albedo: a measure of an
object�s reflecting power; the ratio of the amount of light scattered
by a body to the incident light.
amor: an asteroid with an orbit always
exterior to the Earth�s, but interior to that of Mars.
aphelion: the point at which an object
in orbit around the Sun is furthest from it.
apollo: an asteroid with an orbit that
lies mostly but not entirely outside that of the Earth; its orbit
may cross that of Earth.
asteroid: a minor planet; a body of rock,
carbon, or metal orbiting the Sun. Most asteroids occupy the Main
Belt between the orbits of Mars and Jupiter. The largest asteroid
is Ceres, diameter 950 kilometres, and the size of asteroids ranges
down to countless numbers of boulders.
astronomical unit, or AU: the average distance
between the Earth and the Sun; about 150 million kilometres.
aten: an asteroid with an orbit that is
mostly interior to that of the Earth; its orbit may cross that of
Earth.
CCD: a �charge-coupled device�; an electronic
detector sensitive to visible or infrared light, giving a signal that
can generate a digital image as in a TV camera. CCDs require powerful
computers to analyse and display the data.
comet: a body of dust and ice in orbit
about the Sun. As it approaches the Sun it may develop a �fuzzy� head
and a tail from the gas and dust ejected from the nucleus.
Earth crossing asteroid [or comet]: an
asteroid [or comet] whose orbit crosses that of the Earth if viewed
from the pole of the Earth�s orbit; the asteroid [or comet] may pass
above or below the Earth�s orbit.
Edgeworth�Kuiper Belt: a region beyond
the orbit of Neptune extending to around 1,000 AU from the Sun and
containing perhaps a billion objects; the source of short-period comets.
electromagnetic spectrum: the range of
all wavelengths emitted or absorbed by matter, including not only
visible light, but infrared and radio at longer wavelengths and ultraviolet,
X-rays and gamma rays at shorter wavelengths; analysis of the spectrum
is the main way that astronomers gain insight into the nature of astronomical
bodies.
giant planets: the four outer planets in
the solar system: Jupiter, Saturn, Uranus and Neptune. These planets
are very massive, of great size, yet of low density compared to the
inner or terrestrial planets Mercury,Venus, Earth and Mars. The giant
planets have no solid surface, being composed mainly of hydrogen,
while the terrestrial planets have rocky surfaces.
giant planets:the four outer planets in the solar
system: Jupiter, Saturn, Uranus and Neptune. These planets
are very massive, of great size, yet of low density compared to the
inner or terrestrial planets Mercury, Venus, Earth and Mars. The giant
planets have no solid surface, being composed mainly of hydrogen,
while the terrestrial planets have rocky surfaces.
infrared radiation: light at wavelengths
beyond the red part of the spectrum, to which our eyes do not respond.
Much infrared radiation falling on the Earth is stopped in the atmosphere,
so infrared telescopes must be in orbit or on the tops of high mountains.
K/T boundary: the Cretaceous-Tertiary boundary,
that is the boundary between groups of geological strata, signifying
a global climatic change some 65 million years ago. The K/T event
is whatever caused this global change, and is identified with the
asteroid impact that caused the Chicxulub crater.
light year: the distance travelled by light
in free space in one year; about 100 billion kilometres, or about
60 thousand AU. The nearest star to the Sun is about four light years
distant.
long-period comet: a comet with an orbital period longer than
200 years; such comets are thought to originate in the Oort Cloud,
and can approach the Sun from any direction.
lunar distance: the average distance from
Earth to Moon: 384,400 kilometres, 0.00256955 Astronomical Units (AU)
or about 1.3 light seconds.
magnitude: a measure of the brightness
of astronomical objects; the smaller the number the brighter the object.
A difference of 5 magnitudes corresponds to a change in brightness
of 100 times.
meteor: the bright streak of light � a
�falling star�� that occurs when a solid particle from space (a meteoroid)
enters the atmosphere and is heated by friction.
meteorite: a solid body that partly survives
the passage through the Earth�s atmosphere, and reaches the Earth�s
surface.
near Earth asteroid: an asteroid whose
distance from the Sun at perihelion is less than 1.3 AU.
near Earth comet: a comet whose distance from the Sun at perihelion
is less than 1.3 AU.
Near Earth Object (NEO): a near Earth asteroid or a near Earth
comet.
Oort Cloud: a sphere around the Sun, extending
to about two light years in radius, filled with a huge number (more
than a thousand billion) of cometary nuclei that are the remnants
of the formation of the solar system. Passing stars are thought to
perturb objects from the Cloud, and cause the long-period comets.
perihelion: the point at which an object in orbit around the
Sun is closest to it.
potentially hazardous asteroid: an asteroid
of at least 150 metres diameter, with an orbit that is within 0.05
AU of the Earth�s orbit.
radar: active sensing of an object using
a beam of radio waves directed at the object; the detection of the
reflected radiation from an asteroid or planet can give information
about position, velocity, rotation, and morphology.
short-period comet: a previously detected comet, with period
less than 200 years; such comets tend to approach in the plane that
contains the planets, are thought to originate in the Kuiper Belt,
and their orbits are subject to perturbation by the planets.
spectroscopy: the analysis of radiation,
and inference of its source, by splitting the radiation up into its
constituent wavelengths.
tsunami: a deep water wave generated in
the ocean at the site of an earthquake, underwater landslip, or asteroid
impact; a tsunami amplifies and breaks when it runs into shallows;
incorrectly known as a tidal wave.
visible light: that part of the electromagnetic
spectrum to which our eyes respond.