Distances
In the words of Douglas Adams, the author of The Hitch-Hiker's Guide to the Galaxy:
Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.
The distances involved in the universe are so vast that we must introduce some new units.
The Astronomical Unit A.U.
The most practical units we can use are related to the distance from the Sun to the Earth. This is the A.U. or Astronomical Unit. 1 Astronomical Unit = 149 598 000 km
The Parsec
How to calculate the parsec
Parallax is the apparent shift in the nearest stars due to the motion of the Earth around the Sun. The method of parallax gives rise to a natural distance unit that astronomers call the parsec (which we shall abbreviate as pc). The parsec is defined to be the distance at which a star would have a parallax angle p equal to one second of arc.
parsec in trigonometric terms.
Taking the unit is the parsec. This is defined as
1 Parsec = 3.08568025 × 1016 m. also used are kpc =1000 pc and Mpc =1 million pc
Light Year
The light year is the distance travelled by light in one year. All electromagnetic waves travel at a speed of x 299,792,458 ms-1 and an average year being 365.25 days. One light year is 299,792,458 x 108ms-1 x (365.25 x 24 x 60 x 60) s =
9.46073 x 1015 m. or 9.46073 x 1012 km.
With our new measuring sticks we can give a few examples of the scale of the universe.
The distance from the Earth to the nearest star after our Sun is 4.42 ly.
The Milky Way Galaxy is about 150,000 light-years across
The andromeda galaxy is 2.3 million light-years away.
The edge of the observable universe is 46.5 Giga light years away.
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