The distance to stars are so great that the ordinary units - miles -
are no longer practicable. The nearest star is 25 trillion miles (25,000,000,000,000
miles) away - a number too awkward to write, to remember, or to use.
Three units are commonly used in Astronomy: (1) the Astronomical
Unit; (2) the Light Year; and (3) the Parsec.
(1.) The Astronomical Unit
is, by definition, equal in length to the distance from the earth to the sun - 93 million
miles (93,000,000 miles)
This unit, astronomically speaking, is a fairly small one, and it is
used primarily in stating distances within the Solar System. Thus, the distance of
Pluto is 40 Astronomical Units, or 40 X 93,000,000 miles = 3,720,000,000 miles
(A.U. is the proper abbreviation for Astronomical Unit.)
(2) Another unit of astronomical distance is the Light Year, defined simply as the distance traveled
by a ray of light in a year. Knowing that the speed of light is 186,000 miles in one
second, it is easy to compute that distance. 186,000 is multiplied by the number of
seconds in a year:
186,000 X 60 (seconds per minute) X 60 (min's per hour) X 24 (hours per day) X
365¼ (days per year) = 5,880,000,000,000 miles (5.88 trillion miles) or ( 5.88 X
1012
miles)
Using this unit of distance, Alpha-Centauri is at a distance of 4.2 light years; or,
the light by which we see the star has been traveling 4.2 years.
(3) The definition of the Parsec
is based on a triangle as follows:
If (a), in a triangle, one of the angles is
90º; and if (b), one of the legs at the right angle is 93 million miles long; and if (c),
the angle opposite it is one second;
then, the length of the other 90º leg is equal
to one Parsec. Thus, if angle B is the 90º angle, AB is 1 A.U. long, and angle D is
1 second of angle in size, then BD will have a length of 1 Parsec.
If (a), the angle B is 90º; (b), the side of AB is 1 A.U. (93,000,000
miles) long; (c), the angle D is 1 second, then: the side BD is equal to one Parsec.
updated: