Astronomical Thesaurus
Definitions, formulas and interactive 3D illustrations for the concepts behind Astronomical Algorithms — from Julian Day and precession to perihelion, declination and topocentric coordinates.
Foundations
Jean Meeus' Astronomical Algorithms — the computational methods behind every view on this site.
Time Systems
Julian Day, Dynamic Time, sidereal time and the scales astronomers use to measure cosmic events.
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Julian Day
Continuous day count from noon UT on 1 January 4713 BC — the astronomer's calendar.
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Dynamic Time (ΔT)
The difference between uniform atomic time and Earth's irregular rotation (UT).
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Sidereal Time
Time measured by Earth's rotation relative to the stars, not the Sun.
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Universal Time (UT)
The time scale tied to the Earth's rotation; UTC is its atomic, leap-second version.
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Synodic Period
The time between two identical Sun–Earth–body configurations, such as two oppositions or two full Moons.
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Sidereal Period
The time a body takes to complete one orbit relative to the fixed stars.
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Orbits & Mechanics
Keplerian elements, perihelion, nodes, inclination and the geometry of planetary motion.
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Orbit
The path of a body under gravity — usually an ellipse with the Sun at one focus.
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Perihelion
The point in a solar orbit closest to the Sun.
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Aphelion
The point in a solar orbit farthest from the Sun.
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Semi-major Axis
Half the longest diameter of an orbit — it sets the size, the period and the energy of the orbit.
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Eccentricity
A number from 0 (circle) to 1 (parabola) describing how stretched an orbit is.
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Inclination
The tilt of an orbit relative to a reference plane, usually the ecliptic.
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Ascending and descending nodes
Where an orbit crosses the ecliptic: northward at the ascending node, southward at the descending node.
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Perigee
The point of an orbit around the Earth closest to the Earth — for the Moon, between 356,400 and 370,400 km.
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Apogee
The point of an orbit around the Earth farthest from the Earth — for the Moon, about 404,000 to 406,700 km.
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Coordinate Systems
Equatorial, ecliptic and horizontal coordinates — how we locate objects on the celestial sphere.
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Equatorial Coordinates
The (RA, Dec) system fixed to Earth's equator and the vernal equinox.
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Ecliptic Coordinates
Longitude and latitude measured along and from the ecliptic.
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Horizontal Coordinates
Azimuth and altitude — the observer's local horizon system.
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Right Ascension
The celestial equivalent of longitude — measured eastward from the vernal equinox, in hours.
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Declination
The celestial equivalent of latitude — the angle north or south of the celestial equator.
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Azimuth
The horizontal direction of a celestial object, measured along the horizon from north (or from south in Meeus).
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Altitude
The angular height of a celestial object above the horizon, from 0° at the horizon to 90° at the zenith.
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Topocentric Coordinates
Positions as seen by an observer on the Earth's surface, rather than from the Earth's centre.
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Geocentric Coordinates
Positions as seen from the centre of the Earth — the standard frame of ephemerides.
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Heliocentric Coordinates
Positions measured from the centre of the Sun — the natural frame of planetary orbits.
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Celestial Sphere & Frames
Equinoxes, ecliptic, precession, celestial poles and the reference frames of astronomy.
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Precession
Slow wobble of Earth's axis causing the equinoxes to drift along the ecliptic.
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Ecliptic
The apparent annual path of the Sun on the celestial sphere.
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Celestial Equator
Projection of Earth's equator onto the celestial sphere.
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Vernal Equinox
The point where the Sun crosses the celestial equator going north in March — the origin of celestial coordinates.
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Autumnal Equinox
The September equinox: the Sun crosses the celestial equator southward, at ecliptic longitude 180°.
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Celestial Pole
The points where the Earth's rotation axis meets the celestial sphere, around which the sky turns.
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Zodiac
The band of sky centred on the ecliptic, through which the Sun, Moon and planets move.
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Nutation
A small periodic nodding of the Earth's axis, mainly with an 18.6-year period, superimposed on precession.
Observation & Phenomena
Semidiameter, parallax, refraction, magnitude and what we actually see from Earth.
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Semidiameter
Apparent equatorial radius of a celestial body as an angle.
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Atmospheric Refraction
Bending of light in Earth's atmosphere — objects appear higher than true.
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Apparent Magnitude
Logarithmic measure of brightness — lower numbers are brighter.
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Opposition
When a planet is opposite the Sun in the sky — up all night, at its closest and brightest.
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Conjunction
When two bodies share the same ecliptic longitude or right ascension, appearing close together in the sky.
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Elongation
The angular distance between a planet or the Moon and the Sun, as seen from the Earth.
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Parallax
The apparent shift in position of a nearby object when seen from two different places.
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