There are many different methods used to find stellar
distances, but the earliest one used on the nearest stars
involves the principle of parallax. Hold a pencil upright
in front of your face at about arms length. Now blink
each eye in succesion. You will see an apparent shift in
position of the pencil against the background. That shift
You will also find that the nearer the pencil, the greater
the parallax. As distance increases, parallax is less
apparent. The effect is dependent on both the distance
to the object and the distance between your eyes.
To notice parallax in distant objects, your 'eyes' need
to be further apart. Surveyors accomplish this with two
different measurements, from each location of the simulated
'eyes'. Measuring the distance between these two 'eye'
locations, and the amount of parallax shift, distance can
For stellar distances, two 'eyes' on the earth are still
not far enough apart to create an apparent shift. This is
limited by the earth's diameter--8000 miles. So the two
observation points used are from opposite sides of the
earth's orbit around the Sun. Since we know the diameter
of the earth's orbit, and that it travels from one side
to the other every 6 months, a 'nearby' star's position
shift from six months ago against a background of further
stars can be measured. Using the same calculations that
a surveyor on earth uses, the distance to the nearer
stars can then also be determined.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor
There are lots of ways, but historically the most accurate has been parallax.
This is the same effect that makes distant trees appear to move more slowly than
nearby ones when you drive past them in a car. Basically, as the Earth orbits the
Sun, nearby stars appear to move a teeny bit as a reflection of our motion, while
stars farther away move less. Since we know how big the orbit of the Earth is, we
can use trigonometry to calculate the stars' distances.
I have info about all this at my website: Bitesize Astronomy, part of
my Bad Astronomy website.
Answered by: Phil Plait, Astronomer (http://www.badastronomy.com)
'The atomic bomb ... made the prospect of future war unendurable. It has led us up those last few steps to the mountain pass; and beyond there is different country.'