Penumbra: (1) The comparatively light surrounding
parts of a sunspot. (2) The area of partial shadow
lying to either side of the main cone of shadow cast
by the Earth. During lunar eclipses, the Moon must
move through the penumbra before reaching the
main shadow (or umbra). Some lunar eclipses are
penumbral only.Periastron: The point of the orbit of a member of a
binary system in which the stars are at their closest to
each other. The most distant point is termed apastron.Perigee:The point in the orbit of the Moon or an
artificial satellite at which the body is closest to the
Earth. The most distant point is the apogee.Perihelion: The point in the orbit of a member of the
Solar System in which the body is at its closest to
the Sun. The most distant point is the aphelion. The
Earth reaches perihelion in early January.Periodic times:Seesidereal period.Perturbations: The disturbances in the orbit of a
celestial body produced by the gravitational pulls of
others.Phases:The apparent changes in shape of the Moon
and some planets depending upon the amount of
the sunlit hemisphere turned towards us. The Moon,
Mercury and Venus show complete phases, from new
(invisible) to full. Mars can show an appreciable phase,
since at times less than 90 per cent of its sunlit face is
turned in our direction. The phases of the outer planets
are insignificant.Photometry: The measurement of the intensity of
light. The device now used for accurate determinations
of star magnitudes is the photoelectric photometer,
which consists of a photoelectric cell used together
with a telescope. (A photoelectric cell is an electronic
device. Light falls upon the cell and produces an
electric current; the strength of the current depends
on the intensity of the light.)Photosphere: The bright surface of the Sun.Planet: A non-luminous body moving round a star. It
is likely that other stars have planetary systems similar
to that of the Sun, but as yet there is no definite proof.Planetarium: An instrument used to show an
artificial sky on the inner surface of a large dome,
and to reproduce celestial phenomena of all kinds.
A planetarium projector is extremely complicated,
and is very accurate. The planetarium is an educational
device, and has become very popular in recent years.
Planetaria have been set up in many large cities all over
the world, and are also used in schools and colleges.Planetary nebula: A faint star surrounded by an
immense ‘shell’ of tenuous gas. More than 300 are
known in our Galaxy. They are so called because their
telescopic appearance under low magnification is
similar to that of a planet.Plasma: A gas consisting of ionized atoms (see ion)
and free electrons, together with some neutral particles.Taken as a whole, it is electrically neutral, and is a
good conductor of electricity.Poles, celestial: The north and south points of the
celestial sphere.Populations, stellar: There are two main types of
star regions. Population Iareas contain a great deal
of interstellar material, and the brightest stars are hot
and white; it is assumed that star formation is still in
progress. The brightest stars in Population II areas are
red giants, well advanced in their evolutionary cycle;
there are almost no hot, white giant stars, and there
is little interstellar material, so that star formation has
apparently ceased. Although no rigid boundaries can
be laid down, it may be said that the arms of spiral
galaxies are mainly of Population I; the central parts
of spirals, as well as elliptical galaxies and globular
clusters, are mainly of Population II.Position angle: The apparent direction of one object
with reference to another measured from the north
point of the main object through east (90 degrees),
south (180 degrees) and west (270 degrees).Precession: The apparent slow movement of the
celestial poles. It is caused by the pull of the Moon and
the Sun upon the Earth’s equatorial bulge. The Earth
behaves rather in the manner of a top which is running
down and starting to topple, but the movement is very
gradual; the pole describes a circle on the celestial
sphere, centred on the pole of the ecliptic, which
is 47 degrees in diameter and takes 25,800 years to
complete. Because of precession, the celestial equator
also moves, and this in turn affects the position of the
First Point of Aries (vernal equinox), which shifts
westwards along the ecliptic by 50 seconds of arc
each year. Since ancient times, this motion has taken
the vernal equinox out of Aries into the adjacent
constellation of Pisces (the Fishes). Our present Pole
Star will not retain its title indefinitely. In AD 12,000,
the north polar star will be the brilliant Vega, in Lyra.Prism: A glass block having flat surfaces inclined to
one another. Light passing through a prism will be split
up, since different colours are refracted by different
amounts.Prominences: Masses of glowing gas, chiefly
hydrogen, above the Sun’s bright surface. They are
visible with the naked eye only during total solar
eclipses, but modern equipment allows them to
be studied at any time. They are of two main types,
eruptive and quiescent.Proper motion: The individual motion of a star on
the celestial sphere.Because the stars are so remote,
their proper motions are slight. The greatest known
is that of Barnard’s Star (a red dwarf at a distance of
6 light-years); this amounts to one minute of arc every
six years, so that it will take 180 years to move by an
amount equal to the apparent diameter of the Moon.
The proper motions of remote stars are too slight to
be measured at all.Proton:A fundamental particle with unit positive
electrical charge. The nucleus of a hydrogen atom
consists of one proton. See also neutron.Pulsar: Aneutron star radio source which does not
emit continuously, but in rapid, very regular pulses.
Their periods are short (often much less than one
second).Purkinje effect: An effect inherent in the human
eye, which makes it less sensitive to light of longer
wavelength when the general level of intensity is low.
Consider two lights, one red and one blue, which are
of equal intensity. If the intensity of both are reduced
by equal amounts, the blue light will appear to be the
brighter of the two.Q
Quadrature: The position of the Moon or a planet
when at right angles to the Sun as seen from Earth.
Thus the Moon is in quadrature when it is seen at
half-phase.Quantum: The smallest amount of light-energy which
can be transmitted at any given wavelength.Quasar: A very remote immensely luminous object,
now known to be the core of a very active galaxy –
possibly powered by a massive black hole inside it.
Quasars are also known as QSOs (Quasi-Stellar
Objects). BL Lacertae objects are of the same type,
though less important.R
Radar astronomy:The technique of using radar
pulses to study astronomical objects. Most planets
and some asteroids have been contacted by radar, and
the radar equipment carried in space probes such as
Magellan has provided us with detailed maps of the
surface of Venus.Radial velocity: The towards-or-away movement of
a celestial body, measured by the Doppler effect in
its spectrum. If the spectral lines are red-shifted, the
object is receding; if the shift is to the blue, the object
is approaching. Conventionally, radial velocity is said
to be positive with a receding body, negative with an
approaching body.Radiant: The point in the sky from which the meteors
of any particular shower appear to radiate (for example,
the August shower has its radiant in Perseus, so that
the meteors are known as the Perseids). The meteors
in a shower are really moving through space in
parallel paths, so that the radiant effect is due merely
to perspective.Radio astronomy: Astronomical studies carried out
in the long-wavelength region of the electromagnetic
spectrum.The main instruments used are known as
radio telescopes; they are of many kinds, ranging from
‘dishes’, such as the 76-metre (250-foot) paraboloid
at Jodrell Bank (Cheshire), to long lines of aerials.Radio galaxies: Galaxies which are extremely
powerful emitters of radio radiation.Red shift: The Doppler displacement of spectral
lines towards the red or long-wave end of the spectrum,
indicating a velocity of recession. Apart from the
members of the Local Group, all galaxies show
red shifts in their spectra.ATLAS OF THE UNIVERSE
Glossary
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