Essential astronomical terms explained

APERTURE

The diameter of a telescope’s light-gathering main lens or mirror. The bigger the aperture, the brighter and more detailed the image.

AVERTED VISION

A technique for observing faint objects through a telescope. It involves viewing slightly to the side of the object, allowing its light to fall on an area of the eye more sensitive to light.

BINARY STAR

A system of two stars that orbit a common centre of gravity. The brighter star is classified as the primary, while the dimmer is the secondary.

BLACK HOLE

The collapsed core of a massive star. Its concentration of mass is so dense that nothing, even light, can escape its gravitational pull.

CELESTIAL EQUATOR

A projection of the Earth’s equator into space.

CELESTIAL SPHERE

The projection of the night sky on to an imaginary sphere around the Earth. The astronomical coordinates of right ascension and declination are also mapped onto this sphere.

COLLIMATION

The process of aligning the optical elements of a telescope or binoculars.

COMET

A ball of frozen gases, rock and dust that orbits the Sun.

DECLINATION

The celestial equivalent of latitude, this is the distance of a body north or south of the celestial equator, measured in degrees, minutes and seconds. Objects north of the celestial equator have a positive declination, while those south of the celestial equator have a negative declination.

ECLIPTIC

The apparent annual path of the Sun against the background stars.

ELONGATION

The angle in the sky between a planet and the Sun, as seen from Earth. At its greatest elongation (east or west), a planet is – from our perspective – at its farthest apparent distance from the Sun.

EQUINOX

The two times each year – on or near March 21 and September 22 – at which the Sun crosses the celestial equator.

FOCAL LENGTH

The distance between a telescope’s main lens or mirror and the point at which an image is brought into focus. You can work out magnification with this number; this is calculated by dividing the focal length of a telescope by that of an eyepiece.

GIBBOUS

When the Moon or other body appears more than half, but less than fully, illuminated (from Latin gibbus, meaning ‘hump’).

LIBRATION

The apparent tilting or wobble of the Moon as it orbits Earth, allowing 59 per cent of its surface to be seen over a period of time.

LIGHTYEAR

A unit of measurement equal to the distance that light travels in one year – about 5.8 trillion miles.

MAGNITUDE

The brightness of an astronomical body. The lower the number, the brighter the object. Magnitudes brighter than zero are represented with a negative number.

MERIDIAN

An imaginary line circling the Earth from north to south that marks the point at which the Sun is at its highest in the sky.

METEOR

Also called a shooting star, this is a small particle of dust or rock that burns away on entering Earth’s atmosphere. A meteor shower is when meteors are observed to radiate at regular intervals and from a particular region in the sky as Earth passes through debris left by a disintegrating comet.

NEBULA

A cloud of interstellar dust and gas, usually lit up by stars inside or nearby.

OCCULTATION

When the Moon or a planet conceals or obscures a more distant planet or star.

OPPOSITION

The position of a superior planet when it’s opposite the Sun in the sky.

PHASE

The apparent change in shape of a celestial body as different amounts of its surface are illuminated by the Sun. The Moon displays four main phases: new, first quarter, full and last quarter.

RIGHT ASCENSION

The celestial equivalent of longitude, right ascension is measured in hours, minutes and seconds. The 0h line is measured from the point where the Sun crosses the celestial equator on its apparent path through the sky each year, a point called the vernal equinox.

STAR

A giant ball of hot gas that creates radiation through nuclear fusion. Stars are classified by their spectra (the elements that they absorb) and their temperature. The seven main types, from the hottest to the coolest, are: O, B, A, F, G, K and M.

STAR CLUSTER

A grouping of stars, from a few dozen to a few hundred thousand, bound together by their mutual gravitational attraction.

TRANSIT

The passage of a celestial body across the disk of a larger one, such as Mercury crossing the disk of the Sun or Ganymede’s shadow moving across the surface of Jupiter.

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A timescale based on the rotation of the Earth on its axis, UT is measured from midnight at the Greenwich Meridian. In astronomical use, UT replaced Greenwich Mean Time (GMT), which was measured from the Greenwich Meridian at midday on 1 January 1925.

VARIABLE STAR

A star that fluctuates in brightness over time, such as eclipsing binaries like Algol or pulsating stars like the Cepheids.