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The scale of distances are huge, not for nothing are they referred to as 'astronomical'.
A good way of appreciating the scale of things, suggested by Arthur C Clarke, is to think first of everyday distances; a quarter of a mile is a short walk, ten miles a good walk, a hundred miles would be a car journey, while a thousand miles would be a major journey, probably by plane.
To get the same kind of scale for the Solar System we need to define a new unit, the Mile = one million miles. The Moon is one quarter of a Mile away, a short walk!, the Sun is about 100 Miles and the edge of the orbit of Pluto about 3000 Miles away. For the realm of the stars however even this unit is too small and we need to define the MMile = one million Miles or 1012 miles. On this scale the stars you see in the night sky are only a few MMiles away, and you just have to get used to the idea that on the cosmological scale that is quite close!
Clearly for such huge distances we need units bigger than the humble mile or the SI metre. For distances within the Solar System we use the astronomical unit (AU). This is defined as the average distance between the Earth and the Sun,
Although the light-year is simple to understand is not easy to use. In practice, Astronomers use a different unit the parsec (pc). Hold a pencil out at arms length, view it with your left eye only, then right eye only. Notice how the position of the pencil changes against the background, this effect is called parallax and it is how the eye/brain system gives you 3-D vision. The further away the pencil is, the smaller is the angle between the directions viewd by each eye, and beyond a certain distance the effect becomes too small to measure. Now scale this system up so that the left eye view becomes the view from Earth at one side of its orbit, and the right eye view, the view six months later. Even with such a big baseline (2 AU) the parallax angle can only be measured in seconds of arc. (60 seconds of arc = 1 minute of arc; 60 minutes of arc = 1 degree)
There is one more thing to consider before we can define the parsec, which you may by now have guessed has something to do with a parallax of seconds of arc. Trying to view the same star six months apart is not normally possible because what was the night sky in one season becomes the daytime sky six months later, so 1 parsec is defined as the distance at which a star has a parallax of 1 second of arc when viewed from a baseline of 1 AU, equivalent to viewing three months apart. This makes 1 pc = 3.08 x 1016 m. = 3.26 ly = 206265 AU
There are several ways of measuring the parallax from which the distance an be easily calculated. If the parallax is1/2 arcsec the distance is 2 pc; if1/4
arcsec, it is 4 pc away. For distances to Galaxies this unit is still too
small so the megaparsec (Mpc) is used, equal to one million parsec.
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