Astronomy Dictionary

Browse through Sky at Night Magazine’s comprehensive astronomical dictionary, with over 300 entries.

Schröter effect

The strange observational effect phenomenon in which Venus’s disc reaches half phase a few days before or after the predicted date.


Acting like a lens, the Earth’s atmosphere diffracts the starlight passing through it causing the effect known as scintillation. It can be seen when stars appear to twinkle.

Sea (Moon)

The Moon’s nearside is largely covered in relatively smooth areas, which are known as ‘seas’ or maria. These are not liquid seas like the ones on Earth, but large plains of basalt formed by volcanic eruptions. They appear darker than the surrounding regions because the basalt that they’re made from is less reflective.

Sea (Moon)

A large, iron-rich basalt plain on the lunar surface. Formed by cooling lava, they are generally much smoother than surrounding regions and appear dark compared with the lunar highlands. These regions are known as ‘seas’ because early astronomers believed these features were, in fact, bodies of water. They are more commonly known by their Latin name, mare (plural maria).

Secondary obstruction

The secondary obstruction in a reflecting or catadioptric telescope is a result of the necessary placement of a secondary mirror in the optical path. A large secondary obstruction will generally lower the brightness and contrast of images through the eyepiece at high magnifications.


Seeing is a measure of how steady the atmosphere is. It is particularly important to lunar and planetary observers. The worse seeing the more turbulent the atmosphere and therefore the poorer the view through the telescope.

Seeliger effect

When a reflective object such as a planet is in solar opposition it will appear brighter in the night sky than when it is not. This effect is named after the German astronomer Hugo von Seeliger.


The study of the surface of the Moon.

Semi-apochromatic (APO) refractor

A refracting telescope that uses three or more lenses to bring red, green and blue light to focus at nearly the same point.

Sensor noise

This is noise generated by a digital camera’s sensor. It arises because there is a small amount of electrical activity in the sensor itself. Noise increases with temperature but its effects in images can be reduced by taking many exposures (to increase the signal to noise ratio).

Shadow transit

A shadow transit is when the shadow of a moon passes over the surface of another planet. The most obvious of these to amateur astronomers are the shadow transits of Jupiter’s moons across its disc. They can be observed with a medium to large telescope.


A point in space and time where gravity (amongst other things) becomes infinite and the laws of physics break down.

Solar cycle

The 11-year period over which activity of the Sun, such as sunspots and solar flares, increases and decreases.

Solar cycle

The 11-year period over which the activity of the Sun increases and decreases; phenomena such as sunspots and solar flares are most common during a time of peak activity, called a solar maximum, but may be absent entirely when activity is at a low ebb, a solar minimum.

Solar flares

A violent explosion in the Sun’s atmosphere when energy stored in its magnetic field is released. Flares often occur over sunspots, associated with areas of strong magnetic field.

Solar mass

The mass of stars and other large astronomical bodies is expressed in terms of the mass of the Sun, which weighs in at about 2x10^30 kg.

Solar wind

An emission of charged particles moving at supersonic velocities away from the surface of the Sun.


The name often given to the combined ‘fabric’ of the Universe where both three dimensional space and time are linked in their four dimensions.

Spectral lines

A rainbow shows the small slice of the entire electromagnetic spectrum that humans can see. When astronomers use instruments to break down the light from a distant star or galaxy they also produce spectra. These contain signatures of the chemicals and molecules present in an object

Spectral type

A system of defining a star’s temperature by its colour. For example, class L stars are deep red with temperatures of 1,300–2,500K.


A device that measures the wavelength of light, known as its spectra, and then displays the results as a graph.


An instrument that displays specific features in the light from stars, planets and galaxies. By looking at the spectrum, astronomers can tell what an object is composed of.


A method of determining the chemical composition of a substance by the electromagnetic radiation it absorbs or emits. Through spectroscopy, scientists can discover the chemical components of distant celestial bodies.

Spherical aberration

This is an error in some lenses (and mirrors) that causes light rays coming from the edge of the lens to be brought into focus at a different point to those rays that are passing through the lens’s centre. It causes stars to appear bloated and not as pinpoints of light.

Spiral galaxy

Spiral galaxies are named so because of their spiral arms. The spirals are moving density waves of star formation that propagate around the disk of the galaxy to give them their distinctive shape.

Star diagonal

A mirror on some refracting telescopes that allows more comfortable viewing than you get by looking along the telescope tube. The star diagonal bends the light by 90 degrees.


This is the equivalent of a sunspot on another star. It is a region where magnetic field lines of the star are breaking through the surface slowing the transport of heat and cooling the surface.

Steady State theory

This is a theory that aims to explain the formation and evolution of the Universe where the cosmos has no beginning or end and has been continually creating matter as it expands.


Meaning star like. For example you might hear someone say that a celestial object is “stellar in appearance”.

Stroboscopic imaging

A type of imaging technique that allows studies to be made of oscillating or pulsating celestial object such as a pulsar.

Strong nuclear force

The force that confines the constituents of subatomic particles – called ‘quarks’ – within the particles. The strong nuclear force is one of the four fundamental forces of nature.

Sun pillar

A vertical column of light that appears above or below the Sun at sunset or -rise.


Relatively cool, dark regions on the Sun’s visible surface (the photosphere).


A rocky exoplanet that is more massive than our planet, but generally no larger than 10 Earth masses.


These are rocky worlds found by extrasolar planet searches in recent years. They’re more massive than the Earth, but generally no larger than about 10 Earth masses.


A very luminous, large and massive object that forms when a star with a mass more than 10 times that of the Sun has burned all of the hydrogen in its core. Betelgeuse in the constellation Orion, which is red in colour, is one such supergiant.

Superior conjunction

This is when one of the inferior planets (Mercury or Venus) is positioned directly behind the Sun in its orbit, as seen from the perspective of an observer on Earth. They are therefore hidden from view.

Supermassive black hole

These objects are a million to a billion times more massive than black holes and are found, it is thought, in the centre of every galaxy.


The explosive end to a high-mass star’s life, when it has consumed all its fuel and can no longer support itself against its own gravity. It subsequently contracts and then explodes, expelling its outer layers into space.

Supernova remnant

The relic of a supernova explosion, comprising a neutron star, pulsar or black hole, and an expanding cloud of gas.


These are the faint rainbow bands seen on the inner edge of a rainbow. They appear when rain droplets in the rainbow are comparatively small and of a similar diameter to each other.

Surface brightness

This is a measure of how bright something is (like a galaxy or a nebula) for each unit of area (a square arc-second).

Synodic period

The time between successive oppositions. It is a measure of the orbital period of a planet as viewed from Earth, which differs from the true orbital period because of the movement of our planet along its own orbit.

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