altitude Angular distance above the horizon. A body on the horizon has an altitude of 0[degrees]; one at the zenith (directly overhead) has an altitude of 90[degrees]
angular size The angle subtended by an object on the sky. Measured in degrees ([degrees]), arcminutes O and arcseconds (").
aphelion The point at which a body in orbit around the Sun reaches its farthest distance from the Sun. Earth's aphelion occurs in July.
apogee The point at which a body in orbit around the Earth reaches its farthest distance from the Earth.
Apollo asteroid One of a small group of asteroids whose orbits intersect that of Earth. They are named for the prototype, Apollo.
arcminute One sixtieth of a degree of angular measure. The Moon is about 30 arcminutes across. Symbol: '
arcsecond One sixtieth of an arcminute, or 1/3600 of a degree. Jupiter is about 40 arcseconds across. Symbol: "
ascending node In the orbit of a solar system body, the point where the body crosses the ecliptic from south to north.
asterism A pattern of stars.
asteroid See minor planet.
astrology "The mass cultural delusion that the Sun's apparent position relative to arbitrarily defined constellations at the time of your birth somehow affects your personality" (Sheldon, Big Bang Theory, season 1, episode 1).
astronomical unit (AU) A unit of length, equal to 149 597 870.7 km, which is the mean distance between the Earth and the Sun. Commonly used to specify distances in the solar system. Mercury, the innermost planet, lies on average 0.39 AU from the Sun, while Neptune lies on average 30.1 AU from the Sun.
azimuth Angular distance, measured eastward along the horizon from the north point. A bearing due north is at azimuth 0[degrees], due west is 270[degrees], etc.
BCE 'Before Common Era', secular equivalent of the BC dating convention.
Beehive See Praesepe.
black hole A maximally gravitationally collapsed object, from which not even light can escape. Black holes are one of the possible endpoints of stellar evolution of the most massive stars.
CCD Charge-Coupled Device. Highly sensitive photoelectric devices, made from a crystal of semiconductor silicon, that can electronically record the intensity and point of arrival of tiny amounts of light.
CE 'Common Era', secular equivalent of the AD dating convention.
Cerenkov radiation A bluish light that is emitted when charged particles travel through a medium at a speed that exceeds the speed of light in that medium. It is the electromagnetic analogue to an acoustical shock wave, or sonic boom.
Coal Sack Prominent dark nebula Crux (Southern Cross) and the Pointers.
coelostat A flat mirror that is mechanically steered so that it continuously reflects light from the same area of the sky into the field of view of an instrument that is fixed in position.
conjunction The phenomenon in which two bodies have the same apparent celestial longitude or right ascension as viewed from a third body, usually the Earth. The instant of conjunction does not necessarily correspond to the moment of least angular separation between the two bodies. See also elongation, planetary and Figure 23.
constellation A grouping of stars, usually with pictorial or mythical associations, that serves to identify an area of the celestial sphere.
culmination The instant at which a celestial object crosses the meridian, reaching the point of greatest altitude.
declination The angular distance above (positive) or below (negative) the celestial equator. One of the coordinates, with right ascension, that defines the position of a celestial object. Lines of declination are the celestial equivalent of latitude on Earth.
descending node In the orbit of a solar system body, the point where the body crosses the ecliptic from north to south.
dwarf planet See planet.
eclipse The total or partial obscuration of light from a celestial body as it passes through the shadow of another body. An eclipse of the Sun is, strictly speaking, an occultation.
ecliptic The mean plane of the Earth's orbit around the Sun.
elongation, planetary The angle planet-Earth-Sun. Eastern elongations appear east of the Sun in the evening; western elongations, west of the Sun in the morning. An elongation of 0[degrees] is called conjunction, one of 180[degrees] is called opposition and one of 90[degrees] is called quadrature. See Figure 25.
equator, celestial Projection of the Earth's equator as a line across the sky. To an observer standing on the Earth's equator, such a line would pass through the zenith. The directional bearing of a star is given in terms of its right ascension round the celestial equator.
equinox Either of the two points on the celestial sphere where the celestial equator intersects the
ecliptic (i.e. apparent longitude of the Sun is exactly 0[degrees] or 180[degrees]); also the time at which the Sun passes through either of these intersection points, when the lengths of day and night are approximately the same. The March equinox (around the 20th) is considered the start of autumn in the southern hemisphere, while the September equinox (around the 22nd) marks the start of spring. At the March equinox, also called the vernal equinox, the Sun crosses from south to north of the celestial equator. Compare solstice.
Galaxy, the See Milky Way (a).
Galilean moons The four largest satellites of Jupiter--Io (J I), Europa (J II), Ganymede (J III), and Callisto (J IV)--discovered by Galileo in 1610.
geocentric Having the Earth at the centre.
gibbous (of the Moon, Venus or Mercury) More than half full, but not full.
globular cluster A tightly packed, symmetrical group of thousands of very old stars, probably the oldest stellar formation in the Galaxy. They are generally found in the halo and are high-velocity objects with very elongated orbits around the galactic centre. Bright globular clusters include Omega Centauri and 47 Tucanae.
GMT Greenwich Mean Time. See time.
heliocentric Having the Sun at the centre.
Hyades A young open cluster of more than 200 stars visible to the naked eye in Taurus, about 40 pc distant. Aldebaran is a foreground star in that region of the sky.
Jewel Box Bright open cluster in Crux containing stars with contrasting colours. NGC 4755.
Julian Day (JD) The Julian Day Number is the total number of days that have elapsed since noon (00:00 UT) on January 01, 4713 BCE.
LMC See Magellanic Clouds.
light year A unit of length, 9 460 730 472 580.8 km (about 10 trillion km), the distance that light travels in a vacuum in one year.
Local Group The cluster of galaxies of which our own, the Milky Way, is one. It is a poor, irregular cluster with some 20 certain members: three spirals (the Galaxy, M31, and M33); four irregulars (LMC, SMC, IC 1613, and NGC 6822); and about 13 intermediate or dwarf ellipticals (NGC 147; NGC 185; NGC 205; M32; the Sculptor, Fornax, Leo I and II, UMi, and Draco systems; and the three companions to M31).
longitude, celestial Angular distance along the ecliptic from the vernal equinox eastward.
Magellanic Clouds Two small irregular (or possibly barred spiral) galaxies, satellites of our Milky Way, about 49 000 pc (LMC, in Dorado) and 62 000 pc (SMC, in Tucana) distant, visible to the naked eye from the southern hemisphere.
magnitude, apparent Measure of the observed brightness of a celestial object as seen from the Earth. The brighter the object, the smaller the apparent magnitude.
meridian The great circle passing through the zenith of the observer and the north and south points on his horizon.
Milky Way (a) Our own galaxy, the second largest in the Local Group. The Milky Way is a spiral galaxy, 1010 years old, containing 1011 stars. Its overall diameter is 30 000 pc with a nuclear bulge about 4 000 pc across. The disc is about 750 pc thick and the distance between spiral arms about 1400 pc. (b) A softly glowing band of light that bisects the skies of Earth, produced by light from stars and nebulae in the galactic disc.
minor planet A small planet-like body of the solar system. Most lie between the orbits of Mars and Jupiter. More than 1 800 have been catalogued, and probably millions of smaller ones exist, but their total mass would probably be less than 4% that of the Moon. A few have very elliptical orbits and cross the orbits of several other (major) planets. One or two even have their own satellites. Also called asteroids or planetoids.
Morning Star Any particularly bright star or planet prominently visible in the dawn sky. Often this is Venus, but other bright objects such as Jupiter and Sirius are also sometimes indicated.
nebula (pl. nebulae) Historically, fuzzy patches in the sky. Bright nebulae glow with light emitted by the gas of which they are composed (emission nebulae) or by reflected starlight (reflection nebulae) or both. Dark nebulae consist of clouds of gas and dust that are not so illuminated, giving the cloud the appearance of a region devoid of stars. Planetary nebulae are shells of gas ejected by stars. Spiral nebulae are galaxies.
occultation Complete or partial obscuration of an astronomical object by another. A solar eclipse is, strictly speaking, an occultation.
open cluster Any comparatively loose grouping of stars, and which tend to be found in the spiral arms or the disc of the Galaxy. Examples are the Jewel Box, Pleiades, Hyades and Praesepe.
opposition The instant when the Earth comes directly between a planet and the Sun, when the longitude of that planet differs by 180[degrees] from the longitude of the Sun. Thus only superior planets and minor planets can be in opposition. See also elongation, planetary and Figure 23.
parsec (pc) A unit of length (about 31 trillion km or 3.26 light years), the distance at which an object has a parallax angle of one arcsecond.
perigee The point at which a body in orbit around the Earth is closest to the Earth.
perihelion The point at which a body in orbit around the Sun reaches its closest distance from the Sun. Earth's perihelion occurs early in January.
photometry The measurement of the intensity of light from an astronomical object.
planet A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. A dwarf planet is a celestial body that meets criteria (a) and (b) but (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. All other objects, except satellites, orbiting the Sun shall be referred to collectively as "small solar system bodies".
planetary nebula Shells of illuminated gas that surround an old and dying star (which is evolving to become a white dwarf). The first example was discovered by William Herschel in 1785 and named by him for its superficial resemblance to a planet.
plasma A completely ionized gas; the so-called fourth state of matter (besides solid, liquid, and gas) in which the temperature is too high for atoms as such to exist and which consists of free electrons and free atomic nuclei.
Pleiades A very young (about 70 million years) open cluster of several hundred stars in Taurus, about 125 pc distant. Six members of the cluster are visible to ordinary, naked-eye sight. In Greek mythology the stars represent Pleione and her daughters with Atlas. Also known as isiLimela, Seven Sisters, etc.
position angle (PA) Angular direction, measured eastward from north. The north point has a PA of 0[degrees], directly eastward is 90[degrees].
Praesepe A naked-eye open cluster in Cancer, about 160 pc away. Also known as the Beehive and catalogued as NGC 2632 and Messier 44.
quadrature See elongation, planetary.
quasar The most luminous objects in the Universe, quasars can generate over a trillion times as much light as the Sun from a region little larger than the solar system. Quasars may be the central regions of certain very energetic galaxies at an early stage of their evolution. It is believed that the power of a quasar derives from a massive black hole at its centre.
radial velocity Velocity along the line of sight toward (negative) or away from (positive) the observer. It can be measured from the object's spectrum: one moving toward us has a blue-shifted spectrum, while a receding one is reds-hifted.
radio telescope An instrument used to detect radio waves from cosmic sources, typically consisting of a collector of radiation, a detector, an amplifier (to enhance the weak cosmic signals, by factors of up to around 1015) and a display/storage device. The most familiar type of radio telescope uses a parabolic dish to reflect incoming radiation to a focus but other configurations (arrays of aerials or antennae arranged in rows or grids) are also commonly employed.
SALT Southern African Large Telescope.
SAST South African Standard Time. See time.
satellite (a) A natural body that orbits a planet. The Moon is Earth's satellite. (b) A space vehicle placed in a closed orbit around the Earth, Moon or some other celestial body. Sputnik 1, launched in 1957, was the first Earth satellite.
Seven Sisters See Pleiades.
sidereal Relating to or measured with reference to the apparent motion of the stars. A sidereal period is the time taken for a planet or satellite to complete one orbit of its parent, as measured by its position against the background stars. For the Earth this is 365.2564 days. One sidereal day is 23h 56m 04.1s long.
SMC See Magellanic Clouds.
small solar system body See planet.
solstice Instant when the apparent longitude of the Sun is exactly 90[degrees] or 270[degrees], which is approximately the time of the Sun's standstill in the north-south direction (i.e. extreme solar declination). The June solstice (around the 21st) is considered the start of winter in the southern hemisphere while the December solstice (around the 21st) marks the start of summer. Compare equinox.
spectroscopy Production and interpretation of spectra. This technique is the main source of information on the composition and nature of celestial bodies.
star party A gathering of astronomy enthusiasts in order to observe the sky, discuss various aspects of astronomy, and share in the camaraderie of like-minded individuals. Typically a dark sky site away from light pollution is chosen as a location. Participants bring telescopes and binoculars and spend the nights observing. Lectures, exhibitions of home-built telescopes, contests, tours, raffles, and other similar activities are common. Astrophotography and CCD imaging are also very popular. Commercial vendors selling a variety of astronomical equipment may also be present.
supernova The eruption of a massive star when it reaches an unstable state late in its evolution. The debris consists of an expanding gas shell (the supernova remnant) and possibly a compact stellar object (a neutron star or black hole), which is the original star's collapsed core.
time South African Standard Time (SAST) is the everyday civilian time used in South Africa. It is two hours ahead of Universal Time (UT), which is the equivalent to Greenwich Mean Time (GMT).
transit The passage of a smaller, nearer celestial object across the face of a larger object in the background, as in a transit of Venus across the Sun.
twilight Civil twilight is the period from sunset to the time that the Sun is 6[degrees] below the horizon. Nautical twilight is from the end of civil twilight to the time that the Sun is 12[degrees] below the horizon, which ends when the boundary between sea and sky is no longer discernible. Astronomical twilight is from the end of nautical twilight until the Sun is 18[degrees] below the horizon. The end of astronomical twilight marks the beginning of night, when no part of the atmosphere visible to you is illuminated by the Sun, even indirectly.
UT Universal Time. See time.
zenith The point in the sky directly overhead.
zodiac A band passing around the celestial sphere, centred on the ecliptic, extending about 9[degrees] on either side. It thus includes the apparent annual path of the Sun as seen from the Earth and also the orbit of the Moon and planets.
Table 24. Solar system fact sheet Orbital characteristics Body perihelion aphelion eccent (106 km) (106 km) -ricity Mercury 46 69.8 0.21 Venus 107.5 108.9 0.01 Earth 147.1 152.1 0.02 Mars 206.7 249.2 0.09 Ceres 381.4 447.8 0.08 Jupiter 740.5 816.6 0.05 Saturn 1 352.6 1 514.5 0.06 Uranus 2 741.3 3 003.6 0.05 Neptune 4 452.9 4 553.9 0.01 Pluto 4 436.8 7 375.9 0.25 Makemake 5 760.8 7 939.7 0.16 Haumea 5 194 7 710 0.19 Eris 5 650 14 600 0.44 Body inclination orbital orb. (degrees) period velocity (km x (days) [s.sup.-1]) Mercury 7 88 47.9 Venus 3.4 224.7 35 Earth 0 365.3 29.8 Mars 1.9 687 24.1 Ceres 10.6 1 679.8 17.9 Jupiter 1.3 4 331.6 13.1 Saturn 2.5 10 832.3 9.7 Uranus 0.8 30 799 6.8 Neptune 1.8 60 190 5.4 Pluto 17.1 90 613 4.7 Makemake 29 113 183 4.4 Haumea 28.2 103 468 4.5 Eris 44.2 203 600 3.4 Physical characteristics Body mass radius density (1023 kg) (km) (g/[cm.sup.3]) Mercury 3.302 2440 5.427 Venus 48.685 6052 5.204 Earth 59.736 6378 5.515 Mars 6.4185 3390 3.933 Ceres 0.00943 487 2.077 Jupiter 18981.3 71492 1.326 Saturn 5683.19 60268 0.687 Uranus 868.103 25559 1.318 Neptune 1024.1 24766 1.638 Pluto 0.1314 1151 2.06 Makemake 0.03 717 1.7 Haumea 0.04 770 2.6 Eris 0.167 1163 2.52 Body gravity rotation albedo (m/[s.sup.2]) (hours) Mercury 3.701 1407.5088 0.106 Venus 8.87 -5832.444 0.65 Earth 9.78 23.93419 0.367 Mars 3.71 24.622962 0.15 Ceres 0.27 9.074170 0.09 Jupiter 24.79 9.924913 0.52 Saturn 10.44 10.65622 0.47 Uranus 8.87 17.24 0.51 Neptune 11.15 16.11 0.41 Pluto 0.655 153.293 0.3 Makemake 0.37 7.771 0.81 Haumea 0.44 3.9155 0.7 Eris 0.827 25.9 0.96