The Astronomy & Space Tourism Glossary

Every hobby has its own shorthand, and astronomy and space travel have more than most. Open a telescope review or a space-tourism FAQ and you hit aperture, focal ratio, Bortle, suborbital, apogee — words that each hide a simple idea once someone explains them without the jargon. That is what this page is for: the terms we use across the site, in plain English, with a link to the full guide behind each one.

How to use this glossary

It is split into two lists — the stargazing and telescope words first, then the space-tourism and spaceflight ones — and each list runs alphabetically. Every entry is one or two sentences, enough to understand what you are reading right now, and the Our guide › link takes you to where we cover it in depth. You don’t need to memorise any of it to start; skim it when a word trips you up and come back when it does again.

Astronomy & stargazing

The words that come up when you are choosing a telescope or binoculars, planning a night out, or working out why one scope shows more than another.

Alt-azimuth mount

A mount that swings left-right and up-down, like a camera tripod. It is the simplest way to point a telescope and the easiest for a beginner, though it won’t track a star across the sky on its own the way an equatorial mount can. Our guide ›

Aperture

The width of a telescope’s main lens or mirror, in millimeters or inches. It is the one number that decides how much a scope shows you — a wider aperture gathers more light, so it reaches fainter objects and splits finer detail. It matters far more than magnification. Our guide ›

Apparent magnitude

How bright an object looks from Earth, on a backwards scale where smaller numbers are brighter. The faintest star most people can see from a dark site is about magnitude 6; a bright planet like Venus is negative. Each step of one is roughly 2.5 times the brightness. Our guide ›

Astrophotography

Photographing the night sky, from a phone held to an eyepiece up to a cooled camera stacking hours of exposure. It is a separate hobby from visual observing, with its own gear, and it is where most of the money in amateur astronomy now goes. Our guide ›

Averted vision

Looking slightly to the side of a faint object instead of straight at it, so its light falls on the more sensitive edge of your retina. It is the oldest trick in observing — a galaxy that vanishes when you stare at it pops back when you glance away. Our guide ›

Barlow lens

A lens that drops into the focuser ahead of the eyepiece and multiplies its magnification, usually 2x or 3x. One good Barlow effectively doubles your eyepiece collection for a fraction of the cost of buying more eyepieces. Our guide ›

Bortle scale

A one-to-nine rating of how dark a sky is, from a pristine wilderness (Bortle 1, the Milky Way casts shadows) to an inner city (Bortle 9, a handful of stars). It is the quickest way to judge what a site will actually show you before you drive out. Our guide ›

Collimation

Aligning a telescope’s mirrors so star images come to a clean point. Reflectors need it checked and tweaked now and then — it sounds fiddly but takes a couple of minutes once you’ve done it. It is a characteristic of the design, not a fault. Our guide ›

Dark adaptation

The 20 to 40 minutes your eyes take to reach full night sensitivity. One glance at a white phone screen resets it, which is why observers use dim red light — red disturbs night vision the least. Our guide ›

Deep-sky object

Anything beyond our solar system that isn’t a single star — galaxies, nebulae and star clusters. They are faint and spread out, so a dark sky and a wide, low-power view usually show them better than high magnification. Our guide ›

Dobsonian

A big reflector on a simple wooden rocker base, named for John Dobson, who wanted maximum aperture for minimum money. Inch for inch it is the most light-gathering you can buy, which is why it is the classic recommendation for seeing faint galaxies and nebulae. Our guide ›

Equatorial mount

A mount tilted to match Earth’s axis, so a single slow-motion control follows a star as the sky turns. It has a learning curve but it is what makes long-exposure astrophotography and smooth high-power tracking possible. Our guide ›

Exit pupil

The width of the beam of light leaving an eyepiece or binocular, in millimeters. Too large and you waste light your eye can’t take in; too small and the view dims. It is the number behind why a given binocular suits dusk, deep sky or daytime use. Our guide ›

Eyepiece

The small removable lens you look through, which sets the magnification and field of view. Most telescopes ship with one or two mediocre eyepieces, so a better pair is usually the first upgrade that visibly changes what you see. Our guide ›

Focal length & focal ratio

Focal length is how far the light travels inside the scope to form an image; focal ratio is that length divided by the aperture (the f-number). A low f-number gives bright, wide views for deep sky; a high one gives higher-power views that suit the Moon and planets. Our guide ›

Go-To mount

A motorised mount with a hand controller or app that slews to any object you pick and tracks it. After a short alignment it finds things you’d struggle to star-hop to, at the cost of power and a little setup each night. Our guide ›

Light pollution

The sky glow from streetlights and cities that washes out fainter stars. It is the single biggest thing standing between most people and a good night’s sky — the same telescope shows wildly more from a dark site than a suburban backyard. Our guide ›

Magnification

How much larger a target appears, set by the telescope’s focal length divided by the eyepiece’s. Beginners chase it, but useful magnification is capped by aperture and the steadiness of the air — past about 50x per inch of aperture the image just gets dim and mushy. Our guide ›

Messier catalog

A list of about 110 bright deep-sky objects compiled by Charles Messier in the 1700s. The “M” numbers — M31 the Andromeda Galaxy, M42 the Orion Nebula — are still the standard beginner’s tour of what a backyard scope can show. Our guide ›

Opposition

When a planet sits opposite the Sun in our sky, so it rises at sunset and is up all night at its closest and brightest. It is the best few weeks of the year to look at Mars, Jupiter or Saturn. Our guide ›

Reflector

A telescope that uses a mirror to gather light. Reflectors give the most aperture per dollar, which is why they dominate the budget end, though the open tube needs occasional collimation and cleaning. Our guide ›

Refractor

A telescope that uses lenses, with a sealed tube and sharp, high-contrast views that need no collimation. A good one is superb on the Moon and planets, but aperture gets expensive fast, so refractors tend to be smaller than reflectors at the same price. Our guide ›

Schmidt-Cassegrain (SCT)

A compact catadioptric design that folds a long focal length into a short tube using a mirror and a corrector plate. It packs a lot of aperture into a portable package, which is why it is the go-to all-rounder for both planets and deep sky. Our guide ›

Seeing

How steady the atmosphere is on a given night. Bad seeing makes stars twinkle hard and planets boil; good seeing lets you crank up the magnification. A high, hazy, still night often beats a crisp, gusty one for planetary detail. Our guide ›

Smart telescope

A sealed, motorised scope with a built-in camera that finds a target, stacks a live image and sends it to your phone — no eyepiece. It trades the eyepiece view for easy astrophotography, and it is the fastest-growing corner of the hobby. Our guide ›

Solar filter (ISO 12312-2)

A certified filter that blocks all but a tiny, safe fraction of the Sun’s light. ISO 12312-2 is the only standard that matters for looking at the Sun or an eclipse; anything without verifiable certification can cause permanent eye damage, printed claims included. Our guide ›

Star projector

A home gadget that throws stars, nebulae or a galaxy across a ceiling. It is decor and a mood light, not an astronomy instrument — but it is one of the most-bought “space” gifts, so we test and rank them like any other gear. Our guide ›

Totality

The few minutes during a total solar eclipse when the Moon fully covers the Sun, day turns to dusk and the corona appears. It is the only time it is safe to look without a filter, and the reason people travel across the world for it. Our guide ›

Space tourism & spaceflight

The words that come up when you are reading about flying to space yourself, or watching a launch from the ground.

Apogee

The highest point of a flight’s arc. On a suborbital hop it is the top of the climb — the few minutes of weightlessness and the view of Earth’s curve against black space before the vehicle falls back down. Our guide ›

EVA (spacewalk)

Extravehicular activity — an astronaut working outside the spacecraft in a pressure suit. No tourist flight offers one, but they are the most dramatic thing to watch on a NASA livestream, and we keep a guide to catching them. Our guide ›

G-force

The push you feel as a rocket accelerates or re-enters, measured in multiples of normal gravity. A suborbital flight peaks around 3 to 6 g for short stretches — firm but brief, and the main thing the medical screening and training prepare you for. Our guide ›

Kármán line

The 100-kilometer (62-mile) altitude widely used as the boundary of space. Cross it and you have officially been to space; some US bodies use 50 miles instead, which is why operators sometimes quote different “astronaut” thresholds. Our guide ›

Low Earth orbit (LEO)

The band of space from roughly 160 to 2,000 km up, where the Space Station, most satellites and every proposed space hotel orbit. It is the nearest and cheapest orbit to reach, and the destination for commercial stations. Our guide ›

Microgravity

The near-weightlessness of orbit or the top of a suborbital arc. You are still inside Earth’s gravity — you are falling freely, so there is nothing to push against. It is the floating sensation people mean by “zero gravity”. Our guide ›

Orbital flight

A flight fast enough to circle the Earth rather than fall straight back — days in space, not minutes, and the kind of trip that costs tens of millions. It needs far more speed, and energy, than a suborbital hop. Our guide ›

Parabolic flight

A padded aircraft flying a series of steep up-and-over arcs, each giving 20 to 30 seconds of weightlessness. It is how astronauts train and the only true weightlessness most people can buy today, for a few thousand dollars. Our guide ›

Reusable rocket

A rocket whose booster flies back and lands to be flown again, instead of being thrown away. It is the single change that has driven launch costs down and launch rates up over the past decade. Our guide ›

Scrub

Calling off a launch before liftoff, for weather, a technical hold or a range conflict. Scrubs are routine and can happen minutes before the clock hits zero, which is why launch travellers always keep a backup day. Our guide ›

Spaceport

A site licensed to launch rockets — Cape Canaveral, Vandenberg, Starbase, Wenchang. A few have public viewing areas within a few miles of the pad; most are closed ranges, so where you can actually stand varies a lot by site. Our guide ›

Suborbital flight

A flight that reaches space but not orbit: straight up past the boundary, a few minutes of weightlessness and the view, then back down. It is the cheapest way to space today, in the low-to-mid six figures with Blue Origin and Virgin Galactic. Our guide ›

The bottom line

None of this is as complicated as the jargon makes it sound. The two that unlock the most are aperture (how much a telescope shows you) and the Kármán line (where space begins); get those and most of the rest follows. When a term here raises a bigger question — which telescope, how much a flight costs, where to stand for a launch — follow its guide link and we have the whole answer waiting.