Glossary

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).

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

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

Crystalline sphere: Aristotle’s astronomical model involved a system of rotating crystalline spheres made of aether, the substance of the heavens.

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.

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

Equivalence principle: A fundamental principle of general relativity stating that the force a massive body feels in a gravitational field is the same as the force it feels when experiencing accelerated motion. Thus, a body’s gravitational “mass” is said to be equivalent to its inertial “mass” (see inertia).

Focus of an ellipse: One of two points used to define an ellipse, as the set of points where the sum of the distances to the two foci is constant. Kepler’s first law states that planetary orbits are ellipses with the Sun located at one focus.

Gravitational lensing: Bending of light rays around massive objects as a result of the curvature of space predicted by general relativity.

Inertia: The resistance of any physical body to a change in its state of motion (including its resistance to being accelerated from rest).

Kepler’s laws of planetary motion: Three laws discovered by Johannes Kepler describing the elliptical motion of planets around the Sun.

Length contraction: A phenomenon predicted by Einstein’s relativity theory and confirmed by experiment, which states that the measured length of a moving rod should be less than when the same rod is at rest.

Measurement uncertainty: A non-negative parameter characterizing the dispersion of the values attributed to a measured quantity.

Model calibration: The process of adjusting parameters of a mathematical model (e.g. m and b in the linear model y = mx+b) in order to bring it as close as possible to a set of data points.

Neap tide: A tide just after the first or third quarters of the Moon when there is the least difference between high and low water (see spring tide).

Newton’s laws of motion: Three physical laws described by Sir Isaac Newton in his Principia describing the relation of a body to the forces acting on it, and the resulting motion of the body.

Newton’s law of universal gravitation: The fundamental law of Newton’s theory of gravity, describing the mutual gravitational force between any two massive bodies.

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.

Perihelion precession: The precession of the point of closest approach as a planet’s orbital ellipse precesses around the Sun.

Platonic solids: Five three-dimensional polyhedra composed of congruent regular polygonal faces with the same number of polygons meeting at each vertex.

Postulate: A principle that has been proposed for the purpose of theoretical development; a hypothesis.

Predictive model: A mathematical model whose values accurately predict the locations of data points which were not used for model calibration purposes (see model calibration).

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.

Semi-major axis of an ellipse: One half of the longest diameter of an ellipse (the major axis), which runs through both foci (see Focus of an ellipse).

Spring tide: Occurs when the difference between high and low tide is the greatest, which is when the Sun, Moon and Earth are directly in line and the Sun’s and Moon’s gravitational pull reinforce each other. (see neap tide).

Time-dilation: A phenomenon predicted by Einstein’s relativity theory and confirmed by experiment, which states that time must pass more slowly for a moving clock.