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)
Civil time based on Earth's rotation, referenced to Greenwich meridian.
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Synodic Period
Time between successive identical configurations of two bodies as seen from Earth.
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Sidereal Period
Orbital period relative to the fixed stars — one complete revolution.
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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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Perigee
Closest point to Earth in a satellite or lunar orbit.
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Apogee
Farthest point from Earth in a satellite or lunar orbit.
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Eccentricity
A number from 0 (circle) to 1 (parabola) describing orbital shape.
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Semi-major Axis
Half the longest diameter of an orbit — sets the orbital size and period.
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Orbital Inclination
Angle between an orbit plane and the reference plane (usually the ecliptic).
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Ascending Node
Where an orbit crosses the reference plane moving northward.
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Descending Node
Where an orbit crosses the reference plane moving southward.
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Line of Nodes
The intersection line of an orbit plane with the reference plane.
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Coordinate Systems
Equatorial, ecliptic and horizontal coordinates — how we locate objects on the celestial sphere.
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Right Ascension
Equatorial coordinate analogous to longitude — measured in hours east from the vernal equinox.
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Declination
Equatorial coordinate — angular distance north or south of the celestial equator.
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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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Azimuth
Compass direction of a celestial object measured from north eastward.
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Altitude
Angular height of a celestial object above the horizon.
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Topocentric
Referred to the observer's location on Earth's surface.
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Geocentric
Referred to Earth's centre — standard for many ephemerides.
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Heliocentric
Referred to the Sun's centre — natural for orbital mechanics.
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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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Vernal Equinox
The point where the Sun crosses the celestial equator northward — origin of coordinates.
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Autumnal Equinox
The September equinox — Sun crosses the equator southward.
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Celestial Equator
Projection of Earth's equator onto the celestial sphere.
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Celestial Pole
Points where Earth's rotation axis intersects the celestial sphere.
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Zodiac
The belt of constellations along the ecliptic.
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Nutation
Small oscillation of Earth's axis superimposed on precession.
Observation & Phenomena
Semidiameter, parallax, refraction, magnitude and what we actually see from Earth.
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Conjunction
Two bodies share the same ecliptic longitude as seen from Earth.
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Opposition
A superior planet opposite the Sun in the sky — best time to observe.
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Semidiameter
Apparent equatorial radius of a celestial body as an angle.
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Parallax
Apparent shift in position due to changing viewpoint.
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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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Elongation
Angular separation between the Sun and a planet on the sky.
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