ATLAS OF THE UNIVERSE
D
Day:In everyday language, a day is the amount of
time it takes for the Earth to spin once on its axis.
A sidereal day (see sidereal time) is the rotation
period measured with reference to the stars (23 hours
56 minutes 4.091 seconds). Asolar day is the time
interval between two successive noons; the length
of the mean solar day is 24 hours 3 minutes 56.555
seconds – rather longer than the sidereal day, since
the Sun is moving eastwards along the ecliptic.The
civil dayis, of course, taken to be 24 hours.Declination: The angular distance of a celestial body
north or south of the celestial equator. It may be said
to correspond to latitude on the surface of the Earth.Density:The mass of a substance per unit volume.
Taking water as one, the density of the Earth is 5.5.Dichotomy: The exact half-phase of Mercury, Venus
or the Moon.Diffraction rings: Concentric rings surrounding the
image of a star as seen in a telescope. They cannot
be eliminated, since they are due to the wave-motion
of light. They are most evident in small instruments.Direct motion: Bodies which move around the Sun
in the same sense as the Earth are said to have direct
motion.Those which move in the opposite sense have
retrograde motion. The term may also be applied to
satellites of the planets. No planet or asteroid with
retrograde motion is known, but there are various
retrograde satellites and comets. The terms are also
used with regard to the apparent movements of the
planets in the sky. When moving eastwards against
the stars, the planet has direct motion; when moving
westwards, it is retrograding.Diurnal motion: The apparent daily rotation of the
sky from east to west. It is due to the real rotation of
the Earth from west to east.Doppler effect: The apparent change in the
wavelength of light caused by the motion of the
observer. When a light-emitting body is approaching
the Earth, more light-waves per second enter the
observer’s eye than would be the case if the object
were stationary; therefore, the apparent wavelength
is shortened, and the light seems ‘too blue’. If the object
is receding, the wavelength is apparently lengthened,
and the light is ‘too red’. For ordinary velocities the
actual colour changes are very slight, but the effect
shows up in the spectrum of the object concerned. If
the dark lines are shifted towards the red or long-wave,
the object must be receding; and the amount of the
shift is a key to the velocity of recession. Apart from
the galaxies in our Local Group, all external systems
show red shifts, and this is the observational proof
that the universe is expanding. The Doppler principle
also applies to radiations at radio wavelengths.Double star: A star which is made up of two
components. Some doubles are optical; that is to say,
the components are not truly associated, and simply
happen to lie in much the same direction as seen from
Earth. Most double stars, however, are physically
associated or binary systems.E
Earthshine: The faint luminosity of the night
hemisphere of the Moon, due to light reflected on
to the Moon from the Earth.Eclipses: These are of two kinds: solar and lunar.
(1) A solar eclipse is caused by the Moon passing in
front of the Sun. By coincidence, the two bodies appear
almost equal in size. When the alignment is exact, the
Moon covers up the Sun’s bright disk for a brief period,
either totally or partially (never more than about eight
minutes; usually much less). When the eclipse is total
the Sun’s surroundings – the chromosphere, corona
and prominences– may be seen with the naked eye
(though you should never look directly at the Sun).
If the Sun is not fully covered, the eclipse is partial,
and the spectacular phenomena of totality are not seen.
If the Moon is near its greatest distance from the Earth
(see apogee) it appears slightly smaller than the Sun,
and at central alignment a ring of the Sun’s disk is
left showing around the body of the Moon; this is an
annular eclipse, and again the phenomena of totality
are not seen. (2) A lunar eclipse is caused when the
Moon passes into the shadow cast by the Earth; it may
be either total or partial. Generally, the Moon does not
vanish, as some sunlight is refracted on to it by way of
the ring of atmosphere surrounding the Earth.Eclipsing binary (or Eclipsing variable): A binary
star made up of two components moving around
their common centre of gravity at an angle such that,
as seen from the Earth, the components mutually
eclipse each other. In the case of the eclipsing binary
Algol, one component is much brighter than the other;
every 2^1 / 2 days the fainter star covers up the brighter,
and the star seems to fade by more than a magnitude.Ecliptic:The projection of the Earth’s orbit on to the
celestial sphere. It may also be defined as ‘the apparent
yearly path of the Sun against the stars’, passing
through the constellations of the Zodiac. Since the
plane of the Earth’s orbit is inclined to the equator by
23.5 degrees, the angle between the ecliptic and the
celestial equator must also be 23.5 degrees.Ecosphere: The region around the Sun in which the
temperatures are neither too hot nor too cold for life
to exist under suitable conditions. Venus lies near
the inner edge of the ecosphere; while Mars is near the
outer edge. The ecospheres of other stars will depend
upon the luminosities of the stars concerned.Electromagnetic spectrum: The full range of what
is termed electromagnetic radiation: gamma-rays,
X-rays, ultra-violet radiation, visible light, infra-red
radiation and radio waves. Visible light makes up
only a very small part of the whole electromagnetic
spectrum. Of all the radiations, only visible light and
some of the radio waves can pass through the Earth’s
atmosphere and reach ground-level.Electron:A fundamental particle carrying unit
negative charge of electricity; the orbital components
of the atom.Electron density: The number of free electrons in
unit volume of space. A free electron is not attached
to any particular atom, but is moving independently.Element:A substance which cannot be chemically
split up into simpler substances; 92 elements are
known to exist naturally on the Earth and all other
substances are made up from these fundamental- Various extra elements have been made artificially,
all of which are heavier than uranium (number 92 in the
natural sequence) and most of which are very unstable.
Elongation: The apparent angular distance of a planet
from the Sun, or of a satellite from its primary planet.Emission spectrum: A spectrum consisting of bright
lines or bands. Incandescent gases at low density yield
emission spectra.Ephemeris: A table giving the predicted positions of
a moving celestial body, such as a planet or a comet.Epoch: A date chosen for reference purposes in
quoting astronomical data. For instance, some star
catalogues are given for ‘epoch 1950’; by the year 2000
the given positions will have changed slightly because
of the effects ofprecession.Equation of time: The Sun does not move among
the stars at a constant rate, because the Earth’s orbit
is not circular. Astronomers therefore make use of a
mean sun, which travels among the stars at a speed
equal to the average speed of the real Sun. The interval
by which the real Sun is ahead of or behind the mean
sun is termed the equation of time.It can never exceed
17 minutes; four times every year it becomes zero.Equator, celestial: The projection of the Earth’s
equator on to the celestial sphere divides the sky
into two equal hemispheres.Equatorial mount: A telescope mounting in which
the instrument is set upon an axis which is parallel
to the axis of the Earth; the angle of the axis must be
equal to the observer’s latitude. This means that to
keep an object in view the telescopes were equatorially
mounted, but with the aid of modern computers it has
become possible to make effective drives for altazimuth
telescopes; altazimuth is now the favoured mounting.Equinox: Twice a year the Sun crosses the celestial
equator, once when moving from south to north
(about 21 March) and once when moving from north
to south (about 22 September). These points are
known respectively as the vernal equinox, or First
Point of Aries,and the autumnal equinox, or
First Point of Libra.(The equinoxes are the two
points at which the ecliptic cuts the celestial equator.)Escape velocity: The minimum velocity at which an
object must move in order to escape from the surface
of a planet, or other body, without being given extra
propulsion and neglecting any air resistance. The
escape velocity of the Earth is 11 kilometres per second,
or about 40,200 kilometres per hour; for the Moon it
is only 2.4 kilometres per second; for Jupiter, as much
as 60 kilometres per second.Exosphere: The outermost part of the Earth’s
atmosphere. It is very rarefied, and has no definite
upper boundary, since it simply ‘thins out’ into
surrounding space.Glossary
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