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.
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.
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.
Aphelion
Aphelion is the point farthest
away from the Sun on Earth's elliptical orbit.
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.
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:
- Spring: March equinox to June solstice.
- Summer: June solstice to September equinox.
- Fall (autumn): September equinox to December solstice.
- Winter: December solstice to March equinox.
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.
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.
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.
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.
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
Azimuth
An object's cardinal direction,
such as north, east, south, or west.
Blue Moon
There are two definitions of a
Blue Moon in astronomy; both are a type of Full Moon.
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.
Celestial Horizon
The imaginary horizontal line
separating the two hemispheres of the celestial sphere is called the celestial
horizon.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Elevation (altitude)
Elevation or altitude is the
vertical angle an object makes with the horizon.
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.
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.
Full Moon
The Full Moon is the Moon phase
when the entire face of the Moon is lit up.
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.
Halos
Halos are atmospheric phenomena
created by light which is reflected or refracted by ice crystals in the
atmosphere.
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.
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.
Light Pillars
Light Pillars are an atmospheric
phenomenon created by light which is reflected by ice crystals in the
atmosphere.
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.
Lunar Apogee
The point of the Moon's orbit
farthest from Earth is called apogee.
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.
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.
Lunar Perigee
The point of the Moon's orbit
closest to Earth is called perigee.
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.
Magnetic Declination
The difference between true north
and magnetic north is called magnetic declination or magnetic variation.
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.
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.
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.
A meteorite is the name for a
space rock or meteoroid which has survived falling through the atmosphere and
has landed on Earth.
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.
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.
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.
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: New Moon, Waxing
Crescent Moon, First Quarter Moon, Waxing Gibbous Moon, Full Moon, Waning
Gibbous Moon, Third Quarter Moon, and Waning Crescent Moon.
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.
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.
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.
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.
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.
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.
Parhelic Circle
A parhelic circle is a rare
optical atmospheric phenomenon.
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.
Partial Solar Eclipse
During a partial solar eclipse,
only part of the solar disk is covered by the New Moon.
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.
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. this is when we experience Red Moon in most occasions
Perihelion
Perihelion is the point closest to
the Sun on Earth's elliptical orbit.
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.
Polar Night
Polar night happens when the
entire Sun remains below the horizon all day. It only happens within the polar
circle, and it is the opposite of polar night is midnight Sun or polar day.
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.
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.
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.
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.
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.
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.
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.
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.
Sundogs
Sundogs are an atmospheric
phenomenon created by light which is reflected or refracted by ice crystals in
the atmosphere.
Sunset
As the upper edge of the solar
disk—called the upper limb—disappears below the horizon, the Sun has set.
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.
When full moon occurs in Apogee where the moon is furthest away from Earth.
Micromoon.
When full moon occurs in Apogee where the moon is furthest away from Earth.
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.
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.
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.
Tropical Year
A tropical year is the time it
takes Earth to complete a full 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).
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.
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.
Zenith
In celestial coordinate systems,
the location straight above you is called zenith while the point exactly below
you are referred to as nadir.
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
