Lectures on Stellar Statistics | Page 2

for instance [delta] = -35°.84, we write
([alpha][delta]) = (1744{35}),
so that the last two figures are in italics.
[Transcriber's Note: In this version of the text, the last two figures are
enclosed in braces to represent the italics.]
This notation has been introduced by PICKERING for variable stars
and is used by him everywhere in the Annals of the Harvard
Observatory, but it is also well suited to all stars. This notation gives,
simultaneously, the characteristic numero of the stars. It is true that two
or more stars may in this manner obtain the same characteristic numero.
They are, however, easily distinguishable from each other through
other attributes.
The galactic coordinates l and b are referred to the Milky Way (the
Galaxy) as plane of reference. The pole of the Milky Way has
according to HOUZEAU and GOULD the position ([alpha][delta]) =
(124527). From the distribution of the stars of the spectral type B I
have in L. M. II, 14[2] found a somewhat different position. But having
ascertained later that the real position of the galactic plane requires a
greater number of stars for an accurate determination of its value, I
have preferred to employ the position used by PICKERING in the
Harvard catalogues, namely ([alpha][delta]) = (124028), or

[alpha] = 12h 40m = 190°, [delta] = +28°,
which position is now exclusively used in the stellar statistical
investigations at the Observatory of Lund and is also used in these
lectures.
The galactic longitude (l) is reckoned from the ascending node of the
Milky Way on the equator, which is situated in the constellation Aquila.
The galactic latitude (b) gives the angular distance of the star from the
Galaxy. On plate I, at the end of these lectures, will be found a fairly
detailed diagram from which the conversion of [alpha] and [delta] of a
star into l and b may be easily performed. All stars having an apparent
magnitude brighter than 4m are directly drawn.
Instead of giving the galactic longitude and latitude of a star we may
content ourselves with giving the galactic square in which the star is
situated. For this purpose we assume the sky to be divided into 48
squares, all having the same surface. Two of these squares lie at the
northern pole of the Galaxy and are designated GA1 and GA2. Twelve
lie north of the galactic plane, between 0° and 30° galactic latitude, and
are designated GC1, GC2, ..., GC12. The corresponding squares south
of the galactic equator (the plane of the Galaxy) are called GD1,
GD2, ..., GD12. The two polar squares at the south pole are called GF1
and GF2. Finally we have 10 B-squares, between the A- and C-squares
and 10 corresponding E-squares in the southern hemisphere.
The distribution of the squares in the heavens is here graphically
represented in the projection of FLAMSTEED, which has the
advantage of giving areas proportional to the corresponding spherical
areas, an arrangement necessary, or at least highly desirable, for all
stellar statistical researches. It has also the advantage of affording a
continuous representation of the whole sky.
The correspondence between squares and stellar constellations is seen
from plate II. Arranging the constellations according to their galactic
longitude we find north of the galactic equator (in the C-squares) the
constellations:--

Hercules, Cygnus, Cepheus, Cassiopæa, Auriga, Gemini, Canis Minor,
Pyxis, Vela, Centaurus, Scorpius, Ophiuchus,
and south of this equator (in the D-squares):--
Aquila, Cygnus, Lacerta, Andromeda, Perseus, Orion, Canis Major,
Puppis, Carina, Circinus, Corona australis, Sagittarius,
mentioning only one constellation for each square.
At the north galactic pole (in the two A-squares) we have:--
Canes Venatici and Coma Berenices,
and at the south galactic pole (in the two F-squares):--
Cetus and Sculptor.
3. Changes in the position of a star. From the positions of a star on two
or more occasions we obtain its apparent motion, also called the proper
motion of the star. We may distinguish between a secular part of this
motion and a periodical part. In both cases the motion may be either a
reflex of the motion of the observer, and is then called parallactic
motion, or it may be caused by a real motion of the star. From the
parallactic motion of the star it is possible to deduce its distance from
the sun, or its parallax. The periodic parallactic proper motion is caused
by the motion of the earth around the sun, and gives the annual
parallax ([pi]). In order to obtain available annual parallaxes of a star it
is usually necessary for the star to be nearer to us than 5 siriometers,
corresponding to a parallax greater than 0".04. More seldom we may in
this manner obtain trustworthy values for a distance amounting to 10
siriometers ([pi] = 0".02), or even still greater values. For such large