126 Chapter 2 From the Solar System to the Milky Way
Triangulation Can Give Us the Distance to Stars Beyond
the Solar System
Using the Moon to create a baseline is one way of using Aristarchuss method to calculate
long distances in space. But there is an even better method that uses Earth’s radius of
It goes without saying that Earth revolves once around the Sun in one year. Therefore,
if we observe a heavenly body outside of our solar system at some time and then observe
it six months later, we can see how much the star’s position has changed compared to the
stars around it. Because measuring the change of position gives us the change in the angle
(angle of inclination) required to look up at it, we can use that value to determine the dis-
tance to that heavenly body.
Incidentally, half of the difference in those angles is called the annual parallax, and the
distance to a heavenly body for which this parallax is 1 arc second (1 arc second is 1/3600
of a degree) is a unit called a parsec (short for parallax per one arc second). This unit is
related to other units of distance as follows.
1 pc (parsec) = approximately 3.26 light-years = approximately 206,265 AU = approxi-
mately 3.08568 × 10
m = approximately 31 trillion km = approximately 1.9 × 10
For heavenly bodies that can be observed from Earth by this method, the maximum
parallax is approximately 0.033 arc seconds (approximately 1/100,000 of a degree), and the
maximum distance is approximately 30 parsecs or 100 light-years. In 1989, the European
Space Agency launched the Hipparcos High Precision Parallax Collecting Satellite, which can
accurately measure the distance to stars up to 500 light-years away or up to 1,000 light-
years away within a reasonable margin of error.
Radius of revolution
(1 AU)
Apparent position changes
Earth (summer)
Earth (winter)
Annual parallax
Triangulation using Earth’s radius of revolution

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