Sun at inferior conjunction. Transits of Mercury are
quite frequent (e.g. one in 1973 and the next in 1986),
but the next transit of Venus will not occur until 2004;
the last took place in 1882. Similarly, a satellite of a
planet is said to be in transit when it is seen against
the planet’s disk. Transits of the four large satellites
of Jupiter may be seen with small telescopes; also
visible are shadow transits of these satellites, when
the shadows cast by the satellites are seen as black
spots on the face of Jupiter.Transit instrument: A telescope which is specially
mounted; it can move only in elevation, and always
points to the meridian. Its sole use is to time the
moments when stars cross the meridian, so providing
a means of checking the time. The transit instrument
set up at Greenwich Observatory by Sir George Airy,
in the 19th century, is taken to mark the Earth’s prime
meridian (longitude zero degrees). Although still in
common use it is likely that they will become obsolete
before long.Trojans: Asteroidswhich move around the Sun at
a mean distance equal to that of Jupiter. One group
of Trojans keeps well ahead of Jupiter and the other
group well behind, so that there is no danger of
collision. Hundreds of Trojans are now known.Troposphere: The lowest part of the Earth’s
atmosphere, reaching to an average height of about
11 kilometres above sea-level. It includes most of the
mass of the atmosphere, and all the normal clouds lie
within it. Above, separating the troposphere from the
stratosphere, is the tropopause.Twilight, astronomical: The state of illumination of
the sky when the Sun is below the horizon, but by less
than 18 degrees.Twinkling:Common term for scintillation.U
Ultra-violet radiation:Electromagnetic radiation
which has a wavelength shorter than that of violet
light, and so cannot be seen with the naked eye. The
ultra-violet region of the electromagnetic spectrum
lies between visible light and X-radiation. The Sun is
a very powerful source of ultra-violet, but most of this
radiation is blocked out by layers in the Earth’s upper
atmosphere – which is fortunate for us, since in large
quantities ultra-violet radiation is lethal. Studies of the
ultra-violet radiations emitted by the stars have to be
carried out by means of instruments sent up in rockets
or artificial satellites.Umbra:(1) The dark inner portion of a sunspot. (2) The
main cone of shadow cast by a planet or the Moon.Universal time:The same as Greenwich Mean Time.V
Van Allen Zones (or Van Allen Belts):Zones around
the Earth in which electrically charged particles are
trapped and accelerated by the Earth’s magnetic field.
They were detected by J. Van Allen and his colleagues
in 1958, from results obtained with the first successful
US artificial satellite, Explorer 1. Apparently there are
two main belts. The outer, made up mainly of electrons,is very variable, since it is strongly affected by events
taking place in the Sun; the inner zone, composed
chiefly of protons, is more stable. On the other hand, it
may be misleading to talk of two separate zones; it may
be that there is one general belt whose characteristics
vary according to distance from the Earth. The Van
Allen radiation is of great importance in all geophysical
research, and probably represents the major discovery
of the first years of practical astronautics.Variable stars: Stars which fluctuate in brightness
over short periods of time.Variation:(1) An inequality in the motion of the Moon,
due to the fact that the Sun’s pull on it throughout its
orbit is not constant in strength. (2) Magnetic variation:
the difference, in degrees, between magnetic north
and true north. It is not the same for all places on the
Earth’s surface, and it changes slightly from year to
year because of the wandering of the magnetic pole.Vernal Equinox: SeeFirst Point of Aries.Vulcan:The name given to a hypothetical planet
once believed to move around the Sun at a distance
less than that of Mercury. It is now certain that Vulcan
does not exist.W
White dwarf:A very small, extremely dense star. The
atoms in it have been broken up and the various parts
packed tightly together with almost no waste space,
so that the density rises to millions of times that of
water; aspoonful of white dwarf material would weigh
many tonnes. Evidently a white dwarf has used up
all its nuclear ‘fuel’; it is in the last stages of its active
career, and has been aptly described as a bankrupt
star. Neutron stars are even smaller and denser than
white dwarfs.Widmanstätten patterns:If an iron meteoriteis
cut, polished and then etched with acid, characteristic
figures of the iron crystals appear. These are known as
Widmanstätten patterns. They are never found except
in meteorites.Wolf-Rayet stars:Exceptionally hot, greenish-white
stars whose spectra contain bright emission lines as
well as the usual dark absorption lines. Their surface
temperature may approach 100,000 degrees C, and they
seem to be surrounded by rapidly expanding envelopes
of gas. Attention was first drawn to them in 1867 by the
astronomers Wolf and Rayet, after whom the class is
named. Recently, it has been found that many of the
Wolf-Rayet stars are spectroscopic binaries.X
X-rays: Electromagnetic radiations of very short
wavelength. There are many X-ray sources in the sky;
studies of them must be undertaken by space research
methods.X-ray astronomy:X-rays are very short
electromagnetic radiations, with wavelengths of
from 0.1 to 100 Ångströms.Since X-rays from space
are blocked by the Earth’s atmosphere, astronomical
researches have to be carried out by means of
instruments taken up in rockets. The Sun is a sourceof X-rays; the intensity of the X-radiation is greatly
enhanced by solar flares. Sources of X-rays outside
the Solar System were first found in 1962 by
American astronomers, who located two sources,
one in Scorpius and the other in Taurus; the latter has
now been identified with the Crab Nebula. Since then,
various other X-ray sources have been discovered,
some of which are variable.Y
Year:The time taken for the Earth to go once around
the Sun; in everyday life it is taken to be 365 days
(366 days in Leap Year). (1) Sidereal year: The true
revolution period of the Earth: 365.26 days, or 365 days
6 hours 9 minutes 10 seconds. (2) Tropical year: The
interval between successive passages of the Sun across
the First Point of Aries. It is equal to 365.24 days,
or 365 days 5 hours 48 minutes 45 seconds. The
tropical year is about 20 minutes shorter than the
sidereal year because of the effects of precession,
which cause a shift in the position of the First Point
of Aries. (3) Anomalistic year: The interval between
successive perihelionpassages of the Earth. It is
equal to 365.26 days, or 365 days 6 hours 13 minutes
53 seconds. It is slightly longer than the sidereal year
because the position of the perihelion point moves
by about 11 seconds of arc annually. (4) Calendar year:
The mean length of the year according to the Gregorian
calendar. It is equal to 365.24 days, or 365 days 5 hours
49 minutes 12 seconds.Z
Zenith:The observer’s overhead point (altitude
90 degrees).Zenith distance:The angular distance of celestial
body from the observer’s zenith.Zodiac:A belt stretching right round the sky, 8 degrees
to either side of the ecliptic, in which the Sun, Moon
and bright planets are always to be found. It passes
through 13 constellations, the 12 commonly known
as the Zodiacal groups plus a small part of Ophiuchus
(the Serpent-bearer).Zodiacal constellations: The 12 constellations
used in astrology. They are Aquarius, Aries, Cancer,
Capricornus, Gemini, Leo, Libra, Pisces, Sagittarius,
Scorpius, Taurus and Virgo.Zodiacal light:A cone of light rising from the horizon
and stretching along the ecliptic. It is visible only when
the Sun is a little way below the horizon, and is best
seen on clear, moonless evenings or mornings. It is
thought to be due to small particles scattered near the
main plane of the Solar System. A fainter extension
along the ecliptic is known as the Zodiacal band.GLOSSARY
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