6 
The Astronomical Survey of the Universe [ch. i 
and clouds; they also saw that the extent of this motion would make it 
possible to estimate the distances of the nearer stars. Aristarchus of Samos, 
who anticipated Copemican doctrines as far back as the third century before 
Christ, explained clearly that motion of this kind must be observed unless 
the stars were very remote indeed, and, as no such motion could be detected, 
laid great stress on the extreme distances of the stars. Four centuries later 
Ptolemy argued that the impossibility of detecting such motion proved that 
the earth could not be in motion relative to the stars, and must therefore 
constitute a fixed centre around which the whole universe revolved. When the 
Ptolemaic doctrine was finally challenged by Copernicus and Galileo, it became 
important to detect motion of the kind we have described, both as providing 
final and conclusive proof that the earth was not the unmoving centre of the 
universe, and as giving evidence as to the distances of the stars. 
The apparent motion caused by the swing of the earth in its orbit is 
described as parallactic motion ; the half of the angle swept out by a star as 
the earth moves from one extremity of its orbit to the other (or the angle 
from the mean position to either extreme) is called the “ parallax ” of the 
star. A star whose parallax is one second of arc is at a distance at which the 
mean radius of the earth’s orbit subtends an angle of one second of arc. This 
distance was first introduced as a unit for the measurement of stellar distances 
by Kobold, and was subsequently named the “parsec” by H. H. Turner. 
Since there are 206,265 seconds of arc in a radian, the actual length of the 
parsec is 206,265 times the mean radius of the earth’s orbit. The mean 
radius of the earth’s orbit, commonly called the “astronomical unit” being 
92.870.000 miles, or 149,450,000 kilometres, the parsec is found to be 
19.150.000 million miles or 3083 x 10 18 centimetres. 
Long before the introduction of this unit Herschel had used as unit 
a quantity which he called “the distance of Sirius,” and was supposed to 
represent the mean distance of “ first magnitude ” stars (cf. § 5). Seeliger 
gave precision to this unit, defining it as the distance corresponding to 
a parallax of 5 parsecs, or 1,031,324 astronomical units. Charlier and various 
other continental writers call this unit the Siriometer and define it to be 
1,000,000 astronomical units or 1494 x 10 18 cms. 
Another unit of astronomical distance, especially used in popular exposition, 
is the “ light-year ” or distance which light travels in one year. Since light 
travels 2’998 x 10 10 cms. in a second, and there are 31,557,600 seconds in 
a year, the light-year is found to be equal to 9461 x 10 17 cms. or 5,880,000 
million miles. 
The relation between the three sets of units is as follows : 
One parsec = 3 083 x 10 18 cms. = 3'259 light-years. 
One light-year = 9'461 x 10 17 cms. = 0 3069 parsec. 
One Siriometer = 1494 x 10 18 cms. = 4'848 parsecs.