The Celestial Sphere - Definitions

The Celestial Sphere is the conventional representation of the sky as a spherical shell upon which the celestial bodies are projected. It has all the properties of a sphere of infinite radius with the earth as center. Parallel lines regardless of distance apart are directed toward the same point on the sphere. Each star has its apparent place on the sphere where it appears to be; this specifies only the star's direction (because the stars aren’t actually on a celestial sphere).

The apparent daily rotation or diurnal motion of the heavens is an effect of the earth's rotation on its axis from west to east, within the celestial sphere. Each star describes its diurnal circle around the sky daily. All diurnal circles of stars are parallel to each other and to the celestial equator, and describe the same period of time. The motion of stars rising in the east is fastest - stellar motions become progressively slower as the rising is farther from the east point.

A Great Circle is any circle drawn on a sphere that has the same diameter as the sphere.

The North and South Celestial Poles (NCP & SCP) are projections of the Earth’s north and south poles out to the celestial sphere. They are the two points on the celestial sphere having no diurnal motion.

The Celestial Equator is a projection of the Earth’s equator out onto the celestial sphere. It is the great circle of the celestial sphere halfway between the NCP and SCP. It is the largest diurnal circle.

The Zenith is the point on the celestial sphere that is vertically overhead.

The Nadir is the point opposite the zenith - directly underfoot.

The Ecliptic is the great circle on the celestial sphere traced out annually by the sun, as the Earth orbits it during its 365.24 day period.

Vernal Equinox (First Point in Aries) is the point on the ecliptic where the sun crosses the celestial equator at the beginning of spring, i.e. going from south to north.

Autumnal Equinox is the point on the ecliptic where the sun crosses the celestial equator at the beginning of autumn, i.e. going from north to south.

Hour Circles or Meridians are great circles which pass through the celestial poles and intersect perpendicularly with the celestial equator. They are analogous to meridians on Earth.

The Celestial Meridian or Central Meridian is the meridian that passes through your zenith.

Right Ascension (RA or a) is the celestial coordinate analogous to longitude on the Earth. The RA of an object is its angular distance from the vernal equinox, measured eastward along the celestial equator to the hour circle of the object. It is measured in hr, min, and sec. One hour in RA corresponds to the movement of the stars that occurs in one hour of time because of the Earth’s rotation.

The Declination (d) is the celestial coordinate analogous to latitude on Earth. The declination of an object is its angular distance north (+) or south (-) of the celestial equator, measured along its hour circle. It is measured in degrees and arc minutes.

An object’s location in the sky can be given by its RA and declination coordinates. These coordinates are the same from all observation points on the Earth (equatorial based). Alternatively, an object’s position in the sky can be described by coordinates that are relative to the observer’s location (horizon based).

The Celestial Horizon is the great circle on the celestial sphere halfway between zenith and nadir. (Distinct from the visual horizon where earth and sky seem to meet.)

Vertical Circles are great circles that pass through the zenith and nadir intersecting the horizon at right angles. (Compare to Hour Circles.)

The Altitude of an object is its angular distance above the horizon, measured along a vertical circle. If an object is at zenith its altitude is 90º, at the horizon its altitude is 0º.

The Azimuth of an object is the angular distance measured eastward from the vertical circle passing through north, along the horizon, to the vertical circle of the object. The azimuth of north is 0º, east is 90º, south is 180º, and west it 270º.

In order to find a star in the sky, you need to know not only its celestial coordinates, i.e. RA and declination, but also what time it is, because of the diurnal motion.

The Local Sidereal Time (LST) is defined as the RA of the observer’s zenith. (In other words, the angular distance of the central meridian from the vernal equinox, measured eastward along the celestial equator.)

The Local Hour Angle (HA) of an object is its angular distance from the central meridian (zenith), measured westward along the celestial equator in hr, min, and sec. Therefore, local sidereal time is also the HA of the vernal equinox.

Useful equation (you need to know!): LST = RA + HA. More useful: HA = LST - RA

Equatorial Coordinates

Horizon Coordinates