, Moon, and Sun are lined up such that either the Earth or the Moon casts its shadow on the other body. There are several types of eclipse:
  LUNAR ECLIPSE: The Earth passes between the Moon and Sun, and its shadow darkens the Moon.
  TOTAL SOLAR ECLIPSE: The Moon passes between the Earth and Sun, blocking out the Sun.
  LUNAR ECLIPSE: The Earth passes between the Moon and Sun, and its shadow darkens the Moon.

ECLIPTIC: The plane containing the Earth's orbit about the Sun. Viewed from the Earth, the ecliptic is the line through the sky that the Sun traverses over the course of a year. Since most planets orbit the Sun in nearly the same plane as the Earth, they follow the same path through the sky as the Sun, along the ecliptic.When several planets are visible, you can imagine the line connecting them. (See Figure 1)

EQUINOX: Literally, equinox means "equal night," or the time when there is twelve hours of day and twelve hours of night. The Vernal and Autumnal Equinoxes mark the beginning of Spring and Autumn, respectively. A check of sunrise and sunset times, however, will reveal that on the first days of Spring and Autumn, there is actually slightly more than twelve hours of day. How then is the equinox actually reckoned? Since the celestial equator is inclined with respect to the ecliptic, these two lines (as seen from Earth) will intersect at two points. These two intersections are the Vernal Equinox and the Autumnal Equinox. The former is a point in the sky between the constellations Aquarius and Pisces, and the latter is between Leo and Virgo, 180 degrees away. As the Sun traverses along the ecliptic, it will pass through each equinox. The moment it does so is the time the season officially changes. (See also SOLSTICE). (See Figure 1)

RIGHT ASCENTION: Part of a coordiante system (with "declination") for astronomical objects. Right ascention (abbreviated R.A.) is the distance from the Vernal Equinox (where the ecliptic and celestial equator intersect), measured along the celestial equator. The units for R.A. are hours, minutes, and seconds. One hour corresponds to the distance the Sun travels through the sky in one hour of time, or 15 degrees. As with time, there are 60 minutes to an hour, 60 seconds to a minute. The R.A. for the Sun on the first day of Spring (Vernal Equinox) is 0h, 0m, 0s. At the Autumnal Equinox the R.A. of the Sun is 12h 0m 0s. At the solstices, the Sun's R.A. is 6h 0m 0s (summer) and 18h 0m 0s (winter). Right Ascention is independent of the observer's location. (See Figure 1)

SOLSTICE: The times when the Earth's spin axis points either towards or away from the Sun. Alternatively, the solstice is when the Sun reaches its highest point in the sky, as seen from the pole. The summer solstice is when the Sun reaches its highest point as seen from the North pole, and marks the longest day of the year in the Northern Hemisphere. The winter solstice is when the Sun reaches its highest point as seen from the South pole, and marks the longest day of the year in the Southern Hemisphere. (See Figure 1)

TRANSIT: The time when an astronomical object reaches its hightest altitude. This is also when the object crosses a line going from due North to due South. As with rise and set times, transit time is a function of the observer's location.

ZODIAC: The twelve constellations through which the eliptic passes. They are: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. (See Figure 1)

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October, 2012