Glossary

Archaeoastronomy: The archaeological study of how people in the past have viewed, understood, and incorporated astronomical phenomena within their cultures.

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.

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

Celestial Equator: The projection of the Earth’s equator onto the celestial sphere.

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

Deduction: A process of reasoning in which the conclusion follows necessarily from the premises. Thus, the conclusion cannot be false if the premises are true, though the same conclusions may follow from different premises (see also, first principle, hypothesis, verification, falsification).

Deferent: The orbital path of an imaginary point that planets orbited (along an epicycle) in geocentric models.

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

Eccentric: The off-centre point at which the Earth was sometimes placed in geocentric models. 

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

Epicycle: The second circle, about which planets were supposed to orbit an imaginary point moving along the deferent in geocentric models.

Epistemology: The study of knowledge and understanding.

Equant: A point in the Ptolemaic model, off-centre with respect to the deferent, about which the epicyclic centre was described to move uniformly.

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, solstice).

Falsification: The observation of data that are inconsistent with a given hypothesis, according to which scientists reject the proposal (see also, verification).

First principle: Basic assumption of a physical theory, normally inferred by rational thought based on empirical evidence, which cannot be deduced from any other assumption, and from which observables may be deduced as logical consequences (see also, hypothesis).

Geocentrism: The scientific hypothesis that the Earth rests at the centre of the Universe and is orbited by the celestial objects (see also, heliocentrism).

Heliocentrism: The scientific hypothesis that the Earth and other planets orbit the Sun (see also, geocentrism).

Hypothesis: A potential explanation of phenomena, proposed on the basis of limited evidence and used as a starting point for further investigation (see also, first principle).

Logic: The use of valid reasoning.

Lunar Standstill: An astronomical event that occurs when the 5° tilt of the Moon’s orbital plane aligns with, and therefore adds to (major standstill), or subtracts from (minor standstill) the 23.5° tilt of Earth’s rotational axis. A full cycle, e.g. from major standstill to major standstill, takes 18.6 years to complete. During the month of major standstill, the Moon reaches both its nearest and furthest distances from the celestial poles. During minor standstill, the Moon is as near as it gets to the celestial equator.

Model: A scientific model is a mathematical description of phenomena, which can be compared to data or used to predict phenomena that will occur. In physics and astronomy, we tend to distinguish broadly between physical and empirical models. A physical model is an abstract mathematical description of something that is hypothetically supposed to be occurring, such as a planet orbiting the Sun. The planet’s path is described by a mathematical relation that is, according to modern physics, given by Einstein’s general relativity, a theory of gravitation. In contrast, an empirical model is a mathematical relation used to describe phenomena, which supposes little or nothing about what is really going on, but contains parameters that can be adjusted so that the model fits existing data as closely as possible.

Parallax: The phenomenon in which the position of a foreground object appears to shift with respect to a background when the point of observation has moved. Parallax is important in astronomy because an observed parallax shift is related to the observed object’s distance.

Parsec: The distance to an object whose parallax shift, when observed from two locations separated by the mean annual distance between the Earth and the Sun, is equal to one arcsecond (see also, parallax, arcsecond).

Phenomenon: Observable information about something that happened, which is transmitted through space and time. Phenomena (plural) are scientific observables, the evidence we use in constraining our best scientific theories, and they are distinguished from the things that exist, in and of themselves, which are never directly observed.

Precession: The phenomenon in which the orientation of the celestial sphere changes very slightly with a 26,000 year period.

Retrograde motion: The phenomenon in which planets apparently halt in their regular progression along the ecliptic, and for a while reverse their direction of motion with respect to the background stars.

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

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. 23.5°) towards or away from the Sun (see also, equinox).

Verification: The observation of data that are consistent with a given hypothesis, according to which scientists continue to suspend disbelief in the proposal (see also, falsification).