Astronomical Terms


Horizon and Geographic terms

Angles – Degrees (°), Arcminutes ('), Arcseconds (")

The standard method of measuring apparent distances. There are 360o in a complete circle. Degrees can be approximately measured by knowing that the fist subtends an angle of 10o when the arm is outstretched. The pinky fingernail subtends about 1o. Each degree is subdivided into 60 equal parts, called arcminutes. (There are 60 arcminutes in one degree.) Similarly, each arcminute is subdivided into 60 arcseconds. An arcsecond is the apparent size of a dime at a distance of 1.3 miles. The apparent size of the Moon and Sun are about 30’, or half a degree.

Altitude(also called elevation)

The number of degrees above the horizon an object is. A star on the horizon has an altitude of 0o. A star at zenith has an altitude of 90o.


The direction along the horizon, measured to the right of north. North is 0o azimuth; east is 90o, south is 180o, and west is 270o.


The number of degrees from the Earth’s equator. The latitude of the Earth’s equator is 0o, and the North and South Poles are plus and minus 90o, respectively. The latitude of Los Angeles is 34o.


The number of degrees west of the Prime Meridian, which runs through Greenwich and London, England. LA’s longitude is 118o.

Sky terms

Celestial sphere

The imaginary sphere where all sky objects appear. Due to the Earth’s rotation, the celestial sphere appears to rotate 360o every 23h 56m. Just like the Earth, the celestial sphere has two poles and an equator.


The same as celestial latitude. It measures the distance the celestial equator. The declination that is half-way between the celestial equator and north celestial pole is 45o. Stars south of the celestial equator have negative declinations.

Celestial Equator

The projection of the Earth’s equator onto the celestial sphere. The celestial equator intersects the horizon exactly east and west. The declination of the celestial equator is 0o.

North Celestial Pole (NCP)

The projection of the Earth’s north pole onto the celestial sphere. The NCP is directly above the Earth’s north pole, and does not move in the sky as the Earth rotates. The declination of the NCP is 90 degrees, by definition.

Polaris – The North Star

The medium-bright-star located near the north celestial pole. It will not appear to move (much) throughout the night. It will appear as far above the horizon as your latitude (e.g. 34o above horizon in LA.). The declination of Polaris is +89o.

Right Ascension (RA)

How far east an object is, relative to the sky’s "prime meridian". RA is measured in units hours, minutes, seconds (like a clock, but now these terms refer to angles). One hour equals 15o and 24 hours equals 360o.


The path from west to east through the celestial sphere that the Sun takes through the constellations during the year.


The line that runs from due north, through the zenith, to due south. It divides the sky into east and west. An object rises until it crosses the meridian, and then it starts setting.


The term astronomers use to describe the size of point objects. Seeing is caused by turbulence in the atmosphere. Turbulence can be caused by air motion and temperature variations. Seeing will look like a wavy pattern when looking at an extended (non-point) object. Typical seeing on the El Camino math roof is 1-2 arcseconds. This means that the stars will look like "wavering blobs" 1-2 arcseconds across in the eyepiece. Seeing is what causes the stars to twinkle.


Relative to the stars. Examples: sidereal time, sidereal day, sidereal month. These refer to cycle times as measured relative to the stars instead of the Sun.


The point directly overhead – at an altitude of 90o.


The region around the ecliptic where the Sun (which is always ON the ecliptic), the Moon, and the planets can be found.

Telescope terms

Earth’s rotation rate

The Earth completes one rotation relative to the stars in approximately 23 hours, 56 minutes. This is also called a sidereal day (your watch tells you the position of the Sun, and measures a solar day). This is roughly equivalent to 15 degrees per hour. This causes the stars to appear to move 15 arcseconds every second across the eyepiece unless your telescope is "tracking" them as they move across the sky.

Equatorial mount

A type of mount for a telescope that allows motion in two directions: right ascension and declination. This type of mount allows for easy tracking of the stars as the move across the sky. The telescopes used in ECC’s astronomy laboratory all have equatorial mounts. The other kind of mount is an altitude-azimuth mount, which is cheaper, but does not allow for easy tracking of objects.

Universal Time (UT)

The time zone where the Earth’s prime meridian is located (which runs through Greenwich, England). Pacific Standard Time (PST) is 8 hours earlier than Universal Time, and Pacific Daylight Time (PDT) is 7 hours earlier than it. Sometimes UT is also called "Zulu Time", or just "Zulu".

Field of View

The total angular area visible when looking through a telescope.


The ratio of the angular size of an object when magnified compared to an unmagnified size. For telescopes, magnification can be computed by dividing the telescope’s focal length by the eyepiece’s focal length. Magnification has no meaning without an eyepiece, since there is no apparent size of focused light.


A measure of brightness. Five magnitudes is 100 times. One magnitude is 2.5 times. The system runs backwards, so a 1st magnitude stars is 100 times brighter than a 6th magnitude star (higher numbers mean fainter stars). Really bright objects have negative magnitudes, like the star Sirius (-1.4 mag.) or the full moon (-12.7 mags !).