Skywatch - 2021 - 19

known by its ancient Greek name
Zosma ("girdle"), marks the top of
the powerful hindquarters.
Alpha-Beta. Greek fans might
notice a pattern: Leo's stars are
named in order of brightness in
sync with the Greek alphabet. This
system dates back to 1603, when
German astronomer Johann Bayer
endeavored to instill order and
method into the myriad Arabic,
Greek, and Latin names peppering
the sky. In his Uranometria star
atlas, which consisted of 49 charts
(including one for 12 new southern
constellations, the fi rst star atlas to
do so), he identified the main stars in
each constellation by Greek letters.
Bayer assigned the letter Alpha to all
the brightest stars, the letter Beta to
the second brightest, Gamma to the
third brightest, and so on. However,
he didn't have a systematic method
for gauging stellar brightnesses,
which inevitably led to exceptions
- in fact, in 16 constellations Alpha
isn't actually the brightest star. At
times, he assigned the letters according to stars' positions, as in the
Big Dipper where stars are labeled
according to right ascension (their
placement right to left on the sky).
This Bayer lettering system is
still in use today. In fact, star names
under the Bayer system are a mix of
Greek and Latin. Let's take Regulus
- Alpha Leonis - as an example.
The Greek letter denotes the order
of the star's brightness in the constellation. In this case, Alpha tells us it's
the brightest star (and it really is).
Leonis tells us it's in the constellation Leo - all constellation names
are declined according to the Latin
genitive case, denoting possession; so
Leonis means "of Leo."

to be larger than faint stars, and so we
adopted the word magnitude - from
the Latin magnus ("great" or "large")
- to describe this phenomenon. The
modern magnitude scale, formulated
less than 200 years ago, is logarithmic:
Each magnitude class is about 2.5
times brighter than the next one down.
Hence, fi rst-magnitude stars (i.e., magnitude 1) are 100 times brighter than
sixth-magnitude stars. Stars brighter
than magnitude 0 are assigned negative
magnitudes - Sirius, for example, is
magnitude -1.5. Notice that the brighter
the star the smaller the magnitude. That
takes a bit of getting used to.
In the Sickle we have a handy tool
for understanding magnitudes: There's
one star of each magnitude class from
1 to 5. The figure below labels the stars
of the Sickle with their Bayer letters
along with their magnitudes, but note
that we're "borrowing" Kappa Leonis
for this exercise. One more thing to
bear in mind is that a magnitude class

AKIRA FUJII

Last
Qtr

5 | 8:30 p.m. EST

New
Moon

13 | 5:21 a.m. EST

First
Qtr

21 | 10:40 a.m. EDT

Full
Moon

28 | 2:48 p.m. EDT

brackets the whole number: for example,
a 2nd-magnitude star can lie anywhere
between magnitude 1.5 and 2.4. Starting with Regulus at the bottom, the
magnitudes increase as we climb up
the Sickle and curl along the blade (or
mane). Alpha is magnitude 1.4, which
makes it a fi rst-magnitude star (barely).
Gamma is a second-magnitude star,
Zeta third-magnitude, and Mu fourthmagnitude. Notice that Kappa is magμ
3.9
κ
4.5

ζ
3.4

γ
2.4

Algieba

Sickle

Magnitudes - or how bright
stars appear. The original magni-

tude classification system dates back
several thousand years when either
Hipparchus or Ptolemy (or both;
sources diﬀer) assigned numbers
to stars according to how bright
they looked. Brighter stars appear

MOON PHASES

α
1.4

Regulus

p The

Sickle of Leo might look like a lion's mane to some people. The stars are labeled by their Greek
designations, and their magnitudes are also listed. Technically, Kappa Leonis isn't a Sickle star, but we're
borrowing it to create our magnitude gauge as outlined in the text.
SK Y WATCH 2021

Skywatch - 2021

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