A catalogue or list of stars is a work giving for each star listed its
magnitude and its position on the celestial sphere, with such other
particulars as may be necessary to attain the object of the catalogue.
If the latter includes only the more conspicuous stars, it is common
to add the name of each star that has one; if none is recognized, the
constellation to which the star belongs is frequently given.
The position of a star on the celestial sphere is defined by its
right ascension and declination. These correspond to the longitude
and latitude of places on the earth, in the following way: Imagine a
plane passing through the center of the earth and coinciding with its
equator, to extend out so as to intersect the celestial sphere. The
line of intersection will be a great circle of the celestial sphere,
called the celestial equator. The axis of the earth, being also
indefinitely extended in both the north and the south directions, will
meet the celestial spheres in two opposite points, known as the north
and south celestial poles. The equator will then be a great circle 90°
from each pole. Then as meridians are drawn from pole to pole on the
earth, cutting the equator at different points, so imaginary meridians
are conceived as drawn from pole to pole on the celestial sphere.
Corresponding to parallels of latitude on the earth we have parallels
of declination on the celestial sphere. These are parallel to the
equator, and become smaller and smaller as we approach either pole. The
correspondence of the terrestrial and celestial circles is this:
To _latitude_ on the earth’s surface corresponds _declination_ in the
heavens.
To _longitude_ on the earth corresponds _right ascension_ in the
heavens.
A little study of these facts will show that the zenith of any point on
the earth’s surface is always in a declination equal to the latitude
of the place. For example, for an observer in Philadelphia, in 40°
latitude, the parallel of 40° north declination will always pass
through his zenith, and a star of that declination will, in the course
of its diurnal motion, also pass through his zenith.
So also to an observer on the equator the celestial sphere always spans
the visible celestial hemisphere through the east and west points.
In the case of the right ascension, the relation between the
terrestrial and celestial spheres is not constant, because of the
diurnal motion, which keeps the terrestrial meridians in constant
revolution relative to the celestial meridians. Allowing for this
motion, however, the system is the same. As we have on the earth’s
surface a prime meridian passing from pole to pole through the
Greenwich Observatory, so in the heavens a prime meridian passes from
one celestial pole to the other through the vernal equinox. Then to
define the right ascension of any star we imagine a great circle
passing from pole to pole through the star, as we imagine one to pass
from pole to pole through a city on the earth of which we wish to
designate the longitude. The actual angle which this meridian makes
with the prime meridian is the right ascension of the star as it is the
longitude of the place on the earth’s surface.
There is, however, a difference in the unit of angular measurement
commonly used for right ascensions in the heavens and longitude on the
earth. In astronomical practice, right ascension is very generally
expressed by hours, twenty-four of which make a complete circle,
corresponding to the apparent revolution of the celestial sphere in
twenty-four hours. The reason of this is that astronomers determine
right ascension by the time shown by a clock so regulated as to read 0
hrs., 0 min., 0 sec. when the vernal equinox crosses the meridian. The
hour hand of this clock makes a revolution through twenty-four hours
during the time that the earth makes one revolution on its axis, and
thus returns to 0 hrs., 0 min., 0 sec. when the vernal equinox again
crosses the meridian. A clock thus regulated is said to show sidereal
time. Then the right ascension of any star is equal to the sidereal
time at which it crosses the meridian of any point on the earth’s
surface. Right ascension thus designated in time may be changed to
degrees and minutes by multiplying by 15. Thus, one hour is equal to
15°; one minute of time is equal to 15′ of arc, and one second of time
to 1″ of arc.
It may be remarked that in astronomical practice terrestrial longitudes
are also expressed in time, the longitude of a place being designated
by the number of hours it may be east or west of Greenwich. Thus,
Washington is said to be 5h. 8m. 15s. west of Greenwich. This, however,
is not important for our present purpose.
The first astronomer who attempted to make a catalogue of all the known
stars is supposed to be Hipparchus, who flourished about 150 B.C. There
is an unverified tradition to the effect that he undertook this work
in consequence of the appearance of a new star in the heavens, and a
desire to leave on record, for the use of posterity, such information
respecting the heavens in his time that any changes which might take
place in them could be detected. This catalogue has not come down to
us--at least not in its original form.
Ptolemy, the celebrated author of the ‘Almagest,’ flourished A.D.
150. His great work contains the earliest catalogue of stars which
we have. There seems to be a certain probability that this catalogue
either may be that of Hipparchus adopted by Ptolemy unchanged, or may
be largely derived from Hipparchus. This, however, is little more than
a surmise, due to the fact that Ptolemy does not seem to have been a
great observer, but based his theories very largely on the observations
of his predecessors. The actual number of stars which it contains is
1,030. The positions of these are given in longitude and latitude, and
are also described by their places in the figure of the constellation
to which each may belong. Not unfrequently the longitude or latitude is
a degree or more in error, showing that the instruments with which the
position was determined were of rather rough construction.
So far as the writer is aware, no attempt to make a new catalogue of
the stars is found until the tenth century. Then arose the Persian
astronomer, Abd-Al-Rahman Al-Sufi, commonly known as Al-Sufi, who was
born A.D. 903 and lived until 986. Nothing is known of his life except
that he was a man celebrated for his learning, especially in astronomy.
His only work on the latter subject which has come down to us is a
description of the fixed stars, which was translated from the Arabic
by Schjellerup and published in 1874 by the St. Petersburg Academy of
Science. This work is based mainly on the catalogue of Ptolemy, all
the stars of which he claimed to have carefully examined. But he did
not add any new stars to Ptolemy’s list, nor, it would seem, did he
attempt to redetermine their positions. He simply used the longitudes
and latitudes of Ptolemy, the former being increased by 12° 42′ on
account of the precession during the interval between his time and
that to which Ptolemy’s catalogue was reduced. The translator says of
his work that it gives a description of the starry heavens at the time
of the author and is worthy of the highest confidence. The main body
of the work consists of a detailed description of each constellation,
mentioning the positions and appearances of the stars which it
contains. Here we find the Arabic names of the stars, which were not,
however, used as proper names, but seem rather to have been Arabic
words representing some real or supposed peculiarity of the separate
stars, or arbitrarily applied to them.
Four centuries later arose the celebrated Ulugh Beigh, grandson of
Tamerlane, who reigned at Samarcand in the middle of the fifteenth
century. Bailey says of him: “Ulugh Beigh was not only a warlike and
powerful monarch, but also an eminent promoter of the sciences and
of learned men. During his father’s lifetime he had attracted to his
capital all the most celebrated astronomers from different parts of
the world; he erected there an immense college and observatory, in
which above a hundred persons were constantly occupied in the pursuits
of science, and caused instruments to be constructed of a better form
and greater dimensions than any that had hitherto been used for making
astronomical observations.”
His fate was one which so enlightened a promoter of learning little
deserved; he was assassinated by the order of his own son, who desired
to succeed him on his throne; and in order to make his position the
more secure, also put his only brother to death. A catalogue of the
stars bears the name of this monarch; he is supposed to have made many
or most of the observations on which it is founded. Posterity will
be likely to suppose that a sovereign used the eyes of others more
than his own in making the observations. However this may be, his
catalogue seems to have been the first in which the positions of the
stars given by Ptolemy were carefully revised. He found that there
were twenty-seven of Ptolemy’s stars too far south to be visible at
Samarcand, and that eight others, although diligently looked after,
could not be discovered. It is curious that, like Al-Sufi, he does not
seem to have added any new stars to Ptolemy’s list.
Next in the order of time comes the work of Bayer, whose method of
naming the stars has already been described. The main feature of this
work consists of maps of all the constellations. Previous to his time,
celestial globes, made especially for the use of the navigator, took
the place of maps of the stars. The first edition of this book was
published in 1603, and is distinguished by the fact that a list of
stars in each constellation is printed on the backs of the maps. Bayer
did not confine himself to the northern hemisphere, but extended his
list over the whole celestial sphere, from the north to the south pole.
The catalogue of the celebrated Tycho Brahe, prepared toward the end of
the sixteenth century, though of great historic value, is of no special
interest to the general reader at the present time. A supplement to
it, continuing its list of stars to the south pole, was published by
Halley, who made the necessary observations during a journey to St.
Helena in 1677.
The catalogue of Hevelius, published in 1690, offers no feature of
special interest, except the addition of several new constellations
which he placed between those already known. Having the aid of the
telescope, he was able to include in his catalogue stars which had been
invisible to his predecessors.
Modern catalogues of the stars may be divided into two classes:
Those which include only stars of a special class, or stars of which
the observer sought to determine the position or magnitude with all
attainable precision; and catalogues intended to include all the
stars in any given region of the heavens, down to some fixed order of
magnitude. It may appear remarkable that no attempt of the latter sort
was seriously made until more than two centuries after the telescope
had been pointed at the heavens by Galileo. A reason for the absence
of such an attempt will be seen in the vast number of stars shown by
the telescope, the difficulty of stopping at any given point, and the
seeming impossibility of assigning positions to hundreds of thousands
of stars. The latter difficulty was overcome by the improved methods of
observation devised in modern times.
About the middle of the present century the celebrated Argelander
commenced the work of actually cataloguing all the stars of the
northern celestial hemisphere to magnitude 9½. This work was termed
a _Durchmusterung_ of the northern heavens, a term which has been
introduced into astronomy generally to designate a catalogue in which
all the stars down to a certain magnitude are supposed to be mustered,
as if a census of them were taken. The work fills three quarto volumes
and contains more than 310,000 stars, of each of which the magnitude
and the right ascension and declination are given. This work was
extended by Schönfeld, Argelander’s assistant and successor, to 22° of
south declination.
In the latitudes in which most of the great observatories of the
world are situated, that part of the celestial sphere within 40° or
50° of the south pole always remains below the horizon. Around this
invisible region a belt of somewhat indefinite breadth, 10° or more,
can be only imperfectly observed, owing to the nearness of the stars
to the horizon, and the brevity of the period between their rising
and setting. Up to the middle of the nineteenth century, the few
observatories situated in the southern hemisphere were too ill-endowed
to permit of their undertaking a complete census of this invisible
region.
The first considerable work emanating from the Cordoba Observatory,
under Gould, was a catalogue of all the stars from the south pole
to 10° of north declination which could be seen with the naked eye.
Another work, which was not issued until after Dr. Gould’s death, was
devoted to photographs of southern clusters of stars.
The work of the Cordoba Observatory, with which we are more especially
concerned in the present connection, consists of a ‘Durchmusterung’ of
the southern heavens, commencing at 22° of south declination, where
Schönfeld’s work ended, and continued to the south pole. This work
is still incomplete, but two volumes have been published by Thome,
extending to 41° of south declination. It is expected that the third is
approaching completion. This catalogue is, in one point at least, more
complete than that of Argelander and Schönfeld, as it contains all the
stars down to the tenth magnitude. The two volumes give the positions
and magnitudes of no less than 340,000 stars, and therefore more than
the catalogue of Argelander gives for the entire northern hemisphere.
If the remaining part of the heavens, from 42° to the south pole, is
equally rich, it will contain nearly half a million stars, and the
entire work will comprise more than 800,000 stars.
The Royal Observatory of the Cape of Good Hope, under the able and
energetic direction of Dr. David Gill, has undertaken a work of the
same kind, which is remarkable for being based on photography. The
history of this work is of great interest. In 1882 Gill secured the
aid of photographers at the Cape of Good Hope to take pictures of
the brilliant comet of that year, with a large camera. On developing
the pictures the remarkable discovery was made that not only all the
stars visible to the naked eye, but telescopic stars down to the ninth
or tenth magnitude were also found on the negatives. This remarkable
result suggested to Gill that here was a new and simple method of
cataloguing the stars. It was only necessary to photograph the heavens
and then measure the positions of the stars on the glass negatives,
which could be done with much greater ease and certainty than measures
could be made on the positions of the actual stars, which were in
constant apparent motion.
As soon as the necessary arrangements could be made and the apparatus
put into successful operation, Gill proceeded to the work of
photographing the entire southern heavens from 18° of south declination
to the celestial pole. The results of this work are found in the ‘Cape
Photographic Durchmusterung,’ a work in three quarto volumes, in which
the astronomers of all future time will find a permanent record of the
southern heavens towards the end of the nineteenth century. The actual
work of taking the photographs extended from 1887 to 1891. This,
however, was far from being the most difficult part of the enterprise.
The most arduous task of measuring the positions of a half-million of
stars on the negatives, including the determining of the magnitude of
each, was undertaken by Professor J. C. Kapetyn, of the University of
Groningen, Holland, and brought to a successful completion in the year
1899.
What the work gives is, in the first place, the magnitude and
approximate position of every star photographed. The determining of
the magnitude of a star is an important and delicate question. There
is no difficulty in determining, from the diameter of the image of the
star as seen in the microscope, what its photographic magnitude was
at the time of the exposure, as compared with other stars on the same
plate. But can we rely upon similar photographic magnitudes on a plate
corresponding to similar brightnesses of the stars? In the opinion of
Gill and Kapetyn we cannot. The transparency of the air varies from
night to night, and on a very clear night the same star will give a
stronger image than it will when the air is thick. Besides, slightly
different instruments were used in the course of the work. For these
reasons a scale of magnitude was determined on each plate by comparing
the photographic intensity of the images of a number of stars with the
magnitudes as observed with the eye by various observers. Thus on each
plate the magnitude was reduced to a visual scale.
It does not follow from this that the magnitudes are visual, and not
photographic. It is still true that a blue star will give a much
stronger photographic image than a red star of equal visual brightness.
In a general way, it may be said that the catalogue includes all the
stars to very nearly the tenth magnitude, and on most of the plates
stars of 10.5 were included. In fact, now and then is found a star of
the eleventh magnitude.
A feature of the work which adds greatly to its value is a careful
and exhaustive comparison of its results with previous catalogues of
the stars. When a star is found in any other catalogue the latter is
indicated. Most interesting is a complete list of catalogued stars
which ought to be on the photographic negatives, but were not found
there. Every such case was inexhaustibly investigated. Sometimes the
star was variable, sometimes it was so red in color that it failed
to impress itself on the plate, sometimes there were errors in the
catalogue.
The great enterprise of making a photographic map of the heavens
now being carried on as an international enterprise, having its
headquarters at Paris, is yet wider in its scope than the works we
have just described. One point of difference is that it is intended to
include all the stars, however faint, that admit of being photographed
with the instruments in use. The latter are constructed on a uniform
plan, the aperture of each being 34 centimetres, or 13.4 inches, and
the focal length 343 cm. Two sets of plates are taken, one to include
all the stars that the instrument will photograph near poles, and the
other only to take in those to the eleventh magnitude. Of the latter
it is intended to prepare a catalogue. Some portions of the German and
English catalogues have already been published, and their results will
be made use of in the course of the present work.