Sky & Telescope - February 2023 - 76

BEGINNER'S SPACE by Diana Hannikainen
What Are the Greek
Letters on Star Charts?
TAKE A CLOSE LOOK at the central
star map featured on pages 42-43 - or
at the snippet at right. You'll see that
besides the constellation names (in
purple) and star names (in white) there
are Greek letters accompanying many
of the star symbols. (Messier objects are
prefixed with the letter " M. " ) You might
very well have been wondering what
those curious characters mean and
what purpose they serve. Well, we're
here to clear that up for you.
Higgledy-Piggledy Heavens
There wasn't much organization to the
heavens back in the mists of time, when
early stargazers first cast their eyes skyward.
We owe much of our knowledge
of the ancient skies to the 2nd-century
Alexandrian astronomer, Claudius
Ptolemaeus, better known as Ptolemy.
His legacy is a fascinating compendium
entitled the Almagest.
The Almagest counts 13 books, of
which Books VII and VIII are a star
catalog, one of the earliest known. Ptolemy
expanded on the work of the great
Greek astronomer and mathematician
Hipparchus - in fact, it's largely thanks
to Ptolemy that we know of Hipparchus's
efforts. The Almagest lists 1,022
stars along with their celestial coordinates
and magnitudes. Ptolemy ranked
the brightest stars magnitude 1 and the
faintest visible to the unaided eye magnitude
6, based on Hipparchus's original
system. Each magnitude is twice as
bright as the preceding one, making
it a logarithmic scale. Ptolemy also
arranged the heavens into 48 constellations,
47 of which are still in use today
(see page 24). Such was the influence
of the Almagest that it functioned as a
textbook until the Renaissance, right up
76 FEBRUARY 2023 * SKY & TELESCOPE
to when Nicolaus Copernicus placed the
Sun at the center of our solar system.
During the ensuing centuries following
publication of the Almagest, celestial
cartographers used the information in
Ptolemy's catalog to produce star atlases.
However, more often than not, they
placed stars randomly on constellation
lines, dictated by artistic license rather
than reality. Also, there was neither
order nor method to the types or sizes
of symbols used to denote the stars.
Take a look at the depiction of Orion
from the Leiden Aratea (facing page,
bottom left), dated to the Middle Ages.
What do you see? Your first impression
might be that it portrays Orion as the
mythological hunter, holding up an animal
skin folded over his left arm and his
sword hanging from his belt. You might
notice that the stars are all similar in
size - and if you're familiar with the
constellation, you'll also see that there
appear to be extra stars on the Hunter's
back and several missing in places where
we'd expect to see some. It might be a
pretty picture, but it's not terribly useful
to the serious astronomer.
Thankfully, a systematic-minded
celestial cartographer took the matter
into his own hands and tidied up the
heavens for us.
Enter the Greeks
German magistrate and amateur
astronomer Johann Bayer published a
celestial atlas in 1603 that organized the
stars into 60 constellations. He largely
drew on the meticulous work of Danish
astronomer Tycho Brahe for Northern
Hemisphere star magnitudes and positions.
Bayer's opus, entitled Uranometria,
was the first star atlas to portray the
constellations around the south celestial
pole (gathered together in a single
plate). In addition, Bayer's constellations
reproduce the stars as we see them looking
up into the night sky - until then,
many star atlases and globes showed the
constellations as if we were looking down
onto the celestial sphere.
Most importantly for our discussion,
Bayer introduced the letters of the
Greek alphabet to label the brightest
stars in a constellation. Not only that,
he arranged them in order of magnitude.
So (for the most part) he assigned
Alpha to the brightest star, Beta to the
next brightest, Gamma to the third
brightest, and so on. (When he ran out
of the 24 letters of the Greek alphabet
for larger constellations with many
stars, he continued with a system using
Latin letters.) Auriga, the Charioteer,
Sky & Telescope - February 2023

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