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Astronomy - some useful words

Big Bang Theory - a theory of the origin of the universe. This says that the universe began with a big bang some 15,000 million years ago and has been expanding ever since. Astronomers think that they have discovered the echo of this Big Bang. (Radiation at 3o above absolute zero)

Black Hole - an object with an escape velocity greater than the velocity of light. Anything which falls into it can never get out again. It is thought that black holes are formed by a star collapsing under its own gravity.

Celestial Equator - a line drawn round the middle of the celestial sphere, a projection of the Earth's equator.

Celestial Pole - a point on the celestial sphere in line with the Earth's axis. The pole star is very close to this point.

Celestial Sphere - a sphere surrounding the Earth on which the stars were thought to be. Sometimes called the star sphere it has the same centre as the Earth.

Circumpolar stars - stars close to the pole star that are visible throughout the year; they never go below the horizon.

Direction by the stars - the two stars at the end of the Great Bear (Plough) always point towards the Pole star.

Double star - two stars linked together by gravity, usually rotating about each other.

Ecliptic - the path against the stars along which the Sun and planets appear to move.

Equinoxes - the times of the year when day and night are the same length and when the Sun rises due east and sets due west. The 21st. March and 21st. September.

Galaxy - a large cloud of millions of stars rotating slowly in a giant catherine wheel shape. They may be many thousands of light years in diameter and millions of light years away.

Height of the Sun at Noon -
the maximum height of the Sun during the day. You must measure the time in Greenwich Mean Time (GMT) adjusted for your part of the world.

Horizon - the place where sky and Earth meet. More of the sky will obviously be visible in flat regions than in hilly ones.

Interferomoter - an instrument consisting of two separated telescopes used for measuring the diameter of a star.

Length of daylight - the time the Sun is above the horizon. Minimum in mid-winter and maximun in mid- summer

Light year - the distance that light travels in a year at the velocity of 300 000 km/s (186 000 miles/s).
A light year is 9440 000 000 000 km. or 5880 000 000 000 miles.
Light takes about 8.3 mins to travel from the Sun to the Earth while the nearest star is roughly 4 light years away. The distances of far off galaxies are measured in millions of light years!

Magnitude - a scale on which the apparent brightness of a star is measured. We can see down to the sixth magnitude, while big telescopes can see down to magnitude 22. The lower the number the brighter the star. Magnitude one is two and a half times brighter than magnitude two.

Moon's motion - the Moon orbits the Earth once every 28 days. It also turns on its axis once every 28 days and this is why it always shows the same face to the Earth

Nova - a star that has exploded usually blowing off a large amount of its material. Sometimes the explosion is very big and the star is totally blown apart. This is called a supernova.

Observatory - a place used for observing the heavens. Observatories are sited on the tops of mountains to avoid pollution due to dust in the atmosphere, to get above the clouds and to get away from the lights of large centres of population.

Photometer - a device to measure a star or planets brightness.

Planetary motion - planets closer to the Sun than the Earth will:
(a) show phases like the Moon
(b) never be more than a small angle from the Sun. This means that they will only be visible from Earth just before dawn or dust after sunset.
(c) sometimes be seen to cross in front of the Sun.

Planets further from the Sun than the Earth may show (b) but never (a) or (c).

Planetary orbits - the planets move round the Sun in ellipses, these ellipses are very close to circles except for the orbit of Pluto.

Quasar - a very distant and intense source of radiation of all types. Something the size of a star giving out the energy of a whole galaxy.

Radio telescope - a telescope to detect the radio waves from space. The hydrogen gas that exists between the stars emits radiation with a wavelength of 21cm.

Red shift - the galaxies are receeding from us at enormous speeds.
This causes a reddening of the light that we receive from them. This is called the Doppler effect and may be heared as a charge of pitch as a train whistle or hooter passes you in a station.

Reflector - a telescope with a concave mirror as the objective.

Refractor - a telescope with a convex lens as the objective

Retrograde motion - the way that the planets appear to move backwards along their orbits as seen from the Earth at some times of the year.

Rotation of the stars - the stars appear to rotate in an anticlockwise direction about the pole star.

Schmidt camera - a telescopic camera designed to take pictures of a large area of the sky.

Solar System - the name given to the Sun and its family of nine (ten) planets.

Solar telescope - a special telescope designed for looking at the Sun.

Southern Hemisphere - the sky viewed form the Southern Hemisphere of the Earth. The star patterns will be different. Some of the constellations will be visible but there will be others never seen from the Northern Hemisphere.

Spectroscope - an instrument for splitting light into its various wavelengths, giving information of star structure.

Steady State Theory - a theory about the universe which says that the universe is not changing, matter is being created all the time and the overall density in constant.

Times of rising - the stars take four minutes less than the Sun to return to the same point in the sky. So if a star rises at 10 pm one night then it will rise at 9.56 pm the following night. One year later the star will once again rise at 10 pm.

Variable star - a star whose brightness is not steady but changes from night to night. Some stars change their brightness very slowly and others vary rapidly.

Zodiac - the popular name for the ecliptic, really a band of the sky centred on the ecliptic.


© Keith Gibbs