Reference
Glossary
Plain-language definitions of the astronomy terms used across the site. 62 entries. Filter by topic or search by name.
A
9 entries- Absolute magnitudeHow stars work
- How bright an object would appear at a standard distance of 10 parsecs.
- AlbedoThe neighborhood
- The fraction of incident sunlight a body reflects back into space.
- ApertureOptics & gear
- The clear diameter of a telescope's main optical element.
- AphelionThe neighborhood
- The point in an elliptical orbit farthest from the Sun.
- Apparent magnitudeNaked-eye astronomy
- How bright an object appears from Earth.
- AsterismNaked-eye astronomy
- A recognizable star pattern that is not one of the 88 official constellations.
- Astronomical unit (AU)The neighborhood
- The average Earth–Sun distance: 149,597,870.7 kilometers.
- Averted visionNaked-eye astronomy
- Looking slightly to the side of a faint object so its light falls on more sensitive rod cells.
- AzimuthNaked-eye astronomy
- The compass direction of an object, measured clockwise from north along the horizon.
B
2 entries- Black holeHow stars work
- A region where gravity is strong enough that not even light escapes.
- Bortle scale25° North
- A 1–9 scale of night-sky darkness.
C
6 entries- Celestial equatorNaked-eye astronomy
- The projection of Earth's equator onto the sky.
- Celestial sphereNaked-eye astronomy
- An imaginary sphere surrounding Earth on which stars appear fixed.
- Cepheid variableHow stars work
- A pulsating giant star whose period reveals its true luminosity.
- CollimationOptics & gear
- The precise alignment of a telescope's optical elements.
- ConjunctionThe neighborhood
- When two objects appear close together in the sky.
- ConstellationNaked-eye astronomy
- One of 88 regions the sky is divided into by the IAU.
D
5 entries- Dark-sky site25° North
- A location with minimal artificial light — Bortle 1–3.
- DeclinationNaked-eye astronomy
- The celestial equivalent of latitude, measured in degrees north or south of the celestial equator.
- Deep-sky objectBeyond the Sun
- Any celestial object outside the solar system that is not a single star — nebulae, clusters, and galaxies.
- DobsonianOptics & gear
- A large-aperture Newtonian reflector on a simple azimuth-elevation mount.
- Double starHow stars work
- Two stars that appear close together — either physically bound (binary) or an optical alignment.
E
5 entries- EclipticNaked-eye astronomy
- The apparent yearly path of the Sun through the sky.
- ElongationThe neighborhood
- The angular separation between a planet and the Sun.
- Equatorial mountOptics & gear
- A telescope mount aligned to Earth's rotation axis.
- ExoplanetHow stars work
- A planet orbiting a star other than the Sun.
- EyepieceOptics & gear
- The small lens assembly you look through.
F
2 entries- Focal lengthOptics & gear
- The distance from a telescope's objective to the focused image.
- Focal ratioOptics & gear
- Focal length divided by aperture — a telescope's 'speed'.
G
3 entries- GalaxyBeyond the Sun
- A gravitationally bound system of stars, gas, dust, and dark matter — from ten million to a trillion stars.
- GibbousThe neighborhood
- A Moon or planet phase between quarter and full — more than half but less than fully illuminated.
- Globular clusterBeyond the Sun
- A dense, spherical swarm of hundreds of thousands of ancient stars.
H
1 entry- HR diagramHow stars work
- The Hertzsprung–Russell diagram: luminosity plotted against temperature.
L
2 entries- Light pollution25° North
- Excess artificial sky brightness that washes out stars and deep-sky objects.
- Light-yearHow stars work
- The distance light travels in one year — about 9.46 trillion kilometers.
M
5 entries- MagnificationOptics & gear
- How many times larger the image appears versus the unaided eye.
- MagnitudeNaked-eye astronomy
- The logarithmic brightness scale used in astronomy — lower numbers mean brighter.
- Main sequenceHow stars work
- The diagonal band on the HR diagram where stars spend most of their lives fusing hydrogen.
- Messier objectBeyond the Sun
- One of 110 non-stellar objects catalogued by Charles Messier in the late 18th century.
- MeteorThe neighborhood
- The streak of light from a small piece of interplanetary debris burning up in Earth's atmosphere.
N
2 entries- NebulaBeyond the Sun
- A cloud of gas and dust in interstellar space.
- Neutron starHow stars work
- The compact remnant of a massive star's core collapse.
O
3 entries- OccultationThe neighborhood
- When one celestial body passes in front of another, briefly hiding it.
- Open clusterBeyond the Sun
- A loose group of a few hundred to a few thousand stars born together.
- OppositionThe neighborhood
- When a planet is opposite the Sun in Earth's sky — up all night.
P
4 entries- ParallaxHow stars work
- The apparent shift of a nearby star against the background as Earth orbits the Sun.
- ParsecHow stars work
- 3.26 light-years, or the distance at which 1 AU subtends 1 arcsecond.
- PerihelionThe neighborhood
- The point in an elliptical orbit closest to the Sun.
- PlanisphereNaked-eye astronomy
- A rotating star chart that shows which constellations are above the horizon at a given date and time.
R
4 entries- Red dwarfHow stars work
- A cool, small, faint main-sequence star of spectral class M.
- RedshiftBeyond the Sun
- The stretching of light to longer wavelengths as an object moves away.
- Retrograde motionThe neighborhood
- The apparent westward motion of a planet against the background stars, opposite to its usual eastward drift.
- Right ascensionNaked-eye astronomy
- The celestial equivalent of longitude, measured in hours, minutes, and seconds.
S
3 entries- SeeingOptics & gear
- The steadiness of the atmosphere.
- Sidereal dayNaked-eye astronomy
- One rotation of Earth relative to the stars: 23 h 56 m 4 s.
- SupernovaHow stars work
- The explosive death of a massive star or a runaway white dwarf.
T
3 entries- TerminatorThe neighborhood
- The line dividing the illuminated and dark sides of a moon or planet.
- TransitThe neighborhood
- The passage of a smaller body in front of a larger one — a Mercury or Venus transit across the Sun, or a planet across its host star.
- TransparencyOptics & gear
- How clear the sky is — how much light is absorbed by haze or humidity.
W
1 entry- White dwarfHow stars work
- The exposed carbon-oxygen core left after a Sun-like star sheds its outer layers.
Z
2 entries- ZenithNaked-eye astronomy
- The point directly overhead.
- Zodiac (astronomical)The neighborhood
- The band of sky within about 8° of the ecliptic, where the Sun, Moon, and planets are always found.