As with any field of scientific study, astronomy has a long list of important terminology. So we've put together this glossary where you can check you fully understand the meaning of the common terms, as well as some of the more unusual words.

A -- B -- C -- D -- E -- F -- G -- H -- I -- J -- K -- L -- M -- N -- O -- P -- Q -- R -- S -- T -- U -- V -- W -- X -- Y -- Z

Words beginning with 'S' :

Satellite : a small astronomical body, natural or man-made, which orbits a planet, a minor planet or even another small astronomical body. For example the Moon is Earth’s natural satellite. Seeing : Earth’s atmospheric condition at any one time. Atmospheric turbulence causes image distortion when viewing with a telescope at high magnification, just like water ripples distort and blur images. When there is little disturbance in the air there is good seeing, and the stars don’t twinkle as much. The more atmosphere above the observe the more distortion can be expected, that is why ground based telescopes are located at very high altitudes on mountains.

Semi-major axis : is the radius at its longest two points of an ellipse. It is one of three measurement (the other two being the semi-minor axis and eccentricity) used to describe the shape of an ellipse.

Seyfert galaxies : have bright centers. They are active galaxies, like quasars, but their center don't outshine the host galaxy (the galaxy is clearly visible). At their center can be found a supermassive black hole with an accretion disk.

Sidereal day : the time taken by an astronomical object to complete one rotation on its axis with respect to fixed stars. For example Earth's sidereal day is 4 minutes shorter than its solar day (measured from noon to noon).

Singularity : a region of space-time where the gravitational field of an astronomical object becomes infinite.

Sidereal period : the complete revolution of one celestial object around another. It means that the object returns to the same position relative to fixed stars in the sky.

Slingshot maneuver : also known as gravitational slingshot, gravity assist maneuver and swing-by. It is the use of gravity of an astronomical object to affect the speed or direction of flight of an object.

Solar flares : sudden flashes of increased solar brightness that are occasionally accompanied by solar mass ejections.

Solar wind : a fast moving flow of particle coming from the Sun in all direction. The same applies to all stars; they lose material through the continuous solar wind which travels between 72,000 to 7,200,000 km/h (44,738 to 4,473,872 mph).

Solstice : two days in the year on Earth when the north or south is furthest from the Sun. When the north is furthest away, it’s the winter solstice in the northern hemisphere with the shortest day and longest night of the year. This happens on, or a day or two around, December 21st. On that date the south is at its closest to the Sun, so the Southern hemisphere experiences a summer solstice (the longest day and shortest night of the year). The summer solstice for the northern hemisphere (winter solstice for the southern hemisphere) happens on, or a day or two around, June 21st. After either date the days get longer/shorter depending on which solstice has just taken place. This is all due to the axial tilt of the Earth. If the Earth didn’t have this tilt there would be no seasons.

Star diagonal : angle mirror or prism used with telescopes to allow viewing at a perpendicular direction of the eyepiece axis.

Sunspot : a dark patch on the Sun’s surface which can be planet-size. This is a region of cooler gas compared to the rest of the Sun’s surface.

Superior planets : planets orbiting around the Sun and outside of Earth's orbit.

Supernova : the end of a star life in a massive explosion which produces in a second as much energy as galaxy. The explosion is brief, and the brightness fades rapidly, making supernovae difficult to observe.

Surface pressure : the pressure on the surface of an astronomical object. It is proportional to the mass of atmosphere over a unit of area. For example the surface pressure on Venus is 90 times that of Earth, whereas it’s only 0.6% on Mars compared to Earth.

Synodic period : the time taken for an astronomical object to return to the same location relative to the Sun as seen from Earth.

Resources :

To support the quizzes in each section, we've put together some extra resources to provide further information: