Basic Terminology
You'll often hear the following
words used on our radio program or read them on this site. For more advanced terms and definitions, consult our AstroGlossary.
- Apogee
- The farthest point from Earth in the orbit of the Moon or an artificial satellite.
- Perigee
- The nearest point from Earth in the orbit of the Moon or an artificial satellite.
- Ecliptic
- The path the Sun follows across the sky. The Moon and planets also generally follow the ecliptic.
- Zenith
- The point in the sky directly above an observer.
- Conjunction
- Occurs when a planet or any solar system object forms a straight line with
Earth and the Sun. In the case of Mercury or Venus, if the planet lies between
earth and the Sun, it is in "inferior" conjunction. If the planet is on the
opposite side of the Sun, it is in "superior" conjunction. A close passage
between two planets in Earth's sky is also called a conjunction.
- Opposition
- Occurs when a planet or any solar system object appears opposite the Sun in Earth's sky. Planets rise at sunset when they are at opposition and remain visible for the entire night. Planets are also at their brightest at opposition.
- Equinox
- The moment the Sun crosses the celestial equator (the projection of earth's equator into space). The March equinox marks the beginning of spring in the northern hemisphere, while the September equinox marks the beginning of autumn.
- Solstice
- The moment the Sun stands farthest north (in June) or south (in December) in the sky. The June (summer) solstice is the longest day of the year in the northern hemisphere, while the December (winter) solstice is the shortest day.
- Occultation
- Occurs when a celestial body seems to disappear because another body passes between it and the observer. A solar eclipse is an occultation of the Sun by the Moon.
- Eclipse
- Occurs when one celestial body obscures another, either by passing directly between the observer and the second body, as in a solar eclipse (when the Moon passes between Earth and Sun, obscuring the Sun) or by casting a shadow on the second body, as in the case of a lunar eclipse (when Earth passes between Sun and Moon, shrouding the Moon in Earth's shadow).
- Asterism
- A well-known star pattern that is usually part of, but not a complete, constellation. The Big Dipper in the constellation Ursa Major is an example.
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