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ASTRO-INFO

  Planet Facts   Some basic values for the Solar System bodies
  The Stars   Brightest, biggest, smallest, closest
  The Universe   Info about the main objects in the universe
  Telescope Formulas   Several formulas to help you find information about your telescope
  Links   Handy websites

Glossary

  
Angular measurement 1 degree () = 1/360 of a circle
1 minute  (')  = 1/60 of a degree
1 second ('') = 1/60 of a minute
e.g. The Moon and Sun are (on average) half a degree across or 30 arc minutes
       Mars ranges in apparent diameter from just 3.5 - 25.1 arc seconds
Astronomical Unit (AU) The mean distance of the Earth's orbit from the Sun = 150,000,000 kilometres
This measure is used for distances within a Star System.
e.g. Saturn is 11 AU from the Sun
        Pluto's orbit varies from 29.7 to 49.3 AU from the Sun.
Light Year (LY) The distance light travels in one year.
Light (and all electromagnetic radiation) travels at 300,000 kilometres per second in a vacuum, so:
in one minute it travels 18,000,000 kilometres
in one hour it travels 1,080,000,000  kilometres
in one day it travels 25,920,000,000  kilometres
in one year it travels 9,460,800,000,000  kilometres
e.g. It takes light about 8.3 minutes to travel from the Sun to the Earth.
Magnitude (visual or apparent) The brightness of an astronomical object seen from Earth.
Faint objects have larger numbers and bright objects have smaller numbers.
The faintest stars visible to the naked eye (magnitude 6) are 100 times fainter than the brightest (magnitude 1), and so each magnitude step is roughly 2.5 times brighter or fainter.
e.g. Magnitude 3 is 2.5 times fainter than magnitude 2.
        Magnitude 3 is 6.25 times fainter than magnitude 1.
When an object is brighter than magnitude 0 it is shown with a negative value
e.g. Sirius (Alpha Canis Major) is magnitude -1.44
Magnitude (absolute) Is the intrinsic brightness of an object as seen from a standard distance. For stars it is 10 parsecs.
e.g. Our sun has an apparent magnitude of -26 but seen from 10 pc is +4.83
        Rigel (Beta Orion) has an apparent magnitude of -0.18 but seen from 10 pc is -7.1
Parallax (astronomical) Is the apparent movement of a nearby star with reference to more distant background stars when measured from either side of Earth's orbit. Measurements of such a star taken six months apart reveals its motion back and forth. Using the angle of the stars movement and the radius of the Earth's orbit as the baseline, the distance is derived by trigonometric calculation.
Parsec (pc) A standardised measurement used by astronomers, it is the distance to a celestial object whose 'parallax' is equal to 1 arc second.
1 parsec = 3.26 Light Years.
kiloparsec = 1000 parsecs = 3,260 Light Years.
megaparsec = 1,000,000 parsecs = 3,260,000 Light Years.