Definitions of Astronomical Positions

Definitions of Astronomical Positions

A conjunction is an apparent meeting or passing of two or more celestial bodies. For example, the Moon is in conjunction with the Sun at the phase of new Moon, when it moves between the Earth and Sun and the side turned toward the Earth is dark. Inferior planets—those with orbits smaller than the Earth’s (namely, Venus and Mercury)—have two kinds of conjunctions with the Sun. An inferior conjunction occurs when the planet passes approximately between Earth and Sun; if it passes exactly between them, moving across the Sun’s face as seen from Earth, it is said to be in transit (see below). A superior conjunction occurs when Earth and the other planet are on opposite sides of the Sun, but all three bodies are again nearly in a straight line. Superior planets, those having orbits larger than the earth’s can have only superior conjunctions with the Sun.

When celestial bodies appear in opposite directions in the sky they are said to be in opposition. The Moon, when full, is said to be in opposition to the Sun (the Earth is then approximately between them). A superior planet (one with an orbit farther from the Sun than Earth’s) is in opposition when Earth passes between it and the Sun. The opposition of a planet is a good time to observe it, because the planet is then at its nearest point to the Earth and in its full phase. The inferior planets, Venus and Mercury, can never be in opposition to the Sun.

When a celestial body as seen from the Earth makes a right angle with the direction of the Sun it is said to be in quadrature. The Moon at first or last quarter is said to be at east or west quadrature, respectively. A superior planet is at west quadrature when its position is 90° west of the Sun.

The east–west coordinate by which the position of a celestial body is ordinarily measured is known as the right ascension. Right ascension in combination with declination defines the position of a celestial object. Declination is the angular distance of a body north or south of the celestial equator. North declination is considered positive and south, negative. Thus, +90° declination marks the north celestial pole, 0° the celestial equator, and −90° the south celestial pole. The symbol for right ascension is the Greek letter α (alpha) and for declination
the lowercase Greek letter Δ (delta).

The angular distance in celestial longitude separating the Moon or a planet from the Sun is known as elongation. The greatest elongation possible for the two inferior planets is about 48° in the case of Venus and about 28° in that of Mercury. Elongation may also refer to the angular distance of any celestial body from another around which it revolves or from a particular point in the sky; e.g., the extreme east or west position of a star with reference to the north celestial pole.

The point at which a planet is closest to the Sun is called the perihelion, and the most distant point in that planet’s orbit is the aphelion. The term helion refers specifically to the Sun as the primary body about which the planet is orbiting.

Occultation refers to the obscuring of the light of an astronomical body, most commonly a star, by another astronomical body, such as a planet or a satellite. Hence, a solar eclipse is the occultation of the Sun by the Moon. From occultations of stars by planets, asteroids, and satellites, astronomers are able to determine the precise sizes and shapes of the latter bodies in addition to the temperatures of planetary atmospheres. For example, astronomers unexpectedly discovered the rings of Uranus during a stellar occultation on 10 Mar 1977.

A complete or partial obscuring of a celestial body by another is an eclipse; these occur when three celestial objects become aligned. The Sun is eclipsed when the Moon comes between it and the Earth; the Moon is eclipsed when it moves into the shadow of the Earth cast by the Sun. Eclipses of natural or artificial satellites of a planet occur as the satellites move into the planet’s shadow. When the apparent size of the eclipsed body is much smaller than that of the eclipsing body, the phenomenon is known as an occultation (see above). Examples are the disappearance of a star, nebula, or planet behind the Moon, or the vanishing of a natural satellite or space probe behind some body of the solar system. A transit (see above) occurs when, as viewed from the Earth, a relatively small body passes across the disk of a larger body, usually the Sun or a planet, eclipsing only a very small area: Mercury and Venus periodically transit the Sun, and a satellite may transit its planet. When an object orbiting the Earth is at the point in its orbit that is the greatest distance from the center of the Earth, this point is known as apogee; the term is also used to describe the point farthest from a planet or a satellite (as the Moon) reached by an object orbiting it. Perigee is the opposite of apogee.

The difference in direction of a celestial object as seen by an observer from two widely separated points is termed parallax. The measurement of parallax is used directly to find the distance of the body from the Earth (geocentric parallax) and from the Sun (heliocentric parallax). The two positions of the observer and the position of the object form a triangle; if the base line between the two observing points is known and the direction of the object as seen from each has been measured, the apex angle (the parallax) and the distance of the object from the observer can be determined.


An hour angle is the angle between an observer’s meridian (a great circle passing over his head and through the celestial poles) and the hour circle (any other great circle passing through the poles) on which some celestial body lies. This angle, when expressed in hours and minutes, is the time elapsed since the celestial body’s last transit of the observer’s meridian. The hour angle can also be expressed in degrees, 15° of arc being equal to one hour.