SkyEye

Glossary of Astronomical Terms

A

absolute magnitude (M)
the magnitude of a celestial object would appear if it was exactly 10 parsecs away; a measure of the true luminosity of an object
albedo
surface reflectivity
altitude
the angular distance of an object above or below the horizon
angstrom (Å)
commonly used to measure the wavelength of light; equal to 10-10 meters
aphelion
the outermost point in a solar orbit
apogee
the outermost point in a terrestrial orbit
apparent magnitude (m)
the magnitude of a celestial object as seen by the observer
appulse
the apparent closest approach of two celestial bodies
asterism
a pattern formed by stars in one or more constellations, such as the 'Summer Triangle' or the 'Big Dipper' or the 'Great Square of Pegasus'
asteroid
now called a dwarf planet or small solar-system body, depending on its characteristics
astrology
a belief system which claims that the positions of celestial objects affect or control life on Earth
astronomical unit (AU)
a unit of measurement often used within the solar system; approximately equal to 150,000,000 kilometres, 93,000,000 miles or 8 light minutes, the average distance between the Earth and the Sun
astronomy
the scientific study of celestial objects
axis
an imaginary line about which an object rotates
azimuth
the angular distance of an object around or parallel to the horizon from a predefined zero point

B

Bayer designation
the Greek and Latin letters assigned to stars in a constellation by Johann Bayer in 1603
binary star
two stars gravitationally bound together, orbiting a common centre of mass
blue moon
1. the second full moon to fall within a calendar month
2. the third full moon in a season containing four
bolide
an exceptionally bright meteor; a fireball

C

Cassini's division
the major division in Saturn's rings, between the A- and B-rings
celestial equator
the Earth's equator projected onto the sky
celestial object
an object in space which appears in Earth's sky
celestial poles
the Earth's north and south poles projected onto the sky
celestial sphere
an imaginary sphere centred on the Earth to which are attached the stars and other celestial objects
circumpolar
an object sufficiently close to a celestial pole so that it never sets as seen from the observer's latitude
comet
a small body comprised of a nucleus of dirt and ice; when it nears the Sun, the ice melts, releasing gas and dust, forming a glowing coma around the nucleus and usually a tail pointing away from the Sun.
conjunction
1. when two bodies appear close together in the sky
2. superior conjunction
constellation
an area of sky; originally a grouping of stars in the sky to form some kind of pattern; there are 88 officially recognised constellations
crescent
a phase of the Moon or other planetary object when it is less than half illuminated as seen by the observer

D

declination
the angular distance north or south of the celestial equation and measured in degrees °, minutes ' and seconds "
deep sky object
galaxies, nebulae, star clusters
direct motion
eastward motion in the sky
double star
two stars close together in the sky; they may be a binary star system or simply the result of projection effects on the sky
dwarf planet
a celestial body that
  • is in orbit around the Sun,
  • has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic (nearly round) shape,
  • has not cleared the neighbourhood around its orbit, and
  • is not a satellite
(official IAU definition dated 24 August 2006)

E

Earthshine
sunlight reflected from the Earth which dimly illuminates the unlit part of the Moon
eccentricity
a parameter that describes the shape of an orbit; the closer the eccentricity is to zero, the more circular the orbit
eclipse, lunar
see lunar eclipse
eclipse, solar
see solar eclipse
ecliptic
the path the Sun takes against the background stars; the apparent positions of the Moon and the planets are usually quite close to the ecliptic
elongation
the angular distance of a body from the Sun as seen from the Earth; a planet at greatest eastern elongation is seen in the evening sky and a planet at greatest western elongation is seen in the morning sky
ephemeris
a table of data arranged by date; typically tables of the positions of the Sun, Moon, planets and other solar system objects
equinox
1. the two points where the ecliptic intersects the celestial equator
2. when the Sun appears on the celestial equator

F

fireball
see bolide
first quarter
a phase of the Moon or other planetary object when it is exactly half illuminated as seen by the observer; occurs between the new and full phases
full
a phase of the Moon or other planetary object when it is completely illuminated as seen by the observer; occurs when the object is at opposition

G

galaxy
a gravitationally bound collection of millions or billions of stars, gas, and dust
geocentric
having the Earth as the centre
gibbous
a phase of the Moon or other planetary object when it is more than half illuminated as seen by the observer
globular cluster
a spherical cluster of older stars found in the halos of galaxies

H

heliocentric
having the Sun as the centre

I

inferior conjuction
when an inferior planet passes between the Sun and the Earth so that they are lined up in the order Sun, inferior planet, Earth
inferior planet
a planet whose orbit is inside the Earth's orbit around the Sun, namely, Mercury and Venus
International Astronomical Union (IAU)
a professional group founded in 1919 to promote the science of astronomy through education and research, define fundamental astronomical and physical constants, and act as the sole authority for assigning names to celestial bodies

L

last quarter
a phase of the Moon or other planetary object when it is exactly half illuminated as seen by the observer; occurs between the full and new phases
light year (ly)
a unit of measurement often used for interstellar or intergalactic distances; the distance light travels in a vacuum during a year; approximately equal to 9.5 × 1012 kilometres or 6 × 1012 miles
limb
the edge of the apparent disc of a celestial object
lunar eclipse
occurs when the Moon passes through the shadow cast by the Earth; the three types of lunar eclipses are partial, penumbral and total
lunation
the cycle from new Moon to new Moon

M

magnitude
brightness scale; the smaller the magnitude number, the brighter the object so that the Sun (magnitude -26) is much brighter than Uranus (magnitude +6), see absolute magnitude and apparent magnitude
meridian
an imaginary line connecting a point due south on the horizon with a point due north on the horizon and going through the zenith
meteor
the trail of light left when a meteoroid enters the Earth's atmosphere and burns up; a shooting star
meteorite
a meteoroid that enters the Earth's atmosphere but survives to hit the ground
meteoroid
a chunk of space debris
meteor shower
the appearance of many meteors in a short space of time, all radiating from a common point
Milky Way
the galaxy in which we reside
minor planet
now called a dwarf planet or small solar-system body, depending on its characteristics

N

nadir
the point directly underneath
nebula
a collection of gas and/or dust
new
a phase of the Moon or other planetary object when it is unlit as seen by the observer; occurs when the object is at inferior conjunction
nova
a star which temporarily increases in brightness; thought to be caused by the inflow of matter from a star to its white dwarf companion

O

obliquity
the angle of tilt of a planet's axis of rotation
open cluster
a collection of young stars that formed together; they may or may not be still gravitationally bound; the youngest clusters are still embedded in the gas and dust out of which they formed
opposition
when the Earth passes between the Sun and a superior planet so that they are lined up in the order Sun, Earth, superior planet
orbit
the path of a celestial body through space

P

parallax
the difference in apparent direction of an object as seen from two different vantage points
parsec (pc)
a unit of measurement often used for interstellar or intergalactic distances; approximately equal to 3.2616 light years
penumbra
the incomplete part of a shadow surrounding the umbra
perigee
the innermost point of a terrestrial orbit
perihelion
the innermost point of a solar orbit
phase
the fracton of the illuminated part of the Moon or other planetary object as seen by the observer
phases of the Moon
in order: new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning crescent
planet
a celestial body that
  • is in orbit around the Sun,
  • has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
  • has cleared the neighbourhood around its orbit
(official IAU definition dated 24 August 2006)
plutoid
a celestial body that
  • is in orbit around the Sun at a semi-major axis greater than that of the planet Neptune,
  • has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic (nearly round) shape,
  • has not cleared the neighbourhood around its orbit, and
  • is not a satellite of a plutoid
(official IAU definition dated 11 June 2008)
prograde
direct motion
proper motion
the apparent angular motion across the sky of an object relative to the solar system

Q

quadrature
when a superior planet appears at right angles to the Sun as seen from the Earth

R

radiant
the point in the sky from which meteor showers appear to originate
retrograde
westward motion in the sky
revolution
the movement in an orbit around another body
right ascension
the angular distance around the sky parallel to the celestial equator; measured in hours h, minutes m and seconds s
rotation
the spin on a body's axis

S

satellite
a small body, either natural or artificial, in orbit around a planet
sidereal
with respect to the stars
shooting star
see meteor
small solar-system body
any object except a planet, dwarf planet or satellite which is orbiting the Sun; examples include most asteroids, trans-Neptunian objects and comets (official IAU definition dated 24 August 2006)
solar eclipse
occurs when the Moon occults the Sun and casts a shadow on the Earth; the three types of solar eclipses are annular, partial and total
solar system
the Sun and all the bodies which orbit it
solstice
when the Sun appears furthest north or south of the celestial equator
star
a ball of gas which radiates energy
superior conjuction
when a planet passes behind the Sun as seen from the Earth so that they are lined up in the order planet, Sun, Earth; the term conjunction is often used when the planet involved is a superior planet
superior planet
a planet whose orbit is outside the Earth's orbit around the Sun, namely, Mars, Jupiter, Saturn, Uranus and Neptune
supernova
an exploding star which temporarily outshines the galaxy
synodic
with respect to the alignment of three bodies, usually the Earth, the Sun and a third body such as the Moon

T

terminator
the boundary between the illuminated and dark areas of the Moon or other planetary body
train
a glowing path left in the wake of a passing meteor
trans-Neptunian object
dwarf planets or small solar-system bodies orbiting the Sun beyond the planet Neptune; Pluto was the first one discovered
transit
1. the crossing of a celestial body over the meridian
2. the crossing of a celestial body in front of another

U

umbra
the total part of a shadow
Universal Time (UT)
local time on the Greenwich meridian

V

variable star
a star whose apparent brightness changes over time; this change may regular or irregular

W

waning
getting smaller
waxing
getting larger

Z

zenith
the point directly overhead
zenithal hourly rate (ZHR)
the theoretical maximum number of meteors an ideal observer would see under perfect conditions with the meteor shower radiant directly overhead
zodiac
the twelve constellations (not including Ophiuchus) through which the ecliptic passes: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius and Pisces