SkyEye

When I heard the learn'd astronomer,
When the proofs, the figures, were ranged in columns before me,
When I was shown the charts and diagrams, to add, divide, and measure them,
When I sitting heard the astronomer where he lectured with much applause in the lecture-room,
How soon unaccountable I became tired and sick,
Till rising and gliding out I wander'd off by myself,
In the mystical moist night-air, and from time to time,
Look'd up in perfect silence at the stars.
— Walt Whitman, "When I Heard the Learn'd Astronomer", 1867

Celestial coordinate systems are used to specify positions of objects (moons, planets, stars, galaxies, etc.) in space. The position may be given in three dimensions or simply as a direction plotted on the celestial sphere. In the case of a spherical coordinate system, both a fundamental plane (a plane of reference which divides the sphere into two hemispheres) and a reference direction must be defined. For instance, on Earth, the fundamental plane for the geographic coordinate system of latitude and longitude is taken to be the equator and the reference direction is defined as the Greenwich meridian.

Altitude and Azimuth

The horizontal or altitude-azimuth (alt-az) coordinate system is centred on the position of the observer on Earth, and his/her local horizon.

Altitude is the angular height of an object above the horizon. It ranges from 0° for objects on the horizon to 90° for objects that are directly overhead, at the point known as the zenith. Altitude may also be called elevation, although this can cause confusion because elevation is commonly used to denote the observer's height above mean sea level. Astronomers sometimes replace altitude with zenith distance, which is 0° at the zenith and 90° on the horizon.

It is possible for the zenith distance of the horizon to exceed 90°. This happens if the observer has a sea horizon and is above sea level, such as on the deck of a large ship or on a hilltop on an island. Mariners making sextant sightings of stars or planets from the deck of a large ship must adjust the measured altitude to allow for this effect before using the sightings to calculate their position at sea. The deck of a modern aircraft carrier, for example, is 17 metres (56 feet) above sea level, which adds 7 arc-minutes to the apparent altitude of a star above the sea horizon. This may seem like a small effect, but it corresponds to an offset of 7 nautical miles in the ship's position as determined from that sighting, and therefore cannot be ignored.

The apparent altitudes of celestial objects are also affected by refraction. The Earth's atmosphere acts as a giant lens, bending the incoming light of all celestial objects so that they appear very slightly higher in the sky than they would if the Earth did not have an atmosphere. The effect is zero at the zenith, where the incoming light is hitting the atmosphere at a right angle. It is greatest for objects near the horizon, reaching a maximum of 34 arc-minutes on the horizon itself. A star which appears to be on the horizon has a true zenith distance of 90°34'. This effect must also be taken into account by mariners.

Azimuth is the angle measured around the horizon in a clockwise direction starting from due North. It ranges from 0° for an object that is due North, through 90° (due East), 180° (due South), 270° (due West), to 360° (due North again).

For observers in the Earth's northern hemisphere, the arc on the sky which runs from azimuth 180° on the horizon, through the zenith, to the north celestial pole, is known as the meridian. In the southern hemisphere, the meridian runs from azimuth 0° on the horizon, through the zenith, to the south celestial pole. In both cases, an object attains its greatest altitude above the horizon when it crosses the meridian, an event known as transit.

For an observer at the Earth's North or South Pole, azimuth is undefined, since all directions are due South at the North Pole, and all directions are due North at the South Pole.

Right Ascension and Declination

The positions of celestial objects on the sky are normally specified in terms of right ascension and declination. These are analogous to longitude and latitude on the surface of the Earth, and both coordinate systems use the Earth's equator plane as the fundamental plane.

Declination measures the angular distance of a star or planet north or south of the celestial equator, in the same way that latitude measures the angular distance of a point on the Earth's surface north or south of the Earth's equator. There is a direct correspondence: a star whose declination is 50° north will pass directly overhead as seen by an observer at latitude 50° north on the Earth.

Right ascension measures the 'longitude' of a star or planet eastwards from a zero point known as the First Point of Aries. Unlike the Greenwich meridian on the Earth, the First Point of Aries is not an arbitrary reference point. It is defined by the intersection of the celestial equator with the ecliptic, which is the orbit plane of the Earth.

Right ascension also differs from terrestrial longitude in being measured in hours, minutes and seconds rather than degrees. This is because the right ascensions of stars and planets were determined by recording the time at which they transited the local meridian. Although this method for measuring the positions of stars and planets is no longer used, it is still convenient to measure right ascension as if it were a time, because it simplifies the task of calculating when an object will rise or set or transit. When measured in units of time, right ascension ranges from 0 hours (equivalent to 0° in angular measure) to 24 hours (360°), so that 1 hour of right ascension is equal to 15° in angular measure.

The Earth's equator plane is not fixed in space. The rotation axis of the Earth sweeps out a cone in space once every 25,800 years, maintaining an approximately constant angle to the Earth's orbit plane. This is called precession, and it is caused by the gravitational pull of the Sun and Moon on the Earth's equatorial bulge. It is the same phenomenon which causes the axis of a spinning top or toy gyroscope to sweep out a cone.

As a result of precession, the right ascension and declination of every star changes gradually, as the entire reference system rotates. For this reason, when citing the coordinates of a star, it is essential to specify the instant in time for which they are valid. This is known as the epoch of the coordinates. Star catalogues are usually compiled for a standard epoch such as the year 1950 or 2000.

The rotation axis of the Earth also undergoes much smaller periodic movements over timescales of a few years. This effect is known as nutation. Like precession, it is caused by the gravitational pull of the Sun and Moon on the Earth's equatorial bulge. The largest component has an amplitude of 17 arc-seconds and a period of 18.6 years. Star positions listed in catalogues do not include the effect of nutation.

Ecliptic Longitude and Latitude

The ecliptic plane is the plane in space defined by Earth's orbit about the Sun. The accompanying diagram illustrates how geocentric ecliptic coordinates are defined, with the Earth at the centre. It is also possible to have a heliocentric ecliptic coordinate system which has the Sun at the centre and a barycentric ecliptic coordinate system which has the barycentre of the solar system at its heart.

Ecliptic coordinates are most commonly used when specifying the positions of the Sun, Moon and planets in their orbits. Phenomena such as oppositions, conjunctions and the principal phases of the Moon are defined in terms of differences in ecliptic longitude.

The ecliptic is not a fixed plane. The Earth's orbit plane moves slowly in space, due to the gravitational effects of the other planets. Moreover, precession changes the location of the equinox, which defines the zero point from which ecliptic longitude is measured. The ecliptic longitude of a fixed point in space increases by approximately 50 arc-seconds per year, making one complete revolution in each 25,800-year cycle of precession.

Other Coordinate Systems

The galactic coordinate system is centred on the Sun, with the fundamental plane approximately coincident with the galactic plane. Galactic longitude (l) is measured east along the galactic equator from the galactic centre and is given in degrees °. Galactic latitude (b) is measured north (positive) or south (negative) of the galactic equator. It is also given in degrees.

Supergalactic coordinates are used when studying the distribution of objects in the universe