Richmond dispatch. [volume] (Richmond, Va.) 1884-1903, 1898-08-07


the creation OF a catalogue OF over 1,ooo
stars. prepared on modern principles.
| That catalogue formed the basis of one
! which survives 10 us at the present time.
and was made. some LOO years
ago. by Claudius Ptolemy, the great as
tronomer Of Alexandria, whose work.
which still bears the proud name or
Almagest ''the greatest" remained for
fourteen centuries the one universal as
! Economical textbook.
FOUR COLUMNS OF ENTRY.
| A modern catalogue contains, like that
OF Ptolemy. four columns Of entry. The
| first gives the stars designation; the
\ second an indication OF its brightness;
! the third and fourth. the determinations
| OF It's place. These are expressed in two
| directions which, in modern catalogues,
not in Ptolemy's, correspond on the celes-
/ tial sphere TO longitude and latitude
on the terrestrial. Distance north or
south of the celestial equator is termed
''declination,'' corresponding 10 terres-
| trial latitude Distance in a direction
] parallel TO the equator is termed right
ascension" corresponding to terrestrial
longitude. For geographical purposes we
! conceive the earth TO be encircled by
| two imaginary lines at right angles to
each other-the one the equator, marked
out for Us by the earth Itself; the other,
Longitude nought" the meridian of
Greenwich. fixed for us by general con-
sent, after the lapse of centuries, by
kind of historical evolution. On the
celestial globe In like manner we have
two fundamental lines-one. the celestial
equator. marked out for US by nature:
the other at right angles TO IT, and pass
ing through the poles OF the sky. adopted
as matter of convenience. But a diff
Guilty at once confronts US. Where can
We fix our right ascension nought? What
star has the right to be considered the
Greenwich of the sky?
THE DIFFICULTY MET.
The difficulty is met in the following
manner: For six months OF the year
the summer months-the sun is north of
the celestial equator; for the other six
months OF the year-the months of
winter-it is south OF il. 1t crosses the
equator. therefore, twice in the year
once when moving northward at the
spring equinox; once when moving south
ward at the equinox of autumn. The
point where it crosses the equator at
the first Of these times is taken as the
fundamental point OF the heavens, and
the first sign OF the zodiac-Aries, the
RAMS said TO begin here. and it is
called, therefore, "the first point OF
Aries"
One of the very first facts noticed in the
very early days OF astronomy was that.
as the stars seemed 20 move across the
sky night by night, they seemed to move
in one solid piece, as If they were lamps
rigidly fixed In one and the same solid
vault. OF course, it has long been per-
fectly understood that this apparent
movement was not in the least due TO any
motion of the stars, but simply TO the
rotation Of the earth on its axis. This
rotation is the smoothest, most constant,
and regular movement Of which we know.
1t follows therefore, that the interval OF
time between the passage OF one star
across the meridian of Greenwich ana
that Of any other given star is always
the same This interval Of time is SIM
ply the difference of their right ascension.
II we are able, then, to turn our transit
instrument TO the sun, and 10 number
of stars, each in its proper turn, and by
pressing the tapping-piece on the lnstru-
ment as the sun or star comes up to
each of the ten wires in succession, 10
record the times of it's transit on the
chronograph. we shall have practically
determined their right ascensions-one
OF the two elements of their places.
THE ELEMENT OF DECLINATION
The other element, that Of declination,
is found in a different manner. Inc
celestial equator, like the terrestrial, is
30 degrees from the pole. The bright star
Polaris is not exactly at the North Pole.
but describes a small circle round IT,
Twice in the twenty-four hours 1t translts
over the meridian-once when going from
once when going from west to east below
the pole. The mean between these two
attitudes of Polaris above the horizon
gives the position of the true pole
complete transit observation Of star
consists. therefore, of two operations.
The observer sees star entering the field
of the telescope, and as 1T swims forward
he presses the galvanio button which |
sends a signal TO the chronograph as the
star comes up to each OF the ten vertical
wires in succession. But, besides the ten |
wires. there are others. TWO Verucal
wires lie outside the ten, ana there Is
also horizontal wire. The latter can be
moved by a graduated screw-head just |
above the eye-piece. and as the star |
comes in succession TO these two vertical |
wires. this horizontal wire Is moved by |
the screw-head so as to meet the star
at the moment IL is crossing the Vertlcai |
wire. and the observer presses a second |
little button, which records the position
of the horizontal wire on small paper
covered drum.
SOME SKILFUL CALCULATIONS.
Then. the transit over. the observer \
leaves the telescope and comes round TO |
the outside OF the west pier. Here he finds |
seven large microscopes. which pierce the
whole thickness of the pier, and are d.
rected towards the circumference OF |
large wheel which is rigidly attached to
the telescope and revolves with IL. This |
wheel is six feet in diameter, and has a
silver circle upon both faces. Each circle |
is divided carefully into 1920 divisions
these divisions therefore, being about the
one twentieth of an inch apart. The
lowest microscope is the least powerful,
and shows a large part of the circle, and
enables the observer to see at once TO
what degree and division Of degree the
microscope is pointing. The other six
are very carefully placed 90
degrees apart. These microscopes are all
fitted with movable wires-wires moved by
a very fine and delicate screw; the screw
head having divisions upon II, so that the |
exact amount OF its movements can be
told. Each of the six screw-heads will
read TO the one five thousands part of a
division of the circle-in other words, TO
the one hundred thousandth part of an
inch. Using all six microscopes. and
taking their mean, We are able 10 read 10
the one hundredth of a second of arc
II. therefore. the observations could be
made with perfect certainty down 10 the
MI. # alFarr xtrnf Dl. r.. !