Glossary of Amateur Astronomy Jargon

Astronomy has a lot of specialized terms. Most of them sound more complicated than they actually are. Here's a quick reference guide to the jargon you'll encounter as an amateur observer.

Aberration - Optical defects that distort or blur images. Common types include chromatic aberration (color fringing) and spherical aberration (fuzzy stars).

Aperture - The diameter of a telescope's main lens or mirror. Bigger aperture = more light gathering = fainter objects visible. The most important specification in any telescope.

Apochromatic (APO) - A type of refractor telescope with special lens coatings that eliminate color fringing. Expensive but produces very sharp images.

Apparent Magnitude - How bright an object appears from Earth, measured on a scale where lower numbers are brighter. The Sun is magnitude -27. The faintest stars visible to the naked eye are magnitude +6.

Arcminute - A unit of angular measurement. One degree contains 60 arcminutes. The Moon is about 30 arcminutes wide.

Arcsecond - A smaller unit of angular measurement. One arcminute contains 60 arcseconds. Used for measuring very small details like planetary features.

Asterism - A recognizable pattern of stars that isn't an official constellation. The Big Dipper is an asterism within Ursa Major. The Summer Triangle is an asterism spanning three constellations.

Astigmatism - An optical defect where stars appear elongated or distorted instead of sharp points. Can be caused by poor optics or misaligned telescope components.

Averted Vision - A technique where you look slightly to the side of a faint object instead of directly at it. Uses the more light-sensitive parts of your eye to see dim objects better.

Azimuth - The compass direction of an object, measured in degrees. North = 0°, East = 90°, South = 180°, West = 270°.

Barlow Lens - An optical accessory that increases magnification by 2x or 3x when placed between the telescope and eyepiece. Doubles or triples the effective focal length.

Binaries / Binary Stars - Two stars orbiting each other. Some can be split into separate points of light with a telescope.

Bolide - An extremely bright meteor, often called a fireball. Brighter than magnitude -4.

Bortle Scale - A 9-level scale measuring light pollution and sky darkness. Class 1 is pristine dark sky. Class 9 is inner city with severe light pollution.

Cassegrain - A telescope design using both mirrors and lenses in a compact tube. Schmidt-Cassegrain (SCT) and Maksutov-Cassegrain (MAK) are common types.

Celestial Equator - Earth's equator projected onto the sky. Divides the celestial sphere into northern and southern hemispheres.

Celestial Poles - The two points in the sky where Earth's rotation axis points. The north celestial pole is near Polaris. Objects appear to rotate around these poles.

Celestial Sphere - An imaginary sphere surrounding Earth onto which we project all celestial objects. A useful fiction for mapping the sky.

Chromatic Aberration - Color fringing around bright objects caused by lenses failing to focus all colors at the same point. Common in cheap refractors.

Circumpolar - Objects that never set below the horizon. They circle the celestial pole all night. Which objects are circumpolar depends on your latitude.

Collimation - Aligning the optical elements of a telescope so they're perfectly centered and parallel. Reflectors need regular collimation. Refractors rarely do.

Coma - An optical aberration where stars near the edge of the field of view look like tiny comets instead of sharp points. Common in fast reflectors.

Conjunction - When two or more celestial objects appear close together in the sky. Not necessarily physically close - just aligned from our perspective.

Dark Adaptation - The process of your eyes adjusting to darkness. Takes 20-30 minutes for full adaptation. One glance at a bright light ruins it instantly.

Declination (Dec) - Celestial latitude. Measures how far north or south an object is from the celestial equator. Ranges from +90° (north celestial pole) to -90° (south celestial pole).

Deep Sky Objects (DSOs) - Objects beyond our solar system: galaxies, nebulae, star clusters. Usually faint and require dark skies.

Dew Shield - A tube extension on the front of a telescope that prevents dew from forming on the lens or corrector plate.

Dobsonian - A simple, cheap telescope mount design. A Newtonian reflector on a rotating box. Offers huge aperture for the price.

Double Star - Two stars that appear close together. Can be optical doubles (just happen to align from our view) or true binaries (actually orbiting each other).

Earthshine - Faint glow on the dark part of the Moon caused by sunlight reflecting off Earth onto the lunar surface. Visible during crescent phases.

Ecliptic - The Sun's apparent path across the sky over the course of a year. The zodiac constellations lie along the ecliptic. Planets stay close to it.

Elongation - The angular distance between a planet and the Sun as seen from Earth. Useful for finding Mercury and Venus.

Emission Nebula - A cloud of gas that glows by emitting its own light, usually energized by nearby stars. Appears reddish in photos but gray in telescopes.

Ephemeris - A table or chart showing the predicted positions of celestial objects over time.

Equatorial Mount - A telescope mount aligned with Earth's rotation axis. Allows one-axis tracking. Essential for astrophotography.

Exit Pupil - The diameter of the beam of light exiting an eyepiece. Calculated by dividing aperture by magnification. Should match your eye's pupil size (5-7mm) for optimal brightness.

Eyepiece - The lens you look through on a telescope or binoculars. Different eyepieces give different magnifications and fields of view.

Field of View (FOV) - How much sky you see in your eyepiece, measured in degrees. Wide FOV is good for finding objects. Narrow FOV is good for planetary detail.

Finder Scope - A small, low-power telescope mounted on the main telescope to help aim it. Red dot finders are popular alternatives.

Focal Length - The distance from a lens or mirror to the point where light comes to focus. Longer focal length = higher magnification potential and narrower field of view.

Focal Ratio (f-number) - Focal length divided by aperture. f/5 is "fast" (wide field, bright images). f/10 is "slow" (narrow field, dimmer images).

Galaxy - A massive system of billions of stars held together by gravity. The Milky Way is our galaxy. Most galaxies look like faint smudges in amateur telescopes.

Gegenschein - A faint glow in the sky directly opposite the Sun, caused by sunlight reflecting off interplanetary dust. Extremely faint and only visible under pristine dark skies.

Globular Cluster - A spherical collection of hundreds of thousands of old stars tightly packed together. Look like fuzzy balls in small telescopes, resolve into individual stars in larger apertures.

GoTo Mount - A computerized telescope mount that automatically points to objects when you select them from a database.

Light Pollution - Artificial light from cities that brightens the night sky and washes out faint objects. The enemy of astronomy.

Light-Year - The distance light travels in one year. About 9.5 trillion kilometers. Used to measure distances to stars and galaxies.

Limiting Magnitude - The faintest star you can see under given conditions. Dark skies might reach magnitude +6 or +7. City skies might only reach +3 or +4.

Local Sidereal Time (LST) - The Right Ascension currently crossing your meridian. Tells you which part of the sky is optimally positioned for viewing.

Magnitude - A scale measuring brightness. Lower numbers = brighter. Each magnitude step is about 2.5 times brighter. Negative magnitudes are very bright objects (Sun, Moon, planets).

Messier Object (M) - Objects from Charles Messier's catalog of fuzzy non-comets. 110 objects total, including some of the best deep-sky targets. M31 = Andromeda Galaxy, M42 = Orion Nebula, etc.

Meridian - An imaginary line running from north to south through the zenith (point directly overhead). Objects are highest and best positioned when crossing the meridian.

Meteor - A streak of light caused by a small piece of space debris burning up in Earth's atmosphere. Commonly called a "shooting star."

Meteor Shower - An event where Earth passes through debris left by a comet, producing many meteors radiating from one point in the sky.

Milky Way - The band of faint light stretching across the sky, made up of billions of stars in our galaxy's disk. Only visible from dark sites.

Nebula - A cloud of gas and dust in space. Can be emission nebulae (glowing), reflection nebulae (reflecting starlight), or dark nebulae (blocking background light).

New General Catalogue (NGC) - A catalog of deep-sky objects. Contains nearly 8,000 objects. NGC 224 = Andromeda Galaxy (same as M31).

Newtonian Reflector - A telescope design using a curved primary mirror and a flat secondary mirror. Cheap, simple, and popular for amateurs.

Objective - The main light-gathering element of a telescope. The big lens (refractor) or mirror (reflector) at the front.

Occultation - When one object passes in front of another, blocking it from view. The Moon frequently occults stars and planets.

Open Cluster - A loose group of young stars born from the same gas cloud. The Pleiades and Beehive Cluster are famous examples.

Opposition - When a planet is directly opposite the Sun in the sky. Planets at opposition are at their brightest, and visible all night.

Parallax - The apparent shift in position of a nearby object when viewed from different locations. Used to measure distances to nearby stars.

Planisphere - A rotating star chart showing which constellations are visible at different times and dates.

Plossl Eyepiece - A popular eyepiece design offering good performance at reasonable cost. Standard equipment on many telescopes.

Polar Alignment - Aligning an equatorial mount's axis with the celestial pole. Required for accurate tracking.

Precession - The slow wobble of Earth's rotation axis over 26,000 years. Causes RA and Dec coordinates to shift gradually over time.

Reflection Nebula - A cloud of dust reflecting light from nearby stars. Appears bluish in photos but gray in amateur telescopes.

Refractor - A telescope using lenses to gather and focus light. Sharp views but expensive for large apertures.

Reflector - A telescope using mirrors instead of lenses. Cheaper aperture but requires occasional maintenance.

Resolving Power - The ability to distinguish fine details or split close double stars. Determined primarily by aperture.

Right Ascension (RA) - Celestial longitude, measured in hours, minutes, and seconds. Runs from 0h to 24h around the celestial equator.

Seeing - The steadiness of Earth's atmosphere. Good seeing = sharp, steady views. Poor seeing = shimmering, blurry views. Affects planetary observing more than deep-sky.

Sidereal Day - The time it takes Earth to rotate 360 degrees relative to the stars. About 23 hours 56 minutes. Slightly shorter than a solar day.

Star-Hopping - Finding faint objects by starting at a bright star and "hopping" from one star to another in a pattern until you reach your target.

Super Moon - Media term for a full moon that's slightly closer to Earth than average. Looks marginally bigger. Astronomically insignificant.

Terminator - The line dividing the lit and dark parts of the Moon (or a planet). The best place to observe lunar detail because shadows are longest.

Transparency - How clear the atmosphere is. High transparency = you can see fainter objects. Haze, dust, and humidity reduce transparency.

Vernal Equinox - The point where the Sun crosses the celestial equator moving northward in March. Marks the start of spring. Also the zero point for Right Ascension.

Zenith - The point directly overhead, 90 degrees above the horizon.

Zodiac - The 12 constellations along the ecliptic. The Sun, Moon, and planets move through these constellations over the course of the year.