The herald., 1898-08-07

as a matter Of convenience. But diff
ficulty 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 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 win
ter, it is south of it. 1t crosses the equa
tor, therefore, twice in the year-once
when moving northward at the spring
equinox, once when moving southwark
at the equinox of autumn. The point
where it crosses the equator at the first
Of these times is taken as the fundamen
tai point of the heavens, and the first
sign of the zodiac, Aries the Ram, is
said to begin here, and it is called, there
fore, "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 to move across
the sky night by night, they seemed to
move in one solid piece, as if they were
lamps rigidly fixed at one end and the
same solid vault. OF course it has long
been perfectly understood that this ap-
parent movement was not in the least
due to any motion of the stars, but SIM
ply 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, there
fore, that the interval of time between
the passage of one star across the merid-
ian of Greenwich and that of any other
given star is always the same. This in
terval of time is simply the difference
of their right ascension. If we are able.
then, to turn our transit instrument to
the sun, and to a number of stars, each
in it's proper turn. and by pressing the
tapping piece on the instrument as the
sun or star comes up to each of the ten
wires in succession. to record the time
of its transit on the chronograph, we
shall have practically determined their
right ascensions-one of the two ele
ments of their places.
The other element that of declina
4! ton. is found in different manner. The
celestial equator like the terrestrial,
is 80 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 it
transits over the meridan-once when
going from east to west it passes above
the pole, 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.
A complete transit of a star consists,
therefore, of two operations. The OBJ
server sees star entering the field of
the telescope, and as it swims forward
he presses the galvanic button which
sends signal to the chronograph as the
star comes up to each of the ten verti-
cal wires in succession. But beside the
ten wires, there are others. Two verti-
cal wires lie outside the ten, and there
Is also a horizontal wire. The latter
can be moved by graduated screw head
| just above the eye-piece, and as the star
comes in succession to these two verti-
| cal wires, this horizontal wire is moved
| by the screw head so as to meet the star
at the moment it is crossing the verti-
| cal wire, and the observer presses a
second little button, which records the
| position of the horizontal wire on
small paper-covered drum. Then, the
| transit over the observer leaves the tel
| escope 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 directed
toward the circumference of a large
wheel which is rigidly attached to the
telescope and revolves with it. This
wheel is six feet in diameter, and has
silver circle upon both faces. Each
circle is divided carefully into 1920 di-
visions-these divisions, therefore, be-
| ing about 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 a degree the microscope is pointing.
The other six microscopes are very care
fully placed go degrees apart. These
microscopes are all fitted with movable
wires-wires moved by very fine and
delicate screw; the screw head having