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Astronomy Glossary

Absolute zero – the coldest temperature possible, -273.15 °C, or 0 kelvin (0 K). 0 °C is 273.15 K. Deep space isn't quite absolute zero, but is close, at 3 K.

Absorption lines – see spectral lines.

Aphelion (plural aphelia) – the point in an orbit that a body is furthest from the Sun. Opposite of perihelion.

Astronomical unit (AU) – the distance from the Sun to Earth, about 150 million kilometres.

Barycentre – the centre of mass of two or more bodies orbiting each other.

Binary system – a pair of objects of somewhat similar size which orbit each other. Pluto and Charon are a good example, as are the Sun and Jupiter.

Centaur – an icy body orbiting the Sun between Saturn and Neptune.

Conjunction – when two celestial bodies (stars, planets, Moon etc) appear in the sky very close to each other. Also see planetary conjunction, great conjunction, greatest conjunction on this page. See the Conjunctions page.

Copernican principle – the idea that we are not in any way in a special place in the Universe; named after Nicolaus Copernicus who placed the Sun instead of Earth at the centre of the Universe. The Copernican principle is a philosophical idea, not a scientific one.

Cosmology – the study of the cosmos; where it came from, what shape it is now, etc. Because so much of cosmology depends on untestable ideas it is, strictly speaking, incorrect to call it a science. (Many cosmologists would likely disagree.) James Gunn, Princeton University, co-founder of the Sloan Digital Sky Survey: "Cosmology may look like a science, but it isn’t a science ... A basic tenet of science is that you can do repeatable experiments, and you can’t do that in cosmology."

Cosmological constant – the value of the energy density of free space; a fudge factor introduced to help explain why the Universe's (apparent) expansion appears to be accelerating. As explained here: The biggest problem with the cosmological constant is that the theoretical calculation for what it should be over-predicts the energy density of the cosmic vacuum by 120 orders of magnitude. That is, the value for the cosmological constant determined from cosmology is smaller than the theoretical estimate by a factor of 10−120. This discrepancy has been called “the worst theoretical prediction in the history of physics!” In cosmology this is known as the cosmological constant problem.

Cosmological principle – the idea that on a large enough scale the Universe looks basically the same whereever the viewer is. The cosmological principle is a philosophical idea, not a scientific one, as it cannot be tested independentally of other assumptions about the Universe (because cosmological assumptions affect how data is interpreted).

Dwarf planet – see the dwarf planets section on the Minor Planets page.

Eccentricity – how circular an orbit is. An eccentricity of 0 is a perfect circle, while an eccentricity of 1 is a parabola. In between 0 and 1 an orbit is an ellipse, greater than 1 is a hyperbola.

Eclipse – see solar eclipse and lunar eclipse.

Ecliptic – an imaginary line that traces out the path of the Sun against the background stars. In other words, the plane in which Earth orbits the Sun.

Emission lines – see spectral lines.

Gas giants – large gaseous planets made mostly of hydrogen and helium. There are two in the Solar System; Jupiter and Saturn.

Great conjunction – a conjunction between Jupiter and Saturn.

Greatest conjunction – a conjunction between Jupiter and Saturn that occurs in the sky opposite the Sun.

Ice giants – large gaseous planets made mostly of elements heavier than hydregen and helium. There are two in the Solar System; Uranus and Neptune.

Iridium flare – a particular kind of satellite flare. See Iridium Flares.

Kelvin – a temperature scale starting from abolute zero. Symbol is K with no degrees symbol. The freezing point of water is 273.15 kelvins, or 273.15 K. Note that the scale is the Kelvin scale (uppercase K), but the unit is kelvins (lowercase k).

Lunar eclipse – what happens when the Moon enters Earth's shadow. For more information see the Eclipses page.

Meteor – a small rock (or sometimes lump of iron) that passes through Earth's atmosphere leaving a glowing trail. See iron in the Chemistry section.

Meteorite – a meteor that makes it all the way through the atmosphere without burning up and thus hits Earth's surface. See iron in the Chemistry section.

Minor planet – see the minor planets section on the Solar System page.

Neutron star – a type of old star that has collapsed to form a very dense object made entirely of neutrons.

Parsec – the distance at which one astronomical unit (the distance between Earth and the Sun, about 150,000 km) subtends an arc of one arcsecond (¹/3600°). It's about the angle a 10 cent coin makes when it's over 4.5 km away.

Perihelion (plural perihelia) – the point in an orbit that a body is closest to the Sun. Opposite of aphelion.

Photon – a packet of light. Each photon has a particular energy, which determines its colour.

Planet – a large body orbiting a star that has cleared its orbit of other large bodies. Our Sun has eight planets. The word planet comes from a Greek word meaning wanderer. Astronomers in ancient Greece and ancient China called them wandering stars because the planets look like stars that wander across the sky. For more information see the Planets page.

Plasma – the "fourth state of matter" along with solid, liquid and gas. It's an ionised state of matter similar to a gas. Since it's what stars are made of, plasma is the most common state of matter in the Universe.

Plutino – a TNO with a 2:3 orbital resonance with Neptune; every two orbits a plutino makes, Neptune makes 3. Pluto was the first plutino to be discovered.

Planetary conjunction – a particular type of conjunction between two planets.

Pulsar – a type of neutron star that emits radiation in narrow beams a little like a lighthouse. For more information see neutron stars on the Weird Stars page.

Satellite flare – when a flat panel on a man-made satellite (such as an aerial or solar panel) reflects the Sun's light to us. Iridium flares can be particularly bright.

Solar eclipse – what happens when Earth enters the Moon's shadow. For more information see the Eclipses page.

Spectral lines – lines in the spectrum of a light emitting object. Can be absorption lines – dark lines on a bright continuous spectrum – or emission lines – bright lines on a dark background. See spectral lines in the Chemistry section.

Spectral type – the surface temperature of a star. For more information see the Spectral Type page.

Star – a really big hot ball of mostly hydrogen (as plasma) floating around in space. Held in a roughly spherical shape by its own gravity.

Syzygy – a straight line configuration of three celestial bodies in the same gravitational system. For example, eclipses.

Terrestrial planets – the inner four planets in the Solar System; Mercury, Venus, Earth, and Mars. The term could also apply to rocky planets in other systems.

For a more detailed glossary see www.icq.eps.harvard.edu/ICQGlossary.html.