North Hawaii News Articles from CFHT
Astronomical Distances II - An Astronomical Revolution
In Europe, the years from roughly 1550 to 1650 saw a revolution in
science, and Astronomy was at the forefront. The scientists in this
time began to study the world around them by rigorously testing
theories with observations of the real world and by questioning the
validity of theories which were not supported. For astronomy, this
period was the break between astronomy and astrology, between science
Nicolaus Copernicus studied astronomy because he studied medicine. In
the 1500s, doctors studied astronomy so they would know where the
planets were, to make astrological predictions for the treatment. In
his studies, Copernicus realized that the observations of the motions
of planets made more sense if the Earth and the other planets orbited
the Sun, instead of having the Sun and planets orbit the Earth. His
theory, published in 1543, gained acceptance slowly, but it got a huge
boost from observations made over the next 100 years by Johannes
Kepler and Galileo Galilei. The Church tried to stop these theories,
which it considered heretical, and sentenced Galileo to house arrest
in 1632. But they could not stop a tidal wave.
By the middle of the 1600s, Astronomy had been changed irrevocably.
Astronomers now studied the heavens to understand them, not to support
dubious, mystic predictions. Now they wanted to know hard facts: the
size of the Sun, the distance to the planets, the distance to the
stars. By 1650, the layout of the solar system was widely accepted.
The understanding was deeper than just the fact that the Sun was at
the center. Observations of the motions of the planets made it
possible for the size and shape of all of the orbits to be calculated
exactly. The only problem was, the size depended on the distance
between the Earth and the Sun, which was unknown. In fact, if you can
measure the distance to any planet in the solar system at any time,
then you can get all of the other distances at once!
So, how to measure the distance to a planet? The trick comes from
something called parallax. Hold your finger 12 inches from your face.
Look at your finger with just one eye, then the other. You'll see
that your finger moves compared to the background scenery. The amount
it moves depends on the distance between your eyes and the distance to
your finger. If you know the distance between your eyes, and the
change in position, then you can calculate the distance to your finger
(though it is easier to use a ruler...). Astronomers realized in the
middle of the 1600s that if they could measure the parallax of a
planet, by observing it from two different locations on the Earth at
the same time, they would solve the problem of the solar system scale.
They also realized at the same time that it should be possible to
measure the parallax of the nearest stars as the Earth moves around
the Sun. This would give the distance to the nearest stars, but only
if the distance to the Sun was already known. The race was on!
Actually, it was a very slow race. It was much harder to do either of
these tasks than probably anyone had realized in 1650. The problem of
the Solar System Scale was not solved until 1769. It turns out that
the easiest situation to measure the parallax of a nearby planet is to
watch Venus on the rare occasions that it crosses in front of the face
of the Sun. The disk of the Sun acts as a convenient reference, so it
is easy to measure the different angles even with fairly primitive
equipment. The measurement was tried a few times with inconsistent
results, but for the 1769 Transit, the Astronomy world was ready.
Expeditions were mounted to many places around the world, including
Captain Cook's first voyage to the Pacific, which observed the transit
from Tahiti. After the observations were made and compared, the
distance to the Sun was found to be around 92 million miles, very
close to our best current measurements.
It was even harder to get the distance to the nearest stars. It was
necessary to make more and more careful measurements of the positions
of many stars. Along the way, astronomers discovered several other
effects they had not expected before the parallax could be measured.
They found that the apparent positions of stars change because the
speed of light is not infinite. They found a small wobble in the
rotation of the Earth. They found that some of the nearest stars
actually move across the sky because they move relative to the Sun -
they are not fixed at all. Finally, in 1838, 1839, and 1840, three
astronomers independently measured the parallax of three different
stars, 61 Cygni, Alpha Centaurus, and Vega. The nearest of these is
just about 250,000 times further from the Sun than Earth.
After 200 years, astronomers had finally measured the distance to the
nearest stars. The new measurements showed that the Universe is much
larger than anyone had suspected. In the next article I will discuss
how astronomers since the mid-19th century have learned how to measure
distances to other stars in our Galaxy and also to the other, distant