Glossary

Altitude: The angular distance above the horizon (see also, angular distance, azimuth, horizon).

Angular distance: the angle between two directions originating from an observation point and pointing to two objects is the angular distance between those objects.

Arcminute (’): An angular measure commonly used in Astronomy. 60’ = 1° (see also, degree, arcsecond).

Arcsecond (’’): An angular measure commonly used in Astronomy. 60’’ = 1’; 3600’’ = 1° (see also, degree, arcminute).

Asterism: A recognisable pattern of stars in the night sky.

Astrology: A pseudoscience that interprets the movements and relative positions celestial bodies seen from Earth as having influence on nature.

Astronomical Unit: The average distance between the Earth and the Sun, roughly 1.5 x 10¹¹ m.

Azimuth: The angular distance between the north point on the horizon and the perpendicular projection of a star down to the horizon (see also, altitude, angular distance, horizon)

Celestial Equator: The projection of the Earth’s equator onto the celestial sphere (see also, celestial sphere).

Celestial poles: The projection of the Earth’s rotational axis onto the celestial sphere (see also, celestial sphere, rotation).

Celestial Sphere: The sphere of stars, planets, the Sun and the Moon, as seen from Earth.

Circumpolar stars: Stars that, as viewed from a given geographical latitude, never set below the horizon, day or night (see also, horizon, latitude).

Constellation: (In ancient times) an asterism used by cultures to represent mythical heroes; (in modern times) a well-defined region of the sky used in mapping the celestial sphere.

Declination: The angular distance north or south of the celestial equator (see also, angular distance, celestial equator).

Degree (°): An angular measure. 1° = 1/360 of a circle (see also, arcminute, arcsecond).

Ecliptic: The annual path of the Sun, as seen from Earth, through the celestial sphere (see also, celestial sphere).

Equinox: An astronomical event that occurs when the Sun crosses the celestial equator. The equinoxes occur twice a year (spring and autumn) when the tilt of Earth’s rotational axis is in a direction perpendicular to the Sun (see also, celestial equator, solstice).

Horizon: The line marking the intersection between the Earth and the sky.

Hypothesis: A potential explanation of observable phenomena, proposed on the basis of limited evidence and used as a starting point for further investigation.

International Astronomical Union (IAU): An international society of astronomers that decides on definitions and naming conventions for celestial objects.

Latitude: The angular distance of a geographical point above or below the Earth’s equator, measured relative to the centre of the Earth (see also, angular distance, longitude).

Longitude: The angular distance of a geographical point east or west of the Prime Meridian, measured relative to Earth’s rotational axis (see also, angular distance, latitude).

Light-year: The distance travelled by light in a year, moving at 300,000 km/s.

Magnitude scale: The standard astronomical brightness scale, developed by Hipparchus in the second century BCE.

Milky Way Galaxy: The spiral galaxy containing our Solar System along with 100 billion other stars; visible at night as a band of light called the Milky Way (see also, Solar System).

North star: The closest bright star to the north celestial pole; Polaris (see also, north celestial pole).

Right ascension: The angular distance on the celestial sphere measured eastward along the celestial equator to the vernal equinox (see also, angular distance, celestial equator, celestial sphere, equinox).

Revolution: Here we use the word revolution to refer specifically to a planet’s orbital motion about the Sun or the apparent daily orbital motion of the celestial sphere about the Earth.

Rotation: Here we use the word rotation to refer specifically to a planet’s axial spin.

Scientific Method: The process of data collection, hypothesis formulation, and mathematical analysis through which scientists refine and constrain the most accurate and self-consistent description of nature.

Scientific model: A mathematical description of observable phenomena, which can be compared to data or used to predict phenomena that will occur.

Solar System: The system of planets, comets and asteroids that orbit the Sun.

Solstice: An astronomical event that occurs when the Sun reaches its highest (summer solstice) or its lowest (winter solstice) excursion relative to the celestial equator. The solstices occur when the Earth’s rotational axis is tilted directly (i.e. 23.4°) towards or away from the Sun (see also, celestial equator, equinox).

Zenith: The point directly overhead in the sky.

Zodiac: The band of constellations centred on the ecliptic where the Sun, Moon and planets move (see also, constellation, ecliptic).