Evening star., 1900-09-29
SKIES IN OCTOBER
Stars Above the Horizon and How to
Locate Them.
DEFINING STELLAR MAGNITUDES
Photometry and What 1t Has Done
for Astronomy.
STUDYING TIL E PL A n EFT S

Written for The Evening Star.
Five stars of the first magnitude will be
above the horizon at O'Clock tonight. They
are Fomalhaut, in the Southern Fish, low
in the south; Vega, in the Lyre, and AL
tair, in the Eagle, in midheavens in the
west, the former on our right, the latter
on our left, as We face in that direction;
Capella, in Auriga, low in the northeast,
and Aldebaran, in Taurus, barely above
the horizon, little north of east To this
list might be added Deneb Cygni, in the
tail of the Swan (Cygnus)-head of the
Northern Cross-now nearly overhead at
the hour named, this star being sometimes
reckoned of the first magnitude.
Earlier in the evening may be seen Arc
turus, low in the northwest, and Antares,
low in the southwest. The very brilliant
white star seen above and at the right of
Antares, setting at about the same time
as it, is the planet Jupiter. Farther east
and setting about two hours later, is Sat
urn, appearing as an orange-yellOw star of
the first magnitude.
Stellar Magnitudes.
For convenience of description the stars
were long ago classed according to their
brightness into .'magnitudes,' the first six
classes embracing all the stars visible to
the naked eye. As the magnitudes are now
usually reckoned, there are twenty stars of
the first magnitude-ten in each hemisphere
of which only fourteen are ever visible in
the mean latitude of the United States, the
remaining six being among the south cir
cum polar stars, which never appear above
our horizon About sixty are of the second
magnitude, 200 of the third, 450 of the
fourth, LOO of the fifth and 1900 of the
sixth. These numbers, except the first are
only approximate, the figures being given
differently by different authorities. 1t thus
a lr,rlno hot tIn -hpla
visible to the naked eye is under 6,OOO Of
which two-thirds are SO faint that they
can be seen only on the clearest nights
The telescope magnitudes run down to the
sixteenth or seventeenth, which last en-
braces the faintest stars revealed by the
largest telescopes or by photography.
Photometry.
OF late years attempts have been made
to introduce some system into this original
ly somewhat arbitrary classification of the
stars, and to place the measurement of
stellar magnitudes, or photometry, as this
branch of astronomy is called, upon a sci
entific basis. Many stars are known and
others are suspected to have changed in
brtlliancy since they were first catalogued
and many others have been found to vary
with greater or less regularity in this re-
spect. 1t Is desirable therefore, to have an
exact record of the brightness of every star
both for immediate and for future use 1t
has been agreed that the classes shall be so
formed that typical or average star Of one
magnitude shall be two and half times
Tor trlfe more) as bright as of the
class next below 1t. This ratio has been
adopted partly because it leaves the mag
nlIudes of the naked-eye stars, as formerly
reckoned. practically unchanged, and partly
for the still better reason that an average
star of the first magnitude is thus rendered
exactly one hundred times as bright as a
star of the sixth. Furthermore, the magni-
tudes are now subdivided into tenths and
even hundredths, and are expressed deci-
mally. Thus star which is midway in
point of brightness between average first
and an average second magnitude star is
said to be Of magnitude 1.5. A star of
which the magnitude is IA or 1.3 is nearer
the first than the second magnitude; if its
magnitude is 1.6 or 1.S it is above the aver
age of the second magnitude, but is not
bright enough to be reckoned a f1rst-mag-
nitude star.
Some Examples.
This photometric system can be rendered
more clear by means of a few examples
selected from the stars which are visible
tonight. The magnitudes are those given
in the Harvard Photometry.
Aldebaran. the Bulls Eye" is a typical
star Of the first magnitude. Fomalhaut is
of magnitude 127-that is, 1t is more than
one-fourth of magnitude below the stand
ard of the class to which 1t belongs. AL
tair is little brighter than Aldebaran. Its
magnitude being OBJ Vega is of magni-
tude O19-that is, it is nearly eight-tenths
of a magnitude brighter than Aldebaran-
and Capella is just trifle brighter than
Vega, its magnitude being O.1S
AS typical second-magnitude stars we
may take Beta Aurigae. the star in the
right shoulder of Auriga magnitude 2.0D,
and Dubhe, the upper of the two pointers
In the Dipper magnitude 1866. The five
stars which form the W. Of Cassiepeia
are usually regarded as Of the second mag
nitude but all of them are considerably be-
low the average brightness of stars of this
class. The brightest two are Alpha and
Gamma-the second and third, reckoned
from the right-hand corner of the w
which are of magnitudes 2.25 and 2.SO, re-
spectively. Deneb, Alpha Cygni, is of
magnitude Ia, and Is therefore, nearer the
first than tho second class, while Beta
Cygni, in the foot of the Cross, is Of magni-
tude 2.40, or about midway between the
second and third magnitudes. The Pole
Star is of magnitude 2.15, and is, therefore,
not full second-magnitude star.
Distribution of Magnitudes.
Mr.. J. E. Gore has just published in
knowledge a list of 1.-THE Hundred Bright
est Stars" arranged in the order of their
magnitudes as determined at the Harvard
College observatory. He gives also their
magnitudes as measured at Oxford and
Potsdam. An analysis of the list brings
out some interesting points. The first %
stars in the list are brighter than magni-
tude 1.6 and may, therefore, be classed as
the Harvard photometry' (a1so cited) the
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