PRESS A COSMIC ENTITY ABOVE TO VISIT THE REST OF THE SITE
How many astronomers does it take to change a light bulb?
None, they leave it bust - 'cos they HATE light pollution
How many astro-physicists does it take to change a light-bulb?
One to take it out
One to prove that you can never change it faster than the speed of light
One to ring Patrick Moore to say that it shouldn't be classified as a newly discovered planet
And one to claim that maybe the new bulb has already long ceased to exist but that the light we see is light which came from a bulb 0.00000000000000000000000000000000000000000000000000000000000000001 seconds ago
How many amateur astronomers does it take to change a light bulb?
SEE THE RELATED FAQs:
"What sort of light bulb should I buy?"
How many light bulbs does it take to screw up an astronomer?
How many ears has Captain Kirk got?
The left ear.
The right ear.
Space, the final frontier
There are 10 types of people in this world....
Those that understand binary - and those that don't
What's blueish, unimaginably distant, and conquers Gaul?
Two planets meet. The first one asks: "How are you?"
A neutrino walks into a bar...and keeps right on going....
"They just found an Earth-like planet around Alpha Canis Majoris!"
My sibling kept asking me about the orbits of planets and the amount of area
Q: What's the difference between Max Factor and Quantum Theorist?
A: Max Factor has models that work
Alien Teacher: If I had two apples in one hand and three apples in my other hand, what would I have then?
Alien Pupil: Six more hands with nothing in them!
Copernicus’s father: "Copernicus, young man, when are you going to come to terms with the fact that the world doesn’t revolve around you."
Heisenberg was driving down a road when he got pulled over by the police. The policeman said to him "do you know how fast you were going?"
SCI-PO (thats science poetry)
Everything isnt a Cycle
Their not f***ing circles, alright
KNUCKLEHEAD JPJ’S THEORY
Rock v Metal
NATURE AND NURTURE WENT TO BED
NATURE AND NURTURE PLAYED A GAME
NATURE AN NURTURE HAD A FIGHT
REGULARITY IS KING
Regularity is King
'Cos Regularity is King
You know, Regularity is King
Oh Regularity is indeed King
DOES GOLD HOLD A GRUDGE?
For years now, minerals
Tornadoes have (so far) escaped this onslaught
The battlecry of the non-organic
It all started going wrong when Universal empathy
desire of 'the object'
Capital is a massive victory
Vibration is a form of communication
Agents of anti-life are not all under materials will
ACTUAL PROPER TIPS (so don't start laughing now)
buying telescopes -
It's probably better to just get some decent binoculars - you can see moons of jupiter with binoculars and as some clusters can be the size of the moon, you can check them as well - and they are som much less fiddly than a scope
but if you want a telescope
my best advice for magnification, portability and easy to use:
all you really need is a 3-4 inch newtonian reflector (lightweight n y can see loads with it) (about £100 - 150)
but just as important in my book is an equitorial mount (tripod type thing) (about £60 I think)
and a red dot finderscope is the one (£25ish)
extras - oh and of course, how could i forget you will need to buy
a planispheric astrolabe and planet calendar (£9 in total - and i know a very reliable supplier...blah blah blah)
also there is an incredible book for the first time telescope user called
Turn left at Orion - by Guy Consolmagno and Dan M. Davis
we don't sell it, (perhaps we should) it's getting a plug 'cos its just so useful and easy
airglow a faint background light in the night sky given out by gases in the ionosphere. The sky can therefore never be completely dark as seen from the surface of the Earth.
angular diameter the apparent size of a celestial object, usually expressed in degrees, minutes and seconds of arc.
angular distance the apparent dis tance between two objects on the celestial sphere, such as two stars, usually expressed in degrees, minutes and seconds of arc.
ansae the parts of Saturn's rings that appear like handles on each side of the planet. Singular ansa.
apsides the points in an orbit at which two bodies are closest together (periapsis) and farthest apart (apoapsis). The line join ing these points is called the line of apsides. and is the major axis of the orbit.
arc (measure of) angles on the celestial sphere are measured in degrees, minutes and seconds of arc. The terms arc minute and arc second are used to distinguish these measures from units of time. There are 60 arc minutes in a degree, and 60 arc seconds in an arc minute.
barycentre the centre of mass, or balance point, of a pair of bodies such as a double star or a moon and planet, around which the two bodies orbit.
Big Bang the hypothetical event that is presumed to have marked the origin of the Universe as we know it. The Universe has been expanding since the Big Bang, which is estimated to have occurred between 10000 million and 20000 million years ago.
black hole a volume of space in which gravity is so great that nothing can escape, not even light -hence it is truly black. Black holes are thought to be produced when very massive stars collapse at the end of their life.
Bode's law a series of numbers that roughly describes the average distances of the planets from the Sun in astronomical units, out as far as Uranus. Take the numbers 0, 3, 6, 12, etc., doubling at each step. Add 4 to each number and divide by 10 . The 'law' breaks down for Neptune and Pluto. The German astronomer Johann Bode drew attention to the relationship in 1772, although it had already been pointed out by his countryman Johann Titius; for this reason it is sometimes called the Titius-Bode law.
captured rotation rotation such that a body spins on its axis in the same time as it takes to orbit another body, so that it keeps one face permanently turned towards the object it is orbiting. Our Moon has a cap tured rotation, as do many moons of other planets. Captured rotation is brought about by tidal forces.
CCD an electronic device used instead of photographic film. It consists of a silicon chip which is sensitive to light, divided into sections known as pixels (picture elements). Light falling on each pixel builds up an elec tric charge which can be read off to produce an image. CCDs are much more sensitive to light than is photographic film, so exposures can be much shorter. However, they are also smaller in area and have less resolution than film. The initials stand for charge-coupled device.
central meridian (CM) the imaginary north—south line bisecting the disk of a planet, used as a reference for estimating the longitude of planetary features as the planet rotates. The passage of a feature across the central meridian is called a central meridian transit.
coma (cometary) the cloud of gas and dust, roughly spherical in shape, that makes up the head of a comet. At the centre of the coma is the comet's nucleus, from which the gas and dust escapes. A comet's coma can be between 10000 and 100000 km in diameter.
comes the companion of a double star (plural comites).
coronal hole a cooler and less dense part of the Sun's corona, through which the fastest part of the solar wind flows.
cosmic rays atomic particles that are moving through space at close to the speed of light. They are mostly protons (the nuclei of hydrogen atoms), although the nuclei of most elements are present in small num bers, and also electrons. Some low-energy cosmic rays come from flares on the Sun, but higher-energy cosmic rays are believed to come from outside the Solar System, prob ably from supernovae and their remnants. The highest-energy cosmic rays of all seem to come from distant galaxies and quasars.
cusp one of the two 'horns' of the crescent Moon or of a planet in crescent phase.
Cynthian adjective referring to the Moon. Cytherean adjective referring to Venus.
deep sky that part of space beyond the Solar System. Deep-sky objects include star clusters, nebulae, galaxies, double stars and variable stars.
dichotomy the moment when the Moon, Mercury or Venus is exactly half-illuminated as seen from the Earth.
differential rotation the rotation of a body in which different parts spin at different speeds; for example, a gaseous planet or a star spins faster at the equator than at the poles.
disk the face of a planet, moon or star as seen from the Earth.
Doppler effect a change in the wave length of light caused by the motion of the object emitting the light. If the object is moving towards us the wavelengths are shortened, i.e. moved towards the blue end of the spectrum; this is termed a blueshift. If the object is receding its light is lengthened in wavelength, i.e. moved towards the red end of the spectrum; this is termed a redshift (q.v). The amount of shift is revealed by the position of lines of known wavelength in the object's spectrum.
dwarf star any star on the main sequence of the Hertzsprung-Russell diagram. The Sun is a dwarf star, but many such stars are actually larger than the Sun. The term is also applied to white dwarfs (q.v.), which are not on the main sequence.
early-type star a hot star of spectral type O, B or A.
eccentricity (e) a measure of how non- circular an orbit is. The eccentricity of an ellipse ranges between 0 (a circle) and 1 (a parabola). Eccentricity is calculated by divid ing the distance between the two foci of the ellipse by the length of the major axis.
elongation the angle between the Sun and a planet, or between a planet and a satellite, as seen from the Earth. Elongation is measured along the ecliptic in degrees west or east of the Sun.
emersion the re-emergence of an object after an eclipse or occultation.
emission lines specific wavelengths of light (or other forms of electromagnetic radiation) given out by atoms of a gas. An emission spectrum is a spectrum consisting of bright emission lines, for example as pro duced by the gas of a nebula. Emission lines can appear as bright lines superimposed on a continuous spectrum if given out by hot gas surrounding a star.
ephemeris a table of the predicted posi tions of a celestial object such as the Moon.
the Sun or a planet. Plural cphemerides.
epoch an instant in time, such as the beginning or middle of a year, for which positions of stars, orbital elements and other information are given. Since the coordinates of stars are constantly changing because of precession, star positions are referred to a standard ox fundamental epoch. Currently the standard epoch used by astronomers is 2000 January 1,12h (also written as 2000.0).
equation in astronomy, either a differ ence between two values or a correction, as for instance in the equation of time (difference between mean and apparent solar time) or a personal equation (correction for personal error when measuring or timing something).
escape velocity the speed at which any object, from a rocket to a gas molecule, must move to break away permanently from the gravitational pull of a body. For the Earth, the escape velocity at the surface is 11.2 km s" 1 ; for the Moon it is 2.4 km s
extinction the dimming of starlight by dust in space or by the Earth's atmosphere. Extinction is greater for blue light than it is for red, causing a reddening of starlight. Atmospheric extinction is least at the zenith, where it amounts to a few tenths of a magnitude under clear skies, and increases towards the horizon (see Table 57, on p. 130).
field star a star in the same field of view as an object under study, but which lies at a different distance and hence has no connection with it, for example a foreground star in the same field of view as a distant galaxy
first contact the beginning of an eclipse, transit or occultation. At a solar eclipse, it is when the Moon starts to move across the face of the Sun; at a lunar eclipse it is when the Moon enters the Earth's umbra.
focal length the distance between a lens or mirror and the point at which it brings parallel light rays to a focus.
following objects move across the sky from east to west because of the rotation of the Earth, so the more easterly of a pair of stars, for example (or the easterly side of a planet), is said to be following. The term is also used of features moving across the face of a body as it rotates, such as sunspots, or spots on Jupiter. The other side is described as preceding (q.v.).
fourth contact the end of an eclipse, transit or occultation. At a solar eclipse it is when the Moon moves completely off the face of the Sun; at a lunar eclipse it is when the Moon leaves the Earth's umbra.
frequency (v) the number of waves passing a fixed point in a given time, usually one second. Frequency is measured in hertz, and is equal to the speed of the waves divided by their wavelength. Hence the longer the wavelength, the lower the frequency, and the shorter the wavelength, the higher the frequency.
fundamental star a star whose position is determined as precisely as possible, and against which the positions of other stars can be compared. The positions of funda mental stars are published in fundamental catalogues.
galactic cluster another name for an open star cluster in our Galaxy, so called because they lie in the spiral arms of the Galaxy rather than in the halo around the Galaxy, where the globular clusters lie.
gamma rays radiation of the short est wavelengths, 0.01 nanometres and less, shorter even than X-rays.
giant star a star that is swelling up in size as it approaches the end of its life. Giant stars have similar masses to normal stars such as the Sun, but they are larger in diameter and considerably more luminous.
gibbous the phase of the Moon or a planet when it is between half and fully illuminated.
Gould's Belt a band of young, brilliant stars at an angle of between 15° and 20° to the plane of our Galaxy, stretching around the sky from Perseus, Taurus and Orion, via Carina, to Centaurus and Scorpius. Gould's Belt is believed to be a spur on the local spiral arm of our Galaxy.
great circle a circle that divides a sphere into two equal hemispheres. On the celestial sphere, a great circle has the Earth at its centre; examples are the celestial equator, the ecliptic and lines of right ascension. Compare small circle.
green flash an effect caused by atmo spheric refraction and absorption in which the last visible segment of the setting Sun turns green, sometimes followed by a green ray like a vertical flame at the instant of setting. The phenomenon lasts for only a few seconds, and is best seen over the sea or a distant horizon when the air is clear (i.e. when there is little reddening of the setting Sun). A similar effect can occasionally be seen as the Sun rises.
greenhouse effect the warming of a planet by the trapping of solar radiation in a planet's atmosphere. The greenhouse effect acts particularly strongly on Venus, raising its temperature to very high levels; it operates to a lesser effect in the atmospheres of other planets.
heavy elements in astronomy, all chem ical elements heavier than hydrogen and helium are also called 'metals'.
heliacal rising the occasion on which a star or planet first appears in the dawn sky, after having been too close to the Sun to be visible.
heliacal setting the last occasion on which a star or planet can be seen in the evening sky before it becomes too close to the Sun to be visible.
immersion the entry of a celestial object into a shadow at an eclipse, or the covering of an object at an occultation.
inclination (;') the angle at which an orbit is tilted with respect to a plane of reference. For objects orbiting the Sun the inclination is given relative to the plane of the Earth's orbit; for objects orbiting the Earth, relative to the Earth's equator; and for double stars, relative to the plane of the sky. The axial inclination of a body is the angle at which its axis of rotation is tilted to the perpendicular to the plane of its own orbit.
infrared radiation with wavelengths longer than visible red light but shorter than radio waves, i.e. between about 700 nano metres and 1 millimetre.
interferometer a device in which radio or optical waves collected by two or more apertures are combined to give improved resolution, such as for separating two closely spaced objects.
interpolation the technique of estimat ing a value intermediate between two of a range of given values, for instance the position of a planet on a date between two dates tabulated in an ephemeris.
inverse-square law the law which states that the energy received from a source falls off with the inverse square of the dis tance of the source. For example, a star twice as far away as another identical star appears four times fainter, three times away it appears nine times fainter, and so on. Forces, including gravity, obey the same law.
ion an atom or molecule that has lost one or more electrons (a positive ion) or has gained one or more electrons (a negative ion).
ionization the process by which electrons are added to or removed from an atom or molecule, so turning it into an ion.
irradiation the optical effect in which a bright object seen against a dark background appears larger or brighter than it actually is.
Kirkwood gaps regions of the asteroid belt, corresponding to particular distances from the Sun, where few asteroids are found. The gaps are caused by Jupiter's gravity, which perturbs asteroids out of orbits whose period is an exact fraction of Jupiter's orbital period.
Lagrangian points five places at which small bodies can exist in stable orbits in the plane of two much larger bodies. Three of the points lie on a line joining the two large bodies (one point between the two bodies, and the other two points on either side of them). The two other Lagrangian points lie 60° ahead of and behind one of the larger bodies in its orbit around the other; it is at these places in the orbit of Jupiter that the Trojan asteroids are found. Objects cannot exist permanently at the three other Lagrangian points of Jupiter's orbit because they would be perturbed by the gravitational pulls of the other planets.
late-type star a cool star of spectral type K, M, C or S.
light curve a graph of the changing brightness of an object such as a variable star, or a planet or moon as it rotates.
light, speed of light travels at 299792.5 km s" 1 (often rounded to 300000kms~ l ) in a vacuum; this is the fastest speed in the Universe. All other forms of electromagnetic radiation, from X-rays and gamma rays to radio waves, travel at the same speed.
light-pollution brightening of the night sky caused by artificial sources of illumination such as streetlamps.
light-time the time taken for a beam of light to travel from a celestial body to the Earth. The effect must be taken into account when timing the occurrence of events such as eclipses of the moons of Jupiter, whose times of occurrence are affected by the distance between Jupiter and the Earth.
limb the apparent edge of the disk of a celestial body as seen from the Earth; regions near the visible edge of the Moon are called limb regions. The leading limb of an object crossing the sky as the Earth rotates is called the preceding limb; the trailing limb is called the following limb.
local standard of rest a volume of space extending out to about 100 parsecs from the Sun in which the velocities of all stars relative to the Sun average out to zero.
lunation the time taken by a complete cycle of phases of the Moon, such as from one full moon to the next. A lunation lasts 29.53 days; it is the same as a synodic month.
magnetosphere the extension of the Earth's magnetic field into space. The magnetosphere is like a magnetic bubble around the Earth. The Van Allen radiation belts lie within the magnetosphere. Other bodies with magnetic fields also have magnetospheres. The boundary of the magnetosphere is called the magnetopause.
magnification the amount by which an optical instrument makes an object appear larger. For example, if a line appears ten times longer when viewed through a tele scope, the telescope is said to magnify ten times. The magnification of a telescope depends on the instrument's focal length and on the focal length of the eyepiece in use; eyepieces of shorter focal length produce higher magnifications on a given telescope. Magnification can be calculated by dividing the focal length of the telescope by the focal length of the eyepiece. A magnification of, say, ten is written in the form X10.
major axis the longest diameter of an ellipse, passing through the two foci of the ellipse.
mean the average of a series of values.
meteor the streak of light, lasting no more than a second or so, produced when a speck of dust from space (a meteoroid) burns up in the Earth's atmosphere, usually at a height of about 100 km.
meteorite a chunk of rock or iron from space that reaches the surface of the Earth or of any other body. Large meteorites can pro duce craters when they hit the ground. Most meteorites are thought to be chips from asteroids, but some fragile stony meteorites called carbonaceous chondrites may come from the nuclei of comets.
meteoroid any small solid object in space. When a meteoroid enters the Earth's atmosphere at high speed it produces a meteor.
Metonic cycle the period of 19 calendar years (6939.6 days) after which the Moon's phases recur on the same day of the year. There are 235 lunations in a Metonic cycle.
minor axis the shortest diameter of an ellipse, at right angles to the major axis.
mock Sun an effect caused by ice crystals in the Earth's atmosphere which refract the Sun's light so that two diffuse areas of light occur either side of the Sun, 22° from it. These mock Suns, also known as parhelia or sundogs, usually appear on the rim of a halo surrounding the Sun.
neutron star a tiny, very dense star composed of neutrons. Neutron stars have diameters of only about 20 km, but contain the mass of up to three Suns; if the neutron star had a mass greater than three Suns, gravity would cause it to collapse still further into a black hole. Neutron stars are believed to be left behind after massive stars explode as supernovae at the end of their life; in the explosion, the protons and electrons of the star's core are squeezed together to form neutrons.
node the point at which an orbit crosses a given plane, such as the plane of the Earth's orbit or the Earth's equator. There are two nodes: the ascending node (Q), when the orbiting body moves from south to north, and the descending node (U), when the body moves from north to south. The line of nodes is the straight line joining these two nodes. Regression of the nodes is the westward move ment of the nodes of an orbit caused by the gravitational pull of other bodies, notably the Sun.
oblateness a measure of the amount by which a rotating object such as a star or planet departs from a perfectly spherical shape. Rotation causes the equatorial regions of a sphere to bulge outwards slightly, so that the sphere appears slightly flattened at the poles; hence oblateness is also known as polar flattening. Oblateness is calculated by taking the difference between the equatorial and polar diameters of the object, and dividing by the equatorial diameter. Saturn has the greatest oblateness of any planet in the Solar System, 0.1.
occulting bar a bar that may be moved into the focal plane of an eyepiece so as to obscure a bright object and allow a nearby faint object to be observed.
paraboloid a surface that is curved like a parabola. Main mirrors in telescopes are usually paraboloids, since a paraboloid is free from spherical aberration.
parhelion a mock Sun (q.v.).
penumbra the lighter, outer part of a sunspot or shadow. From within the penum bra of the Moon's shadow, a partial eclipse of the Sun is visible. When the Moon is within the penumbra of the Karth's shadow it is said to be penumbrally eclipsed; but the Earth's penumbral shadow is so faint that in practice a penumbral eclipse is scarcely noticeable.
period the interval between the succes sive occurrences of a cyclical event, such as the time taken for a body to rotate once on its axis or go once around its orbit, or for a variable star to go through one cycle of brightness variations.
perturbation a slight disturbance of the motion of one body caused by the gravita tional pull of other bodies.
phase the proportion of the sunlit side of the Moon or a planet that is visible from Earth. Mercury and Venus go through a com plete cycle of phases similar to those of the Moon. The outer planets show phases only from gibbous to full, being most gibbous at quadrature.
phase angle the angle between the Sun and the observer as seen from the centre of a given object. When the phase angle is 180° the Sun and the observer lie in opposite directions from the object, and the sunlit side of the object is facing away from the observer. At a phase angle of 0° the Sun and the observer are on the same side, and the object appears fully illuminated.
photon the behaviour of light in some situations is best explained by assuming that it is not a wave motion, but a stream of particles. A photon is the name given to such a particle (of light or of other electro magnetic radiation).
planisphere a circular map with a rotat ing mask that can be turned to show the stars as they appear from a given latitude at any time on any date.
position angle the relative position of one object with respect to another, such as the two components of a double star or the position of a star around the Moon's limb at an occultation. Position angle is measured in degrees from north via east, south and west. On the celestial sphere, east is the direction towards the eastern horizon.
preceding term used to describe the side of a planet that leads in its motion across the sky, or of the leading member of a pair of objects such as stars or sunspots. The preceding side can easily be found by watch ing objects drift through the field of view of a telescope. Compare following .
primary the larger body of an orbiting pair (e.g. the Earth is the Moon's primary) or the brighter member of a binary star. Compare secondary.
pulsar a star that, every few seconds or less, gives out a rapid flash of energy at radio and other wavelengths. Pulsars are believed to be rapidly rotating neutron stars (q.v.) that flash each time they spin, like a light house beam.
quasar an object that looks like a star but which emits as much energy as hundreds of normal galaxies. Quasars have high redshifts, and hence must lie far off in the Universe. They are thought to be the bright centres of distant galaxies where matter is falling into a giant central black hole.
radiation belts belts of atomic part icles trapped inside the magnetosphere of a planet. See also Van Allen belts.
radio astronomy the study of radio waves emitted naturally by objects in space. Radio waves are the longest-wavelength radiation, with wavelengths greater than 1 millimetre.
radius vector the imaginary line joining an orbiting body and the object it orbits.
red dwarf a star that is much smaller and cooler than the Sun. Red dwarfs have about one-tenth the mass of the Sun, and are about one-tenth its diameter.
red giant a large, cool star perhaps ten or more times the diameter of the Sun, produced when a normal star swells up near the end of its life.
redshift a lengthening in the wavelengths of light from a body, usually caused by the motion of the emitting body away from us (a Doppler shift), although a redshift can also be caused by the presence of strong gravitational fields. The redshift of galaxies is usually regarded as being directly related to their distance from us in the Universe -hence the greater the redshift, the more distant the galaxy.
refraction (atmospheric) the bending of light by the Earth's atmosphere which increases the apparent altitude of an object above the horizon. It ranges from zero at the zenith to approximately half a degree at the horizon.
residual the difference between observed and calculated values, such as of the position of a planet in its orbit.
retrograde motion of a body from east to west, the opposite of the prevailing direction of motion in the Solar System. The term retrograde can apply to either the orbital motion or the direction of spin of a planet or moon.
revolution the movement of one bodv in orbit around another, or around a centre of mass
rotation the spin of a body on its own axis.
Saros the length of the cycle of solar and lunar eclipses: the period after which the Sun, the Moon and the nodes of the Moon's orbit return to almost the same relative positions. The Saros lasts 6585.32 days (just over 18 years) and contains 223 lunations.
scintillation twinkling (q.v.).
second contact the moment an eclipse becomes total. At a solar eclipse, it is when the Moon completely covers the face of the Sun; at a lunar eclipse, it is when the Moon becomes fully immersed in the Earth's umbra.
secondary a smaller body that orbits around a larger one (e.g. the Moon is the Earth's secondary) or the fainter member of a binary system. Compare primary.
sidereal to do with the stars. Sidereal time is time based on the rotation of the Earth with respect to the stars rather than with respect to the Sun; the sidereal period is the orbital period of a body with reference to a fixed star. Compare synodic.
small circle a circle that does not divide a sphere into two equal hemispheres, unlike a great circle (q.v.). On the celestial sphere, small circles do not have the Earth at their centre — for example, circles of declination (other than the celestial equator) are small circles.
solar wind the tenuous stream of atomic particles from the Sun that flows outwards through the Solar System.
spectral lines narrow lines that cross the spectrum of an object; the lines can be either bright (emission lines) or dark (absorption lines). Each line in the spectrum corresponds to a particular wavelength at which atoms or molecules absorb or emit light.
spectrum, visible the rainbow-like band of colours that is produced when light is split into its constituent wavelengths. Features in the spectrum, such as bright and dark lines, tell astronomers about the composition and motion of gas in the object under study.
supergiant star a star many times the mass of the Sun that is swelling up as it ages. Supergiants are the largest and brightest stars known. Many, perhaps all of them, eventually explode as supernovae.
synodic to do with conjunctions. For example, the synodic period of a planet is the time taken for it to return to conjunc tion with the Sun as seen from the Earth (or conjunction with the Earth as seen from the Sun). The synodic period of a satellite is the mean interval between conjunctions as observed from its parent planet; during this time, the satellite goes through a complete cycle of phases. Compare sidereal.
telluric to do with the Earth, e.g. telluric lines in a star's spectrum are a result of the passage of the star's light through the Earth's atmosphere.
terminator the dividing line between the illuminated and dark portions of a planet or satellite, particularly the Moon. The terminator is the sunrise or sunset line, the boundary between day and night.
third contact the moment when a total eclipse ends. At a solar eclipse, it is when the Sun starts to reappear from behind the Moon; at a lunar eclipse, it is when the Moon starts to emerge from the Earth's umbra.
topocentric as seen from a point on the surface of the Earth. The topocentric coordi nates of a nearby body in space, such as the Moon, are slightly different from those that would be measured from the centre of the Earth (geocentric coordinates).
twinkling the flickering of a star's light caused by air currents in the Earth's atmo sphere which distort the path of light rays, causing the star to change in apparent brightness and to flash different colours, particularly when close to the horizon. Planets do not twinkle as much as stars, because they are not point sources, but under bad conditions even planets can twinkle, especially when low down. A large amount of twinkling is a sign of bad seeing.
ultraviolet radiation with wavelengths shorter than visible violet light but longer than X-rays, from about 10 to 400 nano metres.
umbra the dark central part of a sunspot or shadow. From within the umbra of the Moon's shadow, a total eclipse of the Sun is visible. The Moon is totally eclipsed when it is completely within the umbra of the Earth's shadow; when it is partly immersed in the Earth's umbra, it is partially eclipsed.
Van Allen belts two doughnut-shaped zones of atomic particles around the Earth. They consist of electrons and protons trapped inside the Earth's magnetosphere.
wavelength (A) the distance between a given point on one wave to the same point on the next wave. The wavelength of light is usually measured in either nano metres or angstroms (an angstrom is one-tenth of a nanometre). Wavelength is equal to the speed of the wave divided by its frequency -hence high-frequency waves have a short wavelength, and vice versa.
white dwarf a tiny, hot star that is the end-point in the life of stars like the Sun. A typical white dwarf contains as much mass as the Sun compressed into a ball not much larger than the Earth. They cool with age, so the oldest of them are not actually white. The easiest white dwarf to observe is a member of the triple-star system Omicron-2 (o 2 ) Eridani.
X-rays radiation with wavelengths shorter than ultraviolet light but longer than gamma rays, between about 0.01 and 10 nanometres.
Zeeman effect the splitting of spectral lines into two or more parts by a magnetic field.
zodiac the band of 12 constellations through which the Sun passes each year: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius and Pisces.