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Astronomical Glossary - Terms & Definitions

Astronomical terms can be a little technical and difficult to understand. Below is a list of definitions and explanations to help you navigate astronomical texts and services.

Albedo
In astronomy, the term albedo refers to the brightness of an object in space. Derived from Latin, albedo means "whiteness" (albus= "white"). Albedo is measured on a scale from 0 to 1, where 0 refers to an object that is completely dark, i.e., it does not reflect any light. 1 on the scale refers to a perfectly reflective object. The Moon has an Albedo of 0.12, while Earth's average albedo is 0.3.
Altitude (elevation)
Altitude or elevation is the vertical angle an object makes with the horizon. Read more...
Analemma
A figure of eight pattern produced by plotting the position of the Sun in the sky at the same time of day over the course of a year. Read more...
Annular Solar Eclipse
An annular solar eclipse happens when the Moon covers the Sun's center, leaving the Sun's visible outer edges to form a “ring of fire” or annulus around the Moon. Read more...
Anomalistic Month
The length of time—on average, 27.55 days—from one perigee to the next, where perigee is the point in the Moon’s orbit closest to Earth.
Antumbra
The antumbra is the lighter part of a shadow that forms at a certain distance from the object casting the shadow. It is involved in annular solar eclipses and planet transits. Read more...
Aphelion
Aphelion is the point farthest away from the Sun on Earth's elliptical orbit. Read more...
Apogee
The point in the orbit of the Moon (or other satellite) that is farthest from Earth’s center. Read more...
Apparent Retrograde Motion
Apparent retrograde motion is where a superior planet appears to change direction in the sky for a short time as it is overtaken by Earth in its orbit around the Sun. See also retrograde (below).
Apparent Solar Time
Time measured by the movement of the Sun across the sky; the time shown on a sundial. The difference between apparent solar time and mean solar time is given by the equation of time (see below).
Asterism
A pattern of stars recognizable to observers from Earth. Asterisms may or may not be part of a constellation. The Big Dipper is one of the most well-known asterisms. Its stars belong to the constellation Ursa Major.
Asteroid
Asteroids are rocky objects orbiting the Sun. Their size ranges from a few meters to hundreds of kilometers in diameter. Read more...
Astronomical Season
Astronomers and scientists use the dates of equinoxes and solstices to mark the beginning and end of seasons in a year. In the Northern Hemisphere, the four astronomical seasons are: Read more...
Astronomical Twilight
Astronomical twilight is the darkest of the three twilight phases. It is the earliest stage of dawn in the morning and the last stage of dusk in the evening. Read more...
Atmospheric Phenomena
Atmospheric phenomena occur when light, usually from a natural source, but sometimes from artificial sources, is reflected or refracted as it passes through the atmosphere, for example by air molecules, ice crystals, or different types of particles. Read more...
Aurora Borealis and Aurora Australis
An aurora is a natural phenomenon that creates bright and colorful light displays in the sky. In the Arctic Circle, they are known as aurora borealis or northern lights; in the Antarctic Circle, they are called aurora australis or southern lights. Read more...
Axial Tilt – Obliquity
Earth's axis is tilted by about 23.4 degrees to the perpendicular to the orbital plane (see illustration).
Earth rotates the Sun at a slant. This means that different amounts of sunlight reach the Northern and Southern Hemispheres throughout the year. This is the reason we have seasons on Earth. Read more...
Axis
An axis in astronomy refers to the (imaginary) line that an object, usually a planet, rotates around. Earth's rotational axis is an imaginary straight line that runs through the North and South Poles (see illustration).
Azimuth
An object's cardinal direction, such as north, east, south, or west. Read more...
Blue Moon
There are two definitions of a Blue Moon in astronomy; both are a type of Full Moon. Read more...
Celestial Equator
The celestial equator is the projection of Earth's equator onto the celestial sphere. From our perspective, it is the part of the sky directly above the equator. Read more...
Celestial Horizon
The imaginary horizontal line separating the two hemispheres of the celestial sphere is called the celestial horizon. Read more...
Celestial Pole
The celestial poles are imaginary lines that trace Earth’s rotation axis in space. From our perspective, they are the points in the sky directly above the North Pole and the South Pole. Because of this, objects that lie on the celestial pole do not seem to move at all, while all other objects, mostly stars, seem to move in a circle around the pole.
Celestial Sphere
The celestial sphere is an imaginary sphere that extends infinitely into space with Earth at its center. It is the backdrop the horizontal coordinate system uses to map the sky and describe the positions of its objects. Read more...
Chromosphere
A thin reddish layer surrounding the photosphere of the Sun (or other star). The chromosphere can be briefly glimpsed during a total solar eclipse. Read more...
Circumpolar Stars
Circumpolar stars never set or go below the horizon for observers from specific latitudes. They are visible to observers from these latitudes throughout the year because of their proximity to the celestial pole. Circumpolar objects lie within the circumpolar circle, and stars circumpolar to latitudes in the Northern Hemisphere are not visible in the Southern Hemisphere, and vice versa.
Civil Twilight
Civil twilight is the brightest of the three twilight phases. The Sun is just below the horizon, so there is generally enough natural light to carry out most outdoor activities. Read more...
Comet
Comets are small celestial bodies, primarily made of dust and ice, that orbit the Sun. They are thought to be remnants of the formation of the Solar System. Read more...
Conjunction
A conjunction is when two astronomical objects appear close to each other in the sky. Read more...
Constellation
In the present day, a constellation is a two-dimensional area in space as defined by the International Astronomical Union (IAU). Today, there are 88 officially recognized constellations. This is different from the historical understanding of a constellation dominantly used in astrology which is defined as a group of stars easily identifiable by a pattern to observers on Earth. Read more...
Dawn
Dawn is the transition from night to day as the sky gets brighter. Scientists distinguish between three definitions of dawn: civil, nautical, and astronomical dawn. Each one is a specific moment in time, based on the solar elevation angle. Read more...
Delta T
In eclipse calculations, Delta T is a number that accounts for changes in the speed of Earth’s rotation. It is also written as ΔT, using the Greek letter Δ (Delta). For future eclipses, the value of ΔT is only an estimate, since Earth’s spin varies in unpredictable ways: if Earth rotates more quickly than expected, the eclipse will happen farther to the west than calculated. ΔT is measured in seconds, and is the difference between Terrestrial Time (TT)—a smoothly flowing timescale related to atomic clocks—and Universal Time (UT)—an uneven timescale linked to Earth’s spin. Read more...
Draconic Month
27.21 days on average: a lunar month measured from one ascending node to the next. See node, one of two points where the Moon’s tilted orbit crosses the plane of Earth’s orbit around the Sun. Also called a nodical month.
Dusk
Dusk generally refers to the transition from day to night. Scientists distinguish between three definitions of dusk: civil, nautical, and astronomical dusk. Each one is a specific moment in time, based on the solar elevation angle. Read more...
Dwarf Planet
A celestial body that (a) is in orbit around a star, (b) has enough mass for its own gravity to give it a round shape, (c) has not cleared away other objects that lie in or near its orbit, and (d) is not a moon. Read more...
Earthshine
Earthshine is a dull glow which sometimes lights up the unlit part of the Moon. It usually occurs a few days before and after a New Moon when sunlight reflects off Earth's surface and illuminates the portion of the Moon’s surface which is not lit up by direct sunlight. Read more...
Eclipses
A solar eclipse happens when the New Moon moves between Earth and the Sun. A lunar eclipse occurs when Earth casts a shadow onto the Full Moon. Read more...
Ecliptic — Ecliptic Plane – Orbital Plane
The ecliptic plane, also called orbital plane, is the imaginary line that traces the Sun's apparent path in the sky. In other words, it is the projection of our planet's orbit into the celestial sphere. Any constellations on this line are known as zodiacal or zodiac constellations.
Equation of Time
The difference between apparent solar time and mean solar time. Read more...
Elevation (altitude)
Elevation or altitude is the vertical angle an object makes with the horizon. Read more...
Equinox
There are two equinoxes every year – in March and September – when the Sun is directly above the equator and the length of night and day are nearly equal. Read more...
False Dawn
False dawn or zodiacal light is a rare optical phenomenon that occurs around sunset and sunrise, usually during early spring and late fall. Read more...
Full Moon
The Full Moon is the Moon phase when the entire face of the Moon is lit up. Read more...
Gas Giant
A planet made mostly of gas. Jupiter and Saturn are gas giants.
Great Circle
A great circle is any circle that is formed by a plane that passes through the center of Earth. The equator and the circles created by the meridians form great circles. Read more...
Great Conjunction
A great conjunction is a conjunction involving Jupiter and Saturn. Read more...
Greatest Elongation
The moment when Mercury or Venus reach their farthest distance from the Sun in the morning sky (greatest elongation west) or the evening sky (greatest elongation east), as seen from Earth.
Halos
Halos are atmospheric phenomena created by light which is reflected or refracted by ice crystals in the atmosphere. Read more...
Heliosphere
A region around the Sun filled with a solar wind of charged particles. The heliosphere reaches far beyond Neptune. Read more...
Horizontal Coordinate System
The horizontal coordinate system, also known as the Alt/Az system, is a method for describing the exact position of objects in the sky, such as planets, the Sun, or the Moon. Read more...
Ice Giant
A planet made mostly of icy materials. Uranus and Neptune are ice giants.
Inferior Conjunction
The inferior planets, Mercury and Venus, have two kinds of conjunction. At inferior conjunction, the planet lies between Earth and the Sun. See also superior conjunction.
Inferior Planet
An inferior planet’s orbit is closer to the Sun than Earth’s orbit. Mercury and Venus are inferior planets.
Latitude and Longitude
Cartographers and geographers trace horizontal and vertical lines called latitudes and longitudes across Earth's surface to locate points on the globe. Read more...
Libration
The wobble of a celestial body. For example, although the Moon keeps the same face pointed toward Earth, libration means that—over time—59 percent of the Moon’s surface is visible from Earth.
Light Pillars
Light Pillars are an atmospheric phenomenon created by light which is reflected by ice crystals in the atmosphere. Read more...
Light Year
A light year (also spelled: light-year or lightyear) is a unit of distance and is defined as the distance traveled by light in a vacuum during a Julian year. In astronomy, a Julian year (symbol: a) is a time unit defined as exactly 365.25 days of 86,400 seconds each. The distance is approximately 9.5 trillion kilometers or 6 trillion miles.
Local Midnight
Local midnight typically occurs when the Sun crosses the meridian below the horizon. In areas where there is midnight Sun, local midnight is when the Sun is at its lowest point of the night. Read more...
Lunar Apogee
The point of the Moon's orbit farthest from Earth is called apogee. Read more...
Lunar Eclipse
A lunar eclipse occurs when Earth comes between the Sun and the Moon and blocks the Sun's rays from shining directly on the Moon. Lunar eclipses only happen at Full Moon. Read more...
Lunar Month
A lunar month is the time it takes the Moon to pass through all of the Moon phases, measured from one New Moon to the next. A lunar month is also known as a lunation, while the astronomical term for this period is a synodic month. Read more...
Lunar Perigee
The point of the Moon's orbit closest to Earth is called perigee. Read more...
Lunation
A lunation is the time it takes the Moon to pass through all of the Moon phases, measured from a New Moon to the next New Moon. A lunation is also known as a lunar month, while the astronomical term for this period is a synodic month. Lunations are numbered in several different systems; the most common one is the Brown lunation number system, which we use our Moon phase pages. Read more...
Magnetic Declination
The difference between true north and magnetic north is called magnetic declination or magnetic variation. Read more...
Mean Solar Time
Time measured by clocks and watches, where each day is exactly 24 hours long. The difference between apparent solar time and mean solar time is given by the equation of time (see above).
Meridian
A meridian is an imaginary line that runs from the North Pole to the South Pole, connecting all locations sharing the same longitude. The moment when the Sun or the Moon crosses a location's meridian marks the instant when they reach the highest position in the sky, appearing either due south, due north, or directly overhead. For the Sun, it is the moment of solar noon. Read more...
Meteor
When a meteoroid enters Earth's atmosphere, it starts to glow as it collides with air molecules in the upper atmosphere. The flash of light this generates is called a meteor, a shooting star, or a falling star. Read more...
Meteor Shower
A meteor shower is when an unusual amount of meteors—or shooting stars—flash across the night sky over a period, usually a few days or weeks. Read more...
Meteorite
A meteorite is the name for a space rock or meteoroid which has survived falling through the atmosphere and has landed on Earth. Read more...
Meteoroid
A meteoroid is a block of matter made up of dust particles or fragments from a comet or an asteroid. Meteoroids become meteors, also called shooting stars, when they enter Earth's atmosphere burning a trail of dust and fire which is visible from Earth as a flash of light in the sky. Read more...
Micromoon
When a Full Moon or a New Moon occurs around apogee, which is the point on the orbit farthest from Earth, it's called a Micromoon, Minimoon, or Apogee Moon. When there is a Full or New Moon around perigee, it is called a Supermoon. Read more...
Midnight Sun — Polar Day
Midnight Sun is when at least a part of the Sun's disk is visible above the horizon 24 hours of the day. The scientific name for midnight Sun is polar day, and the opposite is polar night. Read more...
Moon
Another word for a natural satellite, which is a celestial body that orbits a larger one. Earth’s natural satellite is called “the Moon.”
Moon Phase
The sunlight that reflects onto the Moon's surface we call a Moon phase. How much of that light we can see from our point of view on Earth varies every day. The lunar month is generally divided into four primary and four intermediate Moon phases: Read more...
Moonbows
Moonbows or lunar rainbows are rare natural atmospheric phenomena that occur when the Moon’s light is reflected and refracted off water droplets in the air. Read more...
Moonrise and Moonset
Moonrise is defined as the moment the upper edge of the Moon becomes visible above the horizon. The Moon sets as the upper edge disappears below the horizon. Sometimes, the Moon is not visible even if it is above the horizon. This is the case, for example, during the New Moon and when the sky is not clear. Read more...
Nautical Twilight
Nautical twilight is the second twilight phase. Both the horizon and the brighter stars are usually visible at this time, making it possible to navigate at sea. Read more...
Node
One of two points where the Moon’s tilted orbit crosses the plane of Earth’s orbit around the Sun—see illustration. At the ascending node, the Moon travels northward across the plane; at the descending node, it travels southward.
Northern Lights — Aurora Borealis
An aurora is a natural phenomenon that creates bright and colorful light displays in the sky. In the Arctic Circle, they are known as aurora borealis or northern lights. Read more...
Obliquity – Axial tilt
Earth's axis is tilted about 23.4 degrees to the perpendicular to the orbital plane. Our planet rotates the Sun at a slant. This means that different amounts of sunlight reach the Northern and Southern Hemispheres throughout the year. This is the reason we have seasons on Earth. Read more...
Orbit
An orbit is the path of an object around a point or another object in space. In general, this path is repeatedly followed by the object, though, in some cases, different celestial forces like gravitation can change its orbit. An object in orbit is called a satellite. Orbits are formed due to two opposing forces—the orbiting object's momentum and the force of gravity that pulls it towards the object it is orbiting. These two forces have to balance each other for an orbit to be sustained.
Opposition
The moment when a planet (or other solar system body) lies directly opposite the Sun in the sky.
Parhelic Circle
A parhelic circle is a rare optical atmospheric phenomenon. Read more...
Partial Lunar Eclipse
A partial lunar eclipse occurs when Earth moves between the Sun and Moon, but the three bodies do not form a perfectly straight line in space. When this happens, only part of the Moon's surface is covered by the darkest part of the shadow cast by Earth, the umbra. Read more...
Partial Solar Eclipse
During a partial solar eclipse, only part of the solar disk is covered by the New Moon. Read more...
Penumbra
The penumbra is the lighter outer part of a shadow. The Moon's penumbra causes partial solar eclipses, and Earth's penumbra is involved in penumbral lunar eclipses. Read more...
Penumbral Lunar Eclipse
A penumbral lunar eclipse occurs when the Sun, Earth, and the Moon align in an almost straight line. When this happens, Earth covers all or part of the Moon with the outer part of its shadow, also known as the penumbra. Since the penumbra is much fainter than the dark core of the Earth's shadow, the umbra, a penumbral eclipse of the Moon is often difficult to tell apart from a normal Full Moon. Read more...
Perigee
The point in the orbit of the Moon (or other satellite) that is closest to Earth’s center. Read more...
Perihelion
Perihelion is the point closest to the Sun on Earth's elliptical orbit. Read more...
Photosphere
The visible surface of the Sun (or other star). Read more...
Planet
A celestial body that (a) is in orbit around a star, (b) has enough mass for its own gravity to give it a round shape, and (c) has cleared away other objects that lie in or near its orbit. Read more...
Planetary Transit
A planetary transit occurs when a planet passes in front of the Sun. It is then visible from Earth as a tiny black dot silhouetted against the Sun's disk. The only two planets that can be seen transiting the Sun from Earth are Mercury and Venus, because they are the only planets inside Earth's orbit.
Polar Day — Midnight Sun
Polar day or Midnight Sun is when at least a part of the Sun's disk is visible above the horizon 24 hours of the day. The scientific name for midnight Sun is polar day, and the opposite is polar night. Read more...
Polar Night
Polar night happens when the entire Sun remains below the horizon all day. It only happens within the polar circle, and the opposite of polar night is midnight Sun or polar day. Read more...
Prograde
Prograde motion is where a planet or moon (or other solar system body) rotates or orbits in a counterclockwise direction, as seen from above the Sun’s north pole. Earth, the Moon, and most other planets and moons have prograde rotations and orbits.
Reflection
Reflection occurs when light bounces off the surface of an object.
Refraction
Refraction is the bending of light as it moves from one substance to another. For example, it happens when sunlight enters Earth's atmosphere. Read more...
Retrograde
Retrograde motion is where a planet or moon (or other solar system body) rotates or orbits in a clockwise direction, as seen from above the Sun’s north pole. Venus and Uranus have retrograde rotation; Triton (a moon) has a retrograde orbit around Neptune. See also apparent retrograde motion (above).
Ring
A planetary ring is a thin disk of particles orbiting a planet. Saturn has bright rings; Jupiter, Uranus, and Neptune have faint rings.
Saros
An eclipse cycle discovered by the ancient Babylonians. Every 6585.32 days (just over 18 years) the Earth, Moon, and Sun return to almost the exact same alignment, producing a near-identical solar or lunar eclipse.
Shooting Star
A shooting star is a popular term for a meteor, which is a flash of light generated when a meteoroid enters Earth's atmosphere. Read more...
Sidereal Day
The time it takes for a specific star to reach the same position in the sky—about 23 hours, 56 minutes, and 4 seconds. Read more...
Sidereal Month
The time it takes the Moon to complete one orbit around Earth relative to the stars: 27.32 days on average. It is shorter than one orbit of the Moon relative to the Sun—see synodic month.
Sidereal Year
The time it takes Earth to complete one orbital revolution around the Sun, relative to the stars.
Solar Corona
The Sun’s outer atmosphere. During a total solar eclipse, the corona can appear as a ghostly halo around the obscured disk of the Sun. Read more...
Solar Day
Modern timekeeping defines a day as the sum of 24 hours – but that is not quite correct. In solar time, most days are a little longer than 24 hours, the time it takes from one solar noon to the next. Read more...
Solar Noon
Solar noon occurs when the Sun crosses a location's meridian and reaches its highest position of the day. In most locations, it doesn't happen at 12 o'clock. Find Sun times worldwide. Read more...
Solar System
The term used to describe the Sun and everything that orbits it. Read more...
Solar Time
  • Apparent Solar Time / True Solar Time
    A sundial shows the true or apparent solar time. Because Earth's rotation is not constant, solar days vary slightly in length. This means that the speed of true solar time is not constant.
  • Mean Solar Time is based on the length of a mean or average solar day, which is 24 hours long. It moves at a constant speed.
Read more...
Solar Wind
Solar wind is part of space weather. It is a continuous stream of highly energized particles – mostly electrons and protons – that flow out from the Sun through space at very high speeds and high temperature. Northern lights are caused by high solar wind activity. Read more...
Solstice
Solstices happen twice a year — in June and December. The June solstice is around June 21, when the Sun is directly overhead the Tropic of Cancer. The December solstice takes place around December 21. On this day, the Sun is precisely over the Tropic of Capricorn. The winter solstice is the shortest day of the year, while the summer solstice is the longest day of the year. Solstices are opposite on either side of the equator.
Southern Lights — Aurora Australis
An aurora is a natural phenomenon that creates bright and colorful light displays in the sky. In the Antarctic Circle, they are called aurora australis or southern lights. Read more...
Star
A celestial body that produces light and other forms of energy through a process of nuclear fusion. Our home star is the Sun.
Summer Solstice
The summer solstice is the longest day of the year. Solstices are opposite on either side of the equator; the summer solstice in the Northern Hemisphere is the June solstice, while in the Southern Hemisphere, it is the December solstice. Read more...
Sundogs
Sundogs are an atmospheric phenomenon created by light which is reflected or refracted by ice crystals in the atmosphere. Read more...
Sunrise
Sunrise is defined as the moment the upper edge of the solar disk—called the upper limb—becomes visible above the horizon. Read more...
Sunset
As the upper edge of the solar disk—called the upper limb—disappears below the horizon, the Sun has set. Read more...
Superior Conjunction
The inferior planets, Mercury and Venus, have two kinds of conjunction. At superior conjunction, the planet lies on the far side of the Sun to Earth. See also inferior conjunction.
Superior Planet
A superior planet’s orbit is farther from the Sun than Earth’s orbit. Mars, Jupiter, Saturn, Uranus, and Neptune are superior planets.
Supermoon
When a Full Moon or a New Moon occurs around perigee, which is the point on the orbit closest to Earth, it's called a Supermoon. When there is a Full or New Moon around apogee, it is called a Micromoon. Read more...
Synodic Month
A synodic month is the time it takes the Moon to pass through all of the Moon phases, measured from a New Moon to the next New Moon. It is also known as a lunar month or a lunation. Read more...
Terrestrial Planet
A planet made mostly of rock and metal. Mercury, Venus, Earth, and Mars are terrestrial planets.
Tides
The gravitational pull of the Moon and the Sun makes the water in the oceans bulge, causing a continuous change between high and low tide. Read more...
Total Solar Eclipse
During a total eclipse of the Sun, the Moon covers the entire disk of the Sun. A total solar eclipse is only total within the path of totality and only for a short while. Most of the time and in most places, it is visible as a partial solar eclipse. Read more...
Transit
See planetary transit (above).
Transition Region
A thin region of the Sun between the chromosphere and the solar corona where the temperature increases dramatically. Read more...
Tropical Year
A tropical year is the time it takes for the seasons to repeat. It is roughly—but not exactly—the time it takes the Earth to complete one orbit around the Sun. Its duration varies from year to year. Also known as a solar year, an astronomical year, or an equinoctial year, it is, on average, approximately 365 days, 5 hours, 48 minutes and 45 seconds long (365.24219 days). A tropical year is a fraction shorter than a sidereal year. Read more...
Twilight
Twilight is the time between day and night when there is light outside, but the Sun is below the horizon. There are three degrees of twilight: civil, nautical, and astronomical twilight. Read more...
Umbra
The umbra is the darkest portion of a shadow. The Moon's umbra causes total solar eclipses, while Earth's umbra sometimes creates total and partial lunar eclipses. Read more...
Zenith
In celestial coordinate systems, the location straight above you is called zenith while the point exactly below you is referred to as nadir. Read more...
Zodiacal Constellations
Zodiacal constellations are the constellations located within Earth's ecliptic. There are 13, not 12, zodiacal constellations. The 13th constellation is Ophiuchus.
Zodiacal Light
Zodiacal lights or false dawn is a rare optical phenomenon that occurs around sunset and sunrise, usually during early spring and late fall. Read more...

Topics: Astronomy, Stars, Sun, Comets, Eclipses, Equinox, Meteors, Planets, Solstice, Moon, Asteroids, Atmospheric Phenomena, Earth