OUT THERE
6464 DISCOVERMAGAZINE.COMDISCOVERMAGAZINE.COMMANY OBSERVERS (me included) pride
themselves in knowing the brightnesses
of numerous celestial objects. That
measurement, after all, is one of the
basic tools of our science and our
hobby. But how many of us know the
story of the magnitude system?
Let’s dwell a bit and detail how the
system developed, explain the different
kinds of magnitudes, and provide a
couple of nice tools you can use to
deal with them.A BIT OF HISTORY
The Greek astronomer Hipparchus,
who lived in the second century B.C.,
was the first person to write extensively
about stars of different brightnesses.
He divided them into six ranges, which
he called magnitudes. He classified the
brightest as stars of the 1st magnitude
and the faintest as stars of the 6th
magnitude. Astronomers have used his
system, almost unchanged, for almost
2,200 years.
Hipparchus produced a list of
about 850 stars for a catalog he
compiled. That catalog no longer
exists, but Greek philosopher Ptolemy
incorporated all the entries into books
VII and VIII of his massive work,
the Almagest. In the second century,
Ptolemy expanded the catalog to 1,022
stars, but retained the six magnitude
ranges Hipparchus had created.
Fifteen centuries later, Galileo
Galilei pointed an “optic tube” that he
had built toward the sky. In addition
to discovering irregularities on the
moon’s face, that Venus goes through
phases, and that four moons orbitJupiter and not Earth, he noticed
something about his telescope. It
didn’t simply magnify objects — it
literally revealed the invisible.
Writing in the Sidereus Nuncius
(Starry Messenger) in 1610, Galileo
stated, “Indeed, with the glass you
will detect below stars of the sixth
magnitude such a crowd of others that
escape natural sight that it is hardly
believable.” Then the great scientist
coined a term that no one had used
before. He called the brightest of
the stars below naked-eye visibility
“seventh magnitude.”
As telescopes got bigger, astrono-
mers decided it was time to expand
the magnitude system. A multitude
of stars fainter than those listed as
6th magnitude by Hipparchus were
now visible. And there was a problem
on the other end of the scale as well.
The stars designated 1st magnitude
varied greatly in brightness. Some, like
Sirius, dramatically outshone others
like Spica. And what if astronomers
wanted to assign magnitudes to the
planets? Or the moon? Or — would it
be possible — the sun?
By the late 1700s, astronomers were
using an informal system in which
stars that differed by one magnitude
had a brightness difference of about
two and a half. Researchers and
amateurs used that loose definition for
some 70 years.
But during this time, how were they
determining the numbers to assign as
magnitudes? In 1851, English astrono-
mer William R. Dawes demonstrated
one way. He used a wedge of varyingHow Astronomers
Measure Brightness
Whose bright idea was this backward magnitude system anyway?
BY MICHAEL E. BAKICHThe sky is filled with objects
that cover a huge range of
brightnesses. The brightest star,
Sirius, is roughly 4,000 times
brighter than the faintest star
visible to the naked eye.