SkyEye

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

almucantar

a circle of constant altitude, parallel to the horizon

altitude

the angular distance of an object above or below the horizon

Angstrom (Å)

often used in the past to measure the wavelength of light and equal to 10⁻¹⁰ meters; now largely superceded by the nanometer (10⁻⁹ meters)

angular distance

the angle between two sight lines connecting the observer with two celestial objects and usually measured in degrees, arc-minutes or arc-seconds; also called angular separation, apparent distance and apparent separation

angular separation

see angular distance

angular size

the angle between two sight lines connecting the observer with the two opposite sides of a celestial object and usually measured in degrees, arc-minutes or arc-seconds; also called apparent size

anti-transit

the crossing of a planet behind the Sun (or exoplanet behind its star), as seen from Earth (see transit); sometimes called a secondary eclipse

aphelion (plural: aphelia)

the outermost point in a solar orbit

apogee

the outermost point in a terrestrial orbit

apparent distance

see angular distance

apparent separation

see angular distance

apparent magnitude (m)

the magnitude of a celestial object as seen by the observer

apparent size

see angular size

apparition

the period during which a planet visible, beginning and ending with a conjunction, either inferior or superior

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 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

black hole

a region in space with a gravitational field so intense that neither matter nor radiation can escape; primordial black holes formed at the beginning of the universe whereas stellar black holes are the collapsed remains of supermassive stars which have exploded as supernovae

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

Earth's equator projected onto the celestial sphere

celestial object

an object in space which appears in Earth's sky

celestial poles

Earth's north and south poles projected onto the celestial sphere

celestial sphere

an imaginary sphere centred on Earth upon which are attached the stars and other celestial objects

circumpolar

describing an object sufficiently close to a celestial pole so that it never sets as seen from the observer's position

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. inferior conjunction or superior conjunction
2. when two bodies share the same ecliptic longitude
3. when two bodies share the same right ascension
4. (informally) when two bodies are close together in the sky (see appulse)

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

culmination

the instant when a celestial body crosses the observer's local meridian

D

declination

the angular distance north or south of the celestial equation and measured in degrees °, minutes ′ and seconds ″

deep sky object

a celestial object outside the solar system such as a nebula, star cluster and galaxy

dichotomy

the phase of the Moon or inferior planet when it appears exactly half illuminated

direct motion

eastward motion in the sky; also called prograde motion

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, 24 August 2006)

E

Earthshine

sunlight reflected from 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

the obscuring of the light of one celestial body by the passage of another; see lunar eclipse and 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

electromagnetic spectrum

the range of all electromagnetic radiation, from radio waves (long wavelengths) to microwaves to infrared radiation to visible light to ultraviolet radiation to X-rays to gamma rays (short wavelengths)

elongation

the angular distance of a body from the Sun as seen from Earth; an inferior planet at greatest elongation east is seen in the evening sky and an inferior planet at greatest elongation west is seen in the morning sky

ephemeris (plural: ephemerides)

a table of predicted positions of celestial bodies arranged by date; typically tables of the positions of the Sun, Moon, planets and other solar system bodies

equinox

1. the two points where the ecliptic intersects the celestial equator
2. when the Sun appears on the celestial equator

exoplanet

extrasolar planet; a planet in orbit around a star other than the Sun

F

falling star

see meteor

fireball

see bolide

First Point of Aries

the ascending node of the ecliptic and the zero point for several celestial coordinate systems

first quarter

a phase of the Moon when it 90° east of the Sun and is exactly half illuminated as seen from Earth

full

a phase of the Moon when it is completely illuminated as seen from Earth

G

galaxy

a gravitationally bound collection of millions or billions of stars, gas, and dust

geocentric

having Earth as the centre

gibbous

a phase of the Moon or other planetary object when it is more than half but less than fully illuminated as seen by the observer

globular cluster

a spherical gravitationally-bound cluster of older stars usually found in the halos of galaxies

H

heliocentric

having the Sun as the centre

I

inclination

the angle of the orbital plane of a solar system body with respect to the plane of the ecliptic; if the angle is greater than 90°, then the orbit is said to be retrograde

inferior conjuction

when an inferior planet passes between the Sun and Earth so that they are lined up in the order "Sun, inferior planet, Earth"; the longitude of the inferior planet is 0° at this instant

inferior planet

a planet whose orbit is inside 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

J

Julian Date

time measured in days since noon on 1 January 4713 BCE; often used in astronomical calculations involving time intervals

L

last quarter

a phase of the Moon when it 90° west of the Sun and is exactly half illuminated as seen from Earth

latitude

the angular distance of a solar system body above or below the ecliptic

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

longitude

the angular distance of a solar system body along the ecliptic from a pre-determined zero point called the First Point of Aries

lunar

with respect to the Moon

lunar eclipse

occurs when the Moon passes through the shadow cast by Earth; the three types of lunar eclipses are penumbral, partial and total

lunation

a lunar month; the time it takes for the Moon to pass through all of its phases, beginning and ending with new Moon; the Brown lunation number system begins with lunation 1 at the first new Moon of 1923

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

a great circle crossing the celestial sphere 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 Earth's atmosphere and burns up; a falling star or shooting star

meteorite

a meteoroid that enters 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 the observer (directly opposite the zenith)

nebula (plural: nebulae)

a collection of gas and/or dust; types of nebulae include diffuse nebulae (emission, reflection), dark nebulae, planetary nebulae and supernova remnants

neutron star

a small, dense object comprised primarily of tightly packed neutrons and thought to be the remains of a massive star after it has gone supernova

new

a phase of the Moon when it is unlit as seen from Earth

node

the two points of the orbit of a solar system body where it intersects the plane of the ecliptic; the ascending node is where the solar system body cuts through the ecliptic plane south to north and the descending node is where the solar system body cuts through the ecliptic plane north to south

nova (plural: novae)

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

occultation

the temporary obscuring of one celestial body, such as a star, by another, such as the Moon

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 Earth passes between the Sun and a superior planet so that they are lined up in the order "Sun, Earth, superior planet"; the longitude of the superior planet is 180° at this instant

orbit

the path of one celestial object around another under the influence of gravity

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 (plural: perihelia)

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, 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

see direct motion

proper motion

the apparent angular motion across the sky of an object relative to the solar system

pulsar

a rapidly rotating neuton star which emits regular bursts of radiation

Q

quadrature

when a superior planet appears at right angles to the Sun as seen from 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 from a pre-determined zero point call the First Point of Aries; 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 larger body

secondary eclipse

see anti-transit

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 (official IAU definition dated 24 August 2006); examples include most asteroids, trans-Neptunian objects and comets

solar

with respect to the Sun

solar eclipse

occurs when the Moon occults the Sun and casts a shadow on Earth; the four types of solar eclipses are partial, annular, total and hybrid

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 produced through nuclear fusion

star cluster

a group of stars such as a globular cluster or an open cluster

stationary point

the point where a moving body appears to stop its apparent movement and change direction; the point when a solar system body changes from direct to retrograde motion or vice versa

superior conjuction

when a planet passes behind the Sun as seen from Earth so that they are lined up in the order "planet, Sun, Earth"; the longitude of the planet is 0° at this instant; the term conjunction is often used when the planet involved is a superior planet

superior planet

a planet whose orbit is outside Earth's orbit around the Sun, namely, Mars, Jupiter, Saturn, Uranus and Neptune

supernova (plural: supernovae)

an exploding star which temporarily outshines the galaxy

synodic

with respect to the alignment of three bodies, usually Earth, the Sun and a third body such as the Moon or a planet

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 local meridian
2. the crossing of one celestial body (such as a planet) in front of another (such as a star), as seen from Earth (see anti-transit)

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, and may be due to intrinsic variations in the star's luminosity or obscuration by orbiting bodies

W

waning

getting smaller

waxing

getting larger

white dwarf

a small, dense object which forms when a low-mass star ceases nuclear fusion and loses its atmosphere as a planetary nebula

Z

zenith

the point directly overhead the observer on the celestial sphere

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