The Night Site - Glossary

A:

Absorption spectrum - Spectrum crossed by dark lines, these are caused by intervening cool gas.

Albedo - Ratio of light reflected to the light received.

Aphelion - The point of an planetary body's orbit where it is farthest from the Sun.

Apogee - The point of a satellite that is farthest from the planet.

Astronomical Unit - Mean distance of the Earth from the Sun. (149 597 870 kilometres).

B:

Binary Star - Two stars that are bound by each other's gravity, and orbiting each other.

C:

Chromosphere - The inner atmosphere of the Sun.

Cluster - A group of stars that are bound to each other through gravity. There are two different types; an open cluster which hold maybe a few hundred stars, such as the Pleiades (M45, better known as the Seven Sisters). And there are globular clusters, which contain hundreds of thousands of stars, an example of this would be M13 in Hercules.

Coma - The head of a comet.

Conjunction - The point when a planet, the Sun and the earth is a line. For Mercury and Venus who's orbits are smaller then the Earth's, when between the Sun and the Earth it is called inferior conjunction, when on the far side it is superior conjunction.

Constellation - Groups of stars that early astronomers named, because they resembled things. They are basically a way of mapping the sky, like political boundaries on Earth. There are a total of 88 official constellations, and that is unlikely to change until proper motions makes them un-recognisable many thousands of years from now.

Corona - The Sun's outer atmosphere

D:

Declination - A co-ordinate in the sky. It is the equivalent of latitude on the Earth.

Dichotomy - The half-phase of a moon or planet.

Doppler shift - The change of wavelength of light or sound waves, due to motion between the source and the observer. (example: when a car drives past you fast, the sound changes due to the sound waves being squashed when it's coming towards you, and pulled when it's moving away. The same is true for the electromagnetic spectrum).

E:

Eccentricity - Used to describe the difference between an ellipse and a circle.

Eclipse - Occurs when one body passes infront of another blocking Sun's rays. Lunar eclipse when the Earth is between the Sun and the Moon. A solar eclipse when the Moon is between the Sun and the Earth.

Ecliptic - Is the path the Sun travels across the sky in a year.

Electromagnetic spectrum - Encompassing the entire range of electromagnetic radiation (light). The various forms of light; includes radio waves, infrared light, visible light, ultraviolet light, x rays, and gamma rays. Radio waves being the longest wavelength and gamma rays being the shortest.

Ellipse - I the oval path that planets, asteroids and comets trace around the Sun, or other object.

Elongation - The angel of Mercury or Venus from the Sun.

Emission spectrum - The spectrum of bright lines, caused by luminous gas.

Eyepiece - Lens or lenses used to magnify the image produced by the object lens or mirror.

F:

Focal length - The distance between a lens or a mirror and the image it forms.

I:

Ionosphere - The upper layer of the Earth's atmosphere, about 70 kilometres.

L:

Light-year - Distance traveled by light (and all types of electromagnetic radiation) in one year (9 461 000 000 000 kilometres or 9.461 x 10^12 km). This is used to measure distances, for example the nearest star other then the Sun (Proxima Centauri) is 4.2 light-years (also written l.y.) away, this is because the light from this star has taken 4.2 years to reach the Earth. In other words it's 39 735 000 000 000 kilometres away (39.735 x 10^12 km). Light travels at 299 792.5 kilometres per second in a vacuum. This is the fastest possible speed in the universe.

Limb - The edge of the Sun, Moon or planet as seen in the sky.

Luminosity - A measure of light produced from a star.

Lunation - The interval between one New Moon to the next.

M:

Magnetosphere - The shell of charged particles held in place by the Earth's magnetic field.

Magnitude - Is the scale that measures the brightness of an object. Negative values are brighter then positive ones. On a very clear night at a dark sight the human eye can see down to about 6th magnitude (mag). While the brightest star in the sky (Sirius) is -1.5 magnitude. The pole star (Polaris) is about 2.1 magnitude. The Moon is about -12.7 magnitude at opposition.

Meridian - Is an imaginary line that passes through the zenith North and South poles.

N:

Nebula - An area of gas (mainly of hydrogen), an example like the Orion Nebula (M42/43). Stars are born in nebulae where gravity compresses the gas until it's so dense nuclear fusion can begin to fuse hydrogen atoms to form helium, this releases energy. Planetary nebulae are shells of gas that a star gives of in late stages of it's life, and contains many heavier elements that the star made by fusing lighter elements together. An example would be the Ring Nebua (M57) in Lyra.

Node - Is the point of an objects orbit that crosses the Earth's orbit.

O:

Occultation - Occurs when one object passes over another, blocking it from the observer..

Opposition - When a planetary body is opposite in the sky to the Sun.

P:

Parallax - Is the small shift in a stars position against the background stars due to the Earth's movement around the Sun. This used to be used to obtain star distances for closer stars, within 100 light-years.

Perihelion - Is the point where an orbit of a planetary object is closest to the Sun.

Photosphere - The visible surface of the Sun.

Prominence - Eruptions of gas from the surface of the Sun.

Proper motion - Drift of a star across the celestial sphere due to it's own motion.

R:

Radial velocity - The motion of a star or galaxy away or towards the observer.

Red-shift - Happens because the light from an object reddens due to the objects motion away from the observer, due to the light waves being stretched by the objects movement. The opposite is blue shift.

S:

Sidereal day - Length of time for one rotation of the Earth, as measured by a star (23 hours 56 minutes and 4 seconds).

Solar day - Length of time for one rotation of the Earth, as measured by the Sun (24 hours).

Solar wind - The stream of particles that come from the Sun.

Stratosphere - Is the layer of calm air between 15-40 kilometres above the Earth.

T:

Transit - The passage of a smaller disk, over a larger one. Like Mercury crossing the Sun, so you are able to see Mercury as a black disk against the Sun.

U:

Universal Time (UT) - Is basically the same time as GMT, but was renamed on recommendation from the International Astronomical Union for scientific perposes. Several versions of UT exist today. The main 2 are: UT1 and UTC, UT1 (commenly just UT) is a corrected form of UT0, due to variations of the Earth's poles UT0 was not an accurate way of measuring time. UT1 is the time scale used by astronomers. UTC or Coordinated Universal Time, uses the atomic clocks, so one second of UTC is the same as given by International Atomic Time. However because of variations in the Earth's orbit and spin. It must be corrected as needed to keep it in time with UT, so one second is added or dropped, at the end of December, March, June or September as needed. UTC and UT are never more then 0.9 seconds apart from each other.

W:

Wavelength - The distance between a given point on one wave, to the same point on the next wave. Visible light is usually measured in nanometres or angstroms (an angstrom is 10% of a nanometre). Wavelength is equal to the speed of the wave divided by it's frequency, hence high-frequency waves have a short wavelength, and vice versa.

Zodiac - Is centred on the ecliptic, and is about 18� wide. It contains 13 constellations, (mindless astrologers still use 2000 year old charts that only contain 12 constellations, yet another reason why these silly unscientific people should be completely ignored). The Sun and all planets move within the zodiac, the Sun taking one year to complete one rotation across the sky.