Astronomy Glossary

Telescopes Equipment

An altazimuth mount (alt-az) moves in altitude (up-down) and azimuth (side-to-side), like a camera tripod.

Units Distance

Angular size is the apparent angle an object spans in the sky as viewed from Earth, regardless of its physical size or distance.

Telescopes Equipment

Aperture is the diameter of a telescope's primary light-collecting element (lens or mirror), the single most important specification determining teles...

Solar System

Aphelion is the point in an object's orbit farthest from the Sun, where orbital velocity is slowest.

Solar System

Apogee is the point in an object's orbit around Earth that is farthest from Earth (analogous to aphelion for Sun orbits).

Astronomy Concepts

Apparent magnitude measures how bright an object appears from Earth, with lower/negative values being brighter.

Units Distance

An arcminute (symbol: ' or arcmin) is 1/60th of a degree, used for medium-scale angular measurements.

Units Distance

An arcsecond (symbol: " or arcsec) is 1/3600th of a degree, the standard small-angle measurement in astronomy.

Solar System

Asteroids are rocky remnants from the Solar System's formation, too small to be rounded by gravity, predominantly found in the asteroid belt between M...

Units Distance

The average distance from Earth to the Sun.

Units Distance

An Astronomical Unit is the average distance between Earth and the Sun, serving as a fundamental measuring stick for the Solar System.

Telescopes Equipment

A Barlow lens is a negative (diverging) lens placed between telescope and eyepiece to multiply magnification, typically by 2× or 3×.

Telescopes Equipment

Binoculars offer wide-field, two-eyed viewing ideal for casual astronomy, comet hunting, and surveying large sky regions.

Solar System

Comets are icy bodies from the outer Solar System that develop spectacular tails when approaching the Sun as their ices vaporize.

Astronomy Concepts

Declination (Dec, δ) is celestial latitude—angular distance north (+) or south (-) of the celestial equator, analogous to latitude on Earth.

Telescopes Equipment

A Dobsonian mount is a simple altitude-azimuth mount using Teflon bearings on a rotating base, revolutionizing amateur astronomy by making large apert...

Solar System

The ecliptic is the apparent path of the Sun across the celestial sphere over a year, representing Earth's orbital plane projected onto the sky.

Astronomy Concepts

An equinox occurs when the Sun crosses the celestial equator, making day and night approximately equal length everywhere on Earth.

Telescopes Equipment

An eyepiece is the small optical system at the telescope's focus that magnifies the image for viewing, essentially a sophisticated magnifying glass.

Telescopes Equipment

A finder scope is a small, low-power telescope or sighting device mounted on the main telescope to help locate objects in the sky.

Telescopes Equipment

Focal length is the distance from a telescope's primary lens or mirror to the point where light converges to form an image.

Galaxies

A galaxy is a gravitationally bound system of stars, gas, dust, and dark matter, ranging from dwarfs with millions of stars to giants with trillions.

Units Distance

The distance light travels in one year.

Stellar Objects

Main sequence stars are in the stable, longest phase of stellar life, fusing hydrogen into helium in their cores.

Solar System

A meteor is the visible light phenomenon ("shooting star") when a meteoroid enters Earth's atmosphere at high speed and burns due to friction, typical...

Solar System

A meteorite is a meteoroid fragment that survives atmospheric entry and lands on Earth's surface, providing direct samples of Solar System materials.

Solar System

A meteoroid is a small rocky or metallic object in space, typically debris from asteroids or comets, ranging from dust grains to boulder-sized chunks.

Astronomy Concepts

The nadir is the point directly below the observer (altitude = -90°), opposite the zenith.

Cosmology Universe

The observable Universe is the spherical region containing everything we can see or detect, limited by the finite speed of light and Universe's age.

Stellar Objects

Open clusters are loose groups of 100-1,000 young stars born together from the same molecular cloud, weakly gravitationally bound.

Units Distance

A unit of astronomical distance equal to about 3.26 light-years.

Solar System

Perigee is the point in an object's orbit around Earth that is nearest to Earth.

Solar System

Perihelion is the point in an object's orbit nearest to the Sun, where orbital velocity is fastest.

Solar System

A planet is a celestial body orbiting a star, massive enough to be rounded by its own gravity, and has cleared its orbital neighborhood of other debri...

Stellar Objects

A red giant is an evolved star that has exhausted core hydrogen and expanded dramatically while cooling, fusing hydrogen in a shell around an inert he...

Telescopes Equipment

Reflector telescopes use curved mirrors to collect and focus light, offering the most aperture per dollar.

Telescopes Equipment

Refractor telescopes use lenses to bend (refract) light to a focus, the classic "spyglass" design.

Astronomy Concepts

Right Ascension (RA, α) is celestial longitude—angular distance eastward along celestial equator from the vernal equinox, analogous to longitude on Ea...

Astronomy Concepts

A solstice occurs when the Sun reaches its northernmost or southernmost point in the sky, marking the longest and shortest days of the year.

Galaxies

Spiral galaxies are disk-shaped galaxies with prominent spiral arm patterns, active star formation, and significant gas content.

Stellar Objects

A star is a self-luminous sphere of plasma held together by gravity, generating energy through nuclear fusion in its core.

Telescopes Equipment

A star diagonal is a mirror or prism at 90° that redirects the light path, making viewing more comfortable by positioning the eyepiece at right angles...

Telescopes Equipment

A T-ring is a camera-specific adapter that attaches a camera body to telescopes via the standard T-thread (M42×0.75mm) mounting system.

Astronomy Concepts

The zenith is the point directly overhead (altitude = 90°), the highest point in an observer's sky.