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A
Absolute magnitude
The apparent magnitude that a star would have if it could be
observed from a standard distance of 10 parsecs (32.6 light
years).
Altazimuth mounting for a telescope
A mounting on which the telescope may swing freely in any direction.
Altitude
The angular distance of a celestial body above the horizon.
Aphelion
The furthest distance of a planet or other body from the Sun in its
orbit.
Apogee
The furthest point of the Moon from the Earth in its orbit.
Apparent magnitude
The apparent brightness of a celestial body. The lower the magnitude,
the brighter the object: thus the Sun is approximately -27, the Pole
Star +2, and the faintest stars detectable by modern techniques around
+29.
Asteroids
One of the names for the minor planet swarm.
Astronomical unit
The mean distance between the Earth and the Sun. It is equal to
149,598,500km.
B
Binary
star
A stellar system made up of two stars, genuinely associated, and moving
round their common centre of gravity. The revolution periods range from
millions of years for very widely-separated visual pairs down to less
than half an hour for pairs in which the components are almost in
contact with each other. With very close pairs, the components cannot be
seen separately, but may be detected by spectroscopic methods.
Black hole
A region round a very small massive collapsed star from which not even
light can escape.
C
Cassegrain
reflector
A reflecting telescope in which the secondary mirror is convex; the
light is passed back through a hole in the main mirror. Its main
advantage is that it is more compact than the Newtonian reflector.
Celestial sphere
An imaginary sphere surrounding the Earth, whose centre is the same as
that of the Earth’s globe.
Cepheid
A short-period variable star, very regular in behaviour; the name
comes from the prototype star, Delta Cephei. Cepheids are astronomically
important because there is a definite law linking their variation
periods with their real luminosities, so that their distances may be
obtained by sheer observation.
Circumpolar star
A star which never sets. For instance, Ursa Major (the Great Bear) is
circumpolar as seen from England; Crux Australis (the Southern Cross) is
circumpolar as seen from New Zealand.
Conjunction
(1) A planet is said to be in conjunction with a star, or with another
planet, when the two bodies are apparently close together in the sky.
(2) For the inferior planets, Mercury and Venus, inferior conjunction
occurs when the planet is approximately between the Earth and the Sun;
superior conjunction, when the planet is on the far side of the Sun and
the three bodies are again lined up. Planets beyond the earth’s orbit
can never come to inferior conjunction, for obvious reasons!
D
Declination
The angular distance of a celestial body north or south of the celestial
equator. It corresponds to latitude on the earth.
Dewcap
An open tube fitted to the upper end of a refracting telescope. Its role
is to prevent condensation upon the object-glass.
Double star
A star made up of two components - either genuinely associated (binary
systems) or merely lined up by chance (optical pairs).
E
Ecliptic
The apparent yearly path of the Sun among the stars. It is more
accurately defined as the projection of the Earth’s orbit on to the
celestial sphere.
Equator, celestial
The projection of the Earth’s equator on to the celestial sphere.
Equatorial mounting for a telescope
A mounting in which the telescope is set up on an axis which is parallel
with the axis of the Earth. This means that one movement only (east to
west) will suffice to keep an object in the field of view.
Extinction
The apparent reduction in brightness of a star or planet when low down
in the sky, so that more of its light is absorbed by the Earth’s
atmosphere. With a star 1° above the horizon, extinction amounts to 3
magnitudes.
Eyepiece
(Or Ocular) The lens, or combination of lenses, at the eye-end of a
telescope. It is responsible for magnifying the image of the object
under study. With a positive eyepiece (e.g. Ramsden, Orthoscopic or
Monocentric) the image plane lies between the eyepiece and the
object-glass (or main mirror); with a negative eyepiece (such as a
Huyghenian or Tolles) the image plane lies inside the eyepiece. A Barlow
lens is concave, and is mounted in a short tube which may be placed
between the eyepiece and the object-glass (or mirror). It increases the
effective focal length of the telescope, thereby providing increased
magnification.
F
Finder
A small, wide-field telescope attached to a larger one, used for
sighting purposes.
Flares, solar
Brilliant eruptions in the outer part of the Sun’s atmosphere.
Normally they can be detected only by spectroscopic means (or the
equivalent), though a few have been seen in integrated light. They are
made up of hydrogen, and emit charged particles which may later reach
the Earth, producing magnetic storms and displays of auroræ. Flares are
generally, though not always, associated with sunspot groups.
Flare
stars
Faint Red Dwarf stars which show sudden, short-lived increases in
brilliancy, due possibly to intense flares above their surfaces.
G
Galaxies
Systems made up of stars, nebulæ, and interstellar matter. Many, though
by no means all, are spiral in form.
Gibbous phase
The phase of the Moon or planet when between half and full.
H
.
Hubble’s constant
The rate of increase in the recession of a galaxy with increased
distance from the Earth.
I
Inferior
planets
Mercury and Venus, whose distances from the Sun are less than that of
the Earth.
J
Julian day
A count of the days, starting from 12 noon on 1 January 4713 BC.
K
L
Light-year
The distance travelled by light in one year: 9.4607 million million
kilometres.
Lunation
The interval between successive new moons: 29d 12h 44m.
M
Maksutov
telescope
An astronomical telescope involving both mirrors and lenses.
Mass
The quantity of matter that a body contains. It is not the same as
‘weight’.
Meteor
A small particle, friable in nature and usually smaller than a sand
grain, moving round the Sun, and visible only when it enters the upper
atmosphere and is destroyed by friction. Meteors may be regarded as
cometary debris.
Meteorite
A larger object, which may fall to the ground without being destroyed in
the upper atmosphere. A meteorite is fundamentally different from a meteor.
Meteorites are not associated with comets, but may be closely related to
asteroids.
Micrometeorite
A very small particle of interplanetary material, too small to cause a
luminous effect when it enters the Earth’s upper atmosphere.
N
Nebula
A cloud of gas and dust in space. Galaxies were once known as ‘spiral
nebulæ’ or ‘extragalactic nebulæ’.
Neutron star
The remnant of a very massive star which has exploded as a supernova.
Neutron stars send out rapidly-varying radio emissions, and are
therefore called ‘pulsars’. Only two (the Crab and Vela pulsars)
have as yet been identified with optical objects.
Newtonian reflector
A reflecting telescope in which the light is collected by a main mirror,
reflected on to a smaller flat mirror set at an angle of 45º, and
thence to the side of the tube.
Nova
A star which suddenly flares up to many times its normal brilliancy,
remaining bright for a relatively short time before fading back to
obscurity.
O
Occultation
The covering-up of one celestial body by another.
Opposition
The position of a planet when exactly opposite to the Sun in the sky;
the Sun, the Earth and the planet are then approximately lined up.
Orbit
The path of a celestial object.
P
Perigee
The position of the Moon in its orbit when closest to the Earth.
Perihelion
The position in orbit of a planet or other body when closest to the Sun.
Phases
The apparent changes in shape of the Moon and the inferior planets from
new to full. Mars may show a gibbous phase, but with the other
planets there are no appreciable phases as seen from Earth.
Planetary nebula
A small, dense, hot star surrounded by a shell of gas. The name is
ill-chosen, since planetary nebulæ are neither planets nor nebulæ!
Poles, celestial
The north and south points of the celestial sphere.
Precession
The apparent slow movement of the celestial poles. This also
means a shift of the celestial equator, and hence of the equinoxes; the
vernal equinox moves by 50 sec or arc yearly, and has moved out
of Aries into Pisces. Precession is due to the pull of the Moon and Sun
on the Earth’s equatorial bulge.
Proper motion, stellar
The individual movement of a star on the celestial sphere.
Q
Quasar
A very remote, superluminous object. Quasars are now known to be the
cores of very active galaxies, though the source of their energy is
still a matter for debate.
R
Radial
Velocity
The movement of a celestial body toward or away from the observer;
positive if receding, negative if approaching.
Radiant
The point in the sky from which the meteors of any particular shower
seem to radiate.
Right ascension
The angular distance of a celestial body from the vernal equinox,
measured eastward. It is usually given in hours, minutes and seconds of
time, so that the right ascension is the time-difference between the
culmination of the vernal equinox and the culmination of the
body.
S
Scintillation
Twinkling of a star; it is due to the Earth’s atmosphere. Planets may
also show scintillation when low in the sky.
Spectroscopic binary
A binary system whose components are too close together to be seen
individually, but which can be studied by means of spectroscopic
analysis.
Spherical aberration
Blurring of a telescope image; it is due to the fact that the lens (or
mirror) does not bring the light-rays falling on its edge and on its
centre to exactly the same focal point.
Superior planets
All the planets lying beyond the orbit of the Earth in the Solar system
(that is to say, all the principal planets apart from Mercury and
Venus).
Supernova
A colossal stellar outburst, involving (1) The total destruction of the
white dwarf member of a binary system, or (2) The collapse of a very
massive star.
T
Terminator
The boundary between the day-and-night-hemispheres of the Moon or a
planet.
U
V
Variable stars
Stars which change in brilliancy over short periods. They are of various
types.
Variation
An inequality in the Moon’s motion, due to the fact that the pull of
the Sun on the Moon is not constant for all positions in the lunar
orbit.
W
White dwarf
A very small, very dense star which has used up its nuclear energy, and
is in a very late stage of its evolution.
Wolf-Rayet stars
Very hot, greenish-white stars which are surrounded by expanding gaseous
envelopes. Their spectra show bright (emission) lines.
X
Y
Z
Zenith
The observer’s overhead point (altitude 90º).
Zodiac
A belt stretching round the sky, 8º to either side of the ecliptic,
in which the Sun, Moon and principal planets are to be found at any
time. (Pluto is the only planet which can leave the Zodiac, though many
asteroids do so.)
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