Astronomy Glossary
The language of the cosmos can be confusing. Use our comprehensive guide to master the terms used by professional and amateur astronomers.
Aperture
The diameter of the main lens or mirror of a telescope. This is the most important specification of any telescope as it determines how much light it can gather.
Apparent Magnitude
A measure of the brightness of a celestial object as seen from Earth. The smaller or more negative the number, the brighter the object.
Asterism
A recognizable pattern of stars that is not one of the official 88 constellations. The Big Dipper is an asterism within the constellation Ursa Major.
Astronomical Twilight
The period when the sun is between 12 and 18 degrees below the horizon. The sky is dark enough for all astronomical observations.
Azimuth
The angular distance of an object measured clockwise from the North point of the horizon.
Bortle Scale
A nine-level numeric scale that measures the night sky's brightness of a particular location. Class 1 is an extremely dark site, while Class 9 is inner-city sky.
Celestial Equator
The projection of Earth's equator onto the celestial sphere.
Celestial Sphere
An imaginary sphere of infinitely large radius, with Earth at its center, used for describing the positions of stars.
Collimation
The process of aligning the mirrors or lenses of a telescope to ensure it produces sharp images.
Conjunction
When two celestial objects (like two planets or a planet and the moon) appear very close together in the sky.
Constellation
One of the 88 officially recognized areas of the celestial sphere, often containing a traditional star pattern.
Declination (Dec)
The angular distance of an object north or south of the celestial equator. Equivalent to latitude on Earth.
Deep Sky Object (DSO)
Celestial objects that are not individual stars or solar system bodies, such as nebulae, star clusters, and galaxies.
Dew Point
The temperature at which air becomes saturated with water vapor and dew begins to form on surfaces like telescope optics.
Double Star
Two stars that appear very close to each other in the sky. Some are physical pairs (binary stars), others are just line-of-sight alignments.
Ecliptic
The apparent path of the sun across the celestial sphere over the course of a year.
Exit Pupil
The diameter of the beam of light that leaves the eyepiece. It is calculated by dividing the telescope's aperture by its magnification.
Eye Relief
The distance from the last surface of an eyepiece to the point where the eye can see the full field of view.
F-ratio
The focal length of a telescope divided by its aperture. Lower numbers (f/4, f/5) are "fast" and better for deep sky; higher (f/10+) are "slow" and better for planets.
Field of View (FOV)
The area of the sky visible through a telescope or binoculars, usually expressed in degrees.
Focal Length
The distance from the main lens or mirror to the point where the light comes to a focus.
Galaxy
A massive system of stars, stellar remnants, gas, dust, and dark matter bound together by gravity.
GoTo
A computerized mount for a telescope that can automatically locate and track celestial objects.
Light Pollution
Artificial light that brightens the night sky, making it difficult to see faint stars and deep sky objects.
Light-Year
The distance that light travels in a vacuum in one year, approximately 9.46 trillion kilometers.
Messier Catalog
A set of 110 deep sky objects cataloged by French astronomer Charles Messier in the 18th century. Essential targets for amateur astronomers.
Milky Way
The galaxy that contains our Solar System. From Earth, it appears as a hazy band of light stretching across the sky.
Mount
The structure that supports a telescope. The two main types are Alt-Azimuth (moves up-down, left-right) and Equatorial (moves in arcs to track stars).
Nebula
An enormous cloud of dust and gas in interstellar space, often the birthplace of stars.
Opposition
When a planet is on the opposite side of Earth from the sun. This is usually the best time to observe a planet.
Parsec
A unit of distance used in astronomy, equal to about 3.26 light-years.
Planetary Nebula
A type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.
Planisphere
A star chart with a rotating disc that can be adjusted to show the visible stars for any date and time.
Polar Alignment
The process of aligning a telescope mount's axis with the Earth's axis of rotation so it can track stars accurately.
Right Ascension (RA)
The angular distance of an object measured eastward from the vernal equinox. Equivalent to longitude on Earth.
Seeing
A measure of the atmospheric stability. "Good seeing" means the air is steady and images are sharp; "Bad seeing" means the air is turbulent and images boil.
Star Cluster
A group of stars held together by mutual gravitational attraction. Types include open clusters (young, loose) and globular clusters (old, dense).
Terminator
The line between the light and dark sides of the moon or a planet. This is the best area to observe surface detail on the moon.
Transparency
A measure of how clear the atmosphere is, often affected by humidity, dust, or smoke.
Zenith
The point in the sky directly above an observer.