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**The celestial equator** is the great circle that is the projection of the Earth’s equator onto the celestial sphere. Its plane is perpendicular to the axis of rotation of the Earth.

**The celestial poles** are the poles of the celestial equator, or the intersections of the axis of rotation of the Earth with the celestial sphere.

**The ecliptic** is defined to be the plane of the (undisturbed) orbit of the Earth around the Sun.

**The equinox** or, better, the vernal equinox, which is the zero point of both right ascension and celestial longitude, is defined to be in the direction of the ascending node of the ecliptic on the equator. It is that intersection of equator and ecliptic where the ecliptic runs (eastwards) from negative to positive declinations. The other intersection, which is diametrically opposite, is the autumnal equinox.

**The equinoxes** are the instants when the Sun’s apparent longitude is 0° or 180°.

**Solstices:** both the points on the ecliptic 90 degrees away from the equinoxes, and the instants when the apparent longitude of the Sun is 90° or 270°.

**Celestial longitude**, or ectiptical longitude, often called simply longitude, is measured (from 0° to 360°) from the vernal equinox, positive to the east, along the ecliptic.

**Celestial latitude**, or ecliptical latitude, or simply latitude, is measured (from 0° to +90° or to -90°) from the ecliptic, positive to the north, negative to the south.

**Right ascension** is measured (from 0 to 24 hours, sometimes from 0° to 360°) from the vernal equinox, positive to the east, along the celestial equator.

**Declination** is measured (from 0° to ±90°) from the equator, positive to the north, negative to the south.

Owing to the effects of **precession** and **nutation**, the ecliptic and equator, and hence the equinoxes and the poles, are continuously in motion, and so the current celestial coordinates of a “fixed” direction change continuously. The motion of the equator is primarily due to the action of the Sun and the Moon, while the (much slower) motion of the ecliptic is primarily due to the perturbing action of the planets.

**Mean equator:** the instantaneous celestial equator exclusive of the periodic perturbations of the nutation.

**Mean equator and equinox**, or simply mean equinox: an expression used to denote that the reference system takes into account the precession (secular effects) but not the nutation (periodic effects).

**Coordinates:** two (or three) numbers which define the position of a point on a surface (or in space). Examples: longitude and latitude are the two geographical coordinates of a point on the surface of the Earth; right ascension and declination; the rectangular coordinates X, Y, Z of a point in three-dimensional space.

**Heliocentric:** referred to the center of the Sun, for instance a heliocentric orbit, heliocentric coordinates.

**Geocentric:** referred to the center of the Earth, for instance a geocentric observer, geocentric coordinates.

**Topocentric:** referred to the observer on the Earth’s surface, for example the topocentric right ascension and declination of the Moon.

**Aberration** is the apparent displacement of the position of an object due to the finite speed of light. The annual aberration of a star is due to the orbital motion of the Earth around the Sun (or, more exactly, around the barycenter of the solar system).

**Azimuth:** the angular distance measured from the South, positive to the West, along the horizon, to the vertical circle through the point in question. Navigators and meteorologists measure the azimuth from the North, positive to the East.

**Ascending node:** that intersection of the orbital plane with the reference plane where the latitudinal coordinate is increasing (going north). The other intersection is the **descending node**.

**Conjunction:** that configuration of two celestial objects such that either their right ascensions or their celestial longitudes are equal.

**Opposition:** that configuration of two celestial objects such that their celestial longitudes differ by 180°. Most frequently used when one of the objects is the Sun.

**Heliographic coordinate system:** a coordinate system on the surface of the Sun.

**Planetographic coordinate system:** a coordinate system on the surface of a planet. In the case of Mars, the term **areographic** is generally used. For the Moon, the term is **selenographic**. Compare with **geographic** for the Earth.

**Epoch:** a particular fixed instant used as a reference point on a time scale, such as В 1950.0 or J2000.0.

**A Julian century** is a time interval of 36525 days.

**An ephemeris day** is equal to 86400 seconds in the uniform time scale known as Dynamical Time.

**The sidereal time** is the measure of time defined by the motion of the vernal equinox in hour angle; it is the hour angle of that equinox (at a given place and for a given instant). The **true solar time** is the local hour angle of the Sun. The** mean solar time** is the hour angle of the mean Sun, and thus is measured from mean noon. The** civil time** is the mean solar time increased by 12 hours, and thus is measured from mean midnight. — The expression “mean time measured from midnight” is a contradictio in terminis, since the mean (solar) time by definition is measured from noon. Many people erroneously use the expression “Greenwich Mean Time”, when in fact Greenwich Civil Time is meant.

**Universal Time** is the civil time on the meridian of Greenwich.

**The astronomical unit (AU)** is a unit of length used to measure distances in the solar system. It is often called the “mean distance of the Earth to the Sun”. But, rigorously, one AU is the radius of the circular orbit which a particle of negligible mass, and free of perturbations, would describe around the Sun with a period of 2тс/к days, where к is the Gaussian gravitational constant, 0.01720209895. As a consequence, the semimajor axis of the elliptical orbit of the Earth is not exactly 1 AU, but 1.000001018 AU.

**Radius vector:** the straight line connecting a body to the central body around which it revolves, or the distance between these bodies at a given instant. The radius vector of a planet or comet is generally expressed in astronomical units.

**Apsides (plural of apse):** the points of intersection of the major axis with the orbit of a planet, a minor planet, a satellite, or a comet. These are the points of the orbit that are closest (perihelion, perigee, etc.) and farthest (aphelion, apogee, etc.) from the central body.

**Perihelion:** the point of the orbit (of a planet, minor planet, or comet) which is nearest to the Sun. For the corresponding point of the Moon’s orbit with respect to the Earth, the term is perigee. For a satellite of Jupiter with respect to this planet, the traditional term is peryove. For a double star, one says periastron.

**The geometric position of a planet** is the “true” position of that body at the given instant; that is, no allowance is being made for the effects of aberration and light-time.

**Anomalies** The **mean anomaly (M)** of a planet is the angular distance, as seen from the Sun, between the perihelion and the mean position of the planet. The angular distance measured from the perihelion to the true position of the planet is called the **true anomaly (v)**. The **eccentric anomaly** is an auxiliary quantity needed to obtain the true anomaly through solving Kepler’s equation. The equation of the center is the difference between the true and the mean anomalies (C = v - M); it is the difference between the actual position of the body in its elliptic orbit and the position the body would have if its angular motion were uniform.

**Parallax:** the difference in apparent direction of an object as seen from two different locations. For objects in the solar system (Sun, Moon, planet, asteroid, comet), the parallax is the difference in direction between a topocentric observation (by the actual observer at the Earth’s surface) and a hypothetical geocentric observation. For the stars, the (annual) parallax is the difference between geocentric and heliocentric positions.

**Arcminute (') and arcsecond (")** are 1/60 and 1/3600, respectively, of a degree. Not to be confused with minute and second of time (1/60 and 1/3600 of an hour).