Common Stargazing Terms #1

The universe is too big, the kilometre unit for distance is no longer sufficient to conveniently represent the vast distances between stars and galaxies. We need new units, units that are much much larger than the kilometre to avoid too many zeros at the back. Introducing the astronomical unit (AU), light-year (ly), and parsec (pc).

Astronomical Unit (AU)
This is equal to the mean distance between the Earth and the Sun. 1AU = 149,598,500 km, or simply 150 million km.

Light-year (ly)
This is equal to the distance light travels in one year. 1 ly = 9.5 x 10^12 km.
Light travels at about 300,000 km/s, so it travels 300,000 km/s x 3600 s/hr x 24 hr/day x 365.24 days/year = 9.5 x 10^12 km/year.

ParsecParsec (pc)
This is the distance at which an object would have a parallax of one arc-second (1″). 1 pc = 3.26 light-year.

Parallax is a common phenomenon; it is the apparent difference in the position of an object when an observer’s position is changed. The simplest test you can do is close your right eye and place a finger in front of you, now open your right eye and close your left eye, you can see that the position of your finger seems to change. It is not that your finger is moving, it is the eye’s position that had changed (right eye to left eye).

The same thing happens in the sky. Our Earth is constantly revolving around the Sun. Hence the position of some nearby stars will have parallax as we changed our position in space. The distance at which a star would have a parallax of one arc-second is called one parsec. Parsec actually is short for “parallax of one second of arc”.

This distance, d is derived using simple trigonometry. 1″ = 1/3600 deg. Hence tan (1/3600) = 1AU / d => d = 1AU / tan (1/3600) = 206264.8 AU = 3.26 light-years.

For larger distance, kiloparsec (kpc) or even megaparsec (Mpc) is used. 1 kpc = 1000 pc and 1 Mpc = 1000 kpc.


~ by thChieh on April 15, 2008.

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