A practical astronomy glossary

How to use this glossary

Start with sky position when you want to point the map or find an object outdoors; use time and darkness when choosing a session; and use brightness and visibility when judging whether an object may be easy to see. The terms describe an ideal astronomical sky. Weather, buildings, trees, terrain, light pollution, eyesight and optical equipment still matter at a real observing site.

Sky position and coordinate systems

Altitude (Alt.)

The angle of an object above or below the local horizon. 0° is on the geometric horizon and 90° is at the zenith. Higher objects generally pass through less atmosphere and are easier to observe.

Azimuth (Az.)

The compass direction along the horizon, measured clockwise from north: north 0°, east 90°, south 180° and west 270°. Sky Map 2000 uses azimuth with altitude in its Horizontal view.

Cardinal directions

The principal horizon directions: north (N), east (E), south (S) and west (W). NE, SE, SW and NW are the intermediate directions. They make an azimuth easier to use outdoors.

Horizon

The boundary between sky and ground for an ideal, level observer. The map’s horizon is geometric; a hill, roof, tree or haze can hide an object that is mathematically above it.

Zenith and nadir

The zenith is directly overhead; the nadir is directly below your feet. The centre button in the direction pad points a Horizontal view at the zenith.

Meridian

An imaginary north–south line passing through the zenith. An object is usually highest in the sky when it crosses your local meridian.

Right ascension (RA)

A fixed sky-coordinate similar in role to longitude on Earth. It is normally expressed in hours, minutes and seconds, from 0h to 24h, and helps locate an object on a celestial atlas.

Declination (Dec.)

A fixed sky-coordinate similar in role to latitude on Earth, measured north or south of the celestial equator in degrees. Together, RA and Dec identify an object’s position on the celestial sphere.

Celestial equator

The projection of Earth’s equator onto the sky. It divides the sky into northern and southern celestial hemispheres and is the 0° line of declination.

Celestial pole

The point where Earth’s rotation axis appears to meet the sky. The north celestial pole lies close to Polaris; its altitude is approximately your latitude in the Northern Hemisphere.

Sidereal time

A time scale that tracks Earth’s rotation relative to the stars, rather than the Sun. It tells astronomers which right ascension is crossing the local meridian and is used internally to convert fixed sky coordinates into your local view.

Hour angle

The angular distance of an object west of the local meridian. It is one way to express how long before or after meridian crossing an object is.

Field of view (FOV)

How much sky fits across the displayed view, measured in degrees. A smaller FOV is a closer view; a larger FOV shows more of the sky. Your clenched fist at arm’s length is roughly 10° wide.

Time, the Sun and darkness

Local time and UTC

Local time is the civil clock time at the selected place. UTC (Coordinated Universal Time) is the global reference time. The map shows both so a planned observation can be compared across places and dates.

Time zone

The civil-time offset and daylight-saving rules used to convert UTC to local time. A wrong time zone can move the displayed sky by a large amount, even when the coordinates are correct.

Rise, transit and set

Rise is when an object crosses the ideal eastern horizon; transit is its highest crossing of the local meridian; set is its crossing of the western horizon. Actual visibility can differ because of local obstructions and refraction.

Civil twilight

The Sun is between 0° and 6° below the horizon. The sky is still bright enough for most outdoor activity, but faint stars are largely washed out.

Nautical twilight

The Sun is 6° to 12° below the horizon. The horizon is becoming difficult to distinguish, while many stars are visible.

Astronomical twilight

The Sun is 12° to 18° below the horizon. The sky continues to darken, but residual sunlight can still affect the faintest observations.

Astronomical darkness

The Sun is at least 18° below the horizon. It is the conventional threshold for a fully dark astronomical sky; at high latitudes it may not occur on some summer nights.

Solar declination

The Sun’s declination north or south of the celestial equator. It changes through the year because Earth’s axis is tilted, influencing the Sun’s rising direction, noon height and day length.

Solstice and equinox

A solstice occurs near the year’s greatest north or south solar declination; an equinox occurs when the Sun crosses the celestial equator. They mark important seasonal turning points, not identical day and night lengths everywhere.

Brightness, colour and visibility

Apparent magnitude (mag)

A logarithmic measure of how bright an object appears from Earth. Smaller and negative numbers mean brighter: a 1st-magnitude star is brighter than a 4th-magnitude star. The map uses apparent magnitude to size stars in the magnitude display style.

Absolute magnitude

A standardised measure of intrinsic brightness: how bright an object would appear from 10 parsecs away. It is useful for comparing stars independent of distance.

Limiting magnitude

The faintest stellar magnitude the current map setting draws. It is a display limit, not a promise that your eyes, binoculars or telescope will show a star under your local conditions.

Angular diameter

How large an object appears on the sky, measured in degrees, arcminutes or arcseconds. The Moon’s apparent diameter is about half a degree; an arcminute is 1/60 of a degree.

Angular separation

The apparent distance between two objects on the sky. It helps judge close pairings, conjunctions and whether both objects fit in one binocular or telescope field.

Air mass

A rough measure of how much atmosphere you look through. Near the horizon, air mass is larger, so objects can appear dimmer, redder and less sharp.

Atmospheric extinction

The dimming of celestial light by Earth’s atmosphere. It increases near the horizon and with haze, humidity, dust or smoke.

B−V colour index

A numerical comparison of a star’s brightness through blue (B) and visual (V) filters. Smaller or negative values generally indicate bluer, hotter stars; larger values indicate redder, cooler stars.

Spectral class

A classification of stars by temperature and spectrum, commonly O, B, A, F, G, K and M from hotter to cooler. The Sun is a G-type star. Spectral colour mode uses this information visually.

Objects, catalogues and the wider sky

Celestial sphere

An imaginary sphere surrounding the observer on which celestial objects appear projected. It is a useful geometric model, not a physical shell around Earth.

Constellation

One of 88 official regions of the sky defined by the International Astronomical Union. A constellation is an area, not only the familiar line pattern drawn between selected stars.

Asterism

A recognisable star pattern that is not an official constellation, such as the Big Dipper within Ursa Major or the Summer Triangle across three constellations.

Ecliptic

The Sun’s apparent annual path across the celestial sphere. The Moon and planets stay relatively near it because their orbital planes are close to Earth’s orbital plane.

Milky Way

Our home galaxy, seen from within as a diffuse band of unresolved stars, dust and gas. Its appearance depends strongly on darkness, latitude, season and light pollution.

Deep-sky object (DSO)

A general observing term for objects beyond the Solar System, including star clusters, nebulae and galaxies. The map draws a selected set of Messier and NGC/IC objects.

Open cluster

A relatively loose, young group of stars born from the same cloud. The Pleiades are a well-known example.

Globular cluster

A dense, roughly spherical collection of old stars orbiting a galaxy. They often require binoculars or a telescope to appreciate fully.

Nebula

An interstellar cloud of gas and dust. It may emit light, reflect starlight, block background light, or be material expelled by an ageing star.

Galaxy

A vast gravitationally bound system of stars, gas, dust and dark matter. The Andromeda Galaxy is the most prominent external galaxy for many northern observers.

Messier, NGC and IC

Catalogue designations used to identify deep-sky objects. M refers to Charles Messier’s catalogue; NGC and IC are the New General Catalogue and Index Catalogue.

Light-year and parsec

A light-year is the distance light travels in one year. A parsec is about 3.26 light-years and is commonly used in professional astronomy.

Parallax

The apparent shift of a nearby star against distant background stars as Earth moves around the Sun. It provides a direct method for measuring stellar distance.

Proper motion

A star’s slow apparent movement across the sky over years and centuries. Constellation shapes are therefore not permanently fixed on very long timescales.

The Moon and planets

Moon phase

The changing sunlit fraction of the Moon’s visible disk. The main phases are new Moon, first quarter, full Moon and last (third) quarter, with crescent and gibbous phases between them.

Illumination

The percentage of the Moon’s apparent disk lit by the Sun. A high percentage can brighten the night sky and make faint deep-sky objects harder to see.

Waxing and waning

The Moon is waxing when its illuminated portion is increasing after new Moon and waning when it is decreasing after full Moon.

Synodic month

The average interval between the same Moon phases, about 29.5 days. It differs from the Moon’s orbital period relative to the stars because Earth is moving around the Sun too.

Conjunction

A close apparent meeting of two celestial objects in the sky, often the Moon and a planet or two planets. It does not mean that the objects are close together in space.

Opposition

When a Solar System object is approximately opposite the Sun in the sky as seen from Earth. Outer planets are often near their best visibility around opposition.

Elongation

The angular distance between an object and the Sun in the sky. Large elongation makes Mercury or Venus easier to spot away from twilight.

Retrograde motion

An apparent temporary westward reversal of a planet against the background stars, caused by the changing viewpoint as Earth and the planet move in their orbits.

Practical observing terms

Dark adaptation

The process by which your eyes become more sensitive in low light. Protect it by reducing bright white light and lowering device brightness; Night Vision in the app is a helpful interface mode, not a complete solution.

Light pollution

Artificial outdoor light that brightens the night sky and reduces contrast. Its effect varies greatly by location, direction, transparency and the object you are observing.

Transparency and seeing

Transparency describes sky clarity and faint-object contrast; seeing describes atmospheric steadiness and the sharpness of stars at high magnification. A clear night can have poor seeing, and vice versa.

Binoculars and aperture

Binoculars provide a wide, bright field and are excellent for learning the sky. Aperture is the diameter of an optical instrument’s light-gathering opening; more aperture can reveal fainter detail, but conditions and skill still matter.

Star atlas

A map of the sky used to identify and navigate among stars and deep-sky objects. Sky Map 2000’s Star Atlas mode gives the interactive map a light, print-like display style.

Star hopping

A practical technique for finding a faint object by moving from a recognisable bright star or pattern through a sequence of nearby guide stars.