Glossary
Key astronomy terms used across the exhibits, with definitions and links.
A
Angular size
The apparent size of an object as seen from a given viewpoint, measured as an angle. Two objects of different physical size can subtend the same angular size if they are at different distances.
Aphelion
The point in an orbit farthest from the central body (the Sun for planets in the solar system).
B
Blackbody radiation
Electromagnetic radiation emitted by an idealized object that absorbs all incident radiation. Its spectrum depends only on temperature and peaks at a wavelength given by Wien’s law.
C
Conjunction
A configuration where two celestial bodies have the same ecliptic longitude as seen from Earth. Superior conjunction places the body behind the Sun; inferior conjunction places it between Earth and the Sun.
E
Eccentricity
A measure of how much an orbit deviates from a perfect circle. A circle has eccentricity 0; a parabolic escape orbit has eccentricity 1.
Ecliptic
The plane of Earth’s orbit around the Sun, and by extension the apparent path of the Sun on the sky over a year.
Equation of state
A relationship among thermodynamic variables (pressure, density, temperature) that describes the material properties of a gas or plasma. Stellar interiors require combining ideal gas, radiation, and degeneracy pressure contributions.
K
Kepler’s laws
Three laws describing planetary motion: (1) orbits are ellipses with the Sun at one focus, (2) a line from planet to Sun sweeps equal areas in equal times, (3) the square of the orbital period is proportional to the cube of the semi-major axis.
L
Lunar phase
The shape of the illuminated portion of the Moon as seen from Earth, determined by the Moon-Earth-Sun geometry. Phases cycle from new to full and back over about 29.5 days.
O
Obliquity
The angle between a planet’s rotational axis and the perpendicular to its orbital plane. Earth’s obliquity (~23.4°) causes the seasons.
P
Parallax
The apparent shift in position of an object when viewed from two different vantage points. Stellar parallax uses Earth’s orbital diameter as a baseline to measure distances to nearby stars.
Perihelion
The point in an orbit closest to the central body (the Sun for planets in the solar system).
R
Rayleigh criterion
The angular resolution limit of a circular aperture, approximately 1.22 times the wavelength divided by the aperture diameter. Two point sources closer than this angle cannot be distinguished.
Retrograde motion
The apparent backward (westward) drift of a planet against the background stars, caused by the relative orbital motion of Earth and the planet.
S
Semi-major axis
Half the longest diameter of an ellipse. For an orbit, it determines the size of the orbit and (via Kepler’s third law) the orbital period.
Synodic period
The time between successive identical configurations (e.g., conjunctions) of two orbiting bodies as seen from a reference body. It depends on both orbital periods.
W
Wavelength
The spatial period of a wave, the distance between successive crests. For electromagnetic radiation, wavelength determines the type of light (radio, infrared, visible, ultraviolet, X-ray, gamma ray).
Wien’s displacement law
The peak wavelength of a blackbody spectrum is inversely proportional to the temperature: λ_max = b / T, where b ≈ 2.898 × 10⁻³ m·K.