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l ight sphere. It is found by the following 
proportion. As the radius to the cosine of 
the sun or star’s greatest declination, so is 
the tangent of the distance from Aries to 
Libra, to the tangent of right ascension. 
O blique ascension is that degree and mi- 
nute of the equinoctial, counting from the 
beginning of Aries, which rises with the cen- 
tre of the sun or a star, or which comes to 
the horizon at the same time as die sun or 
star, in an oblique sphere. In order to find 
the oblique ascension we must first find the 
ascensional difference. 
The arch of right ascension coincides with 
the right ascension itself, and is the same in 
all parts of the globe. The arch ol oblique 
ascension coincides with the oblique ascen- 
sion, and changes according to the latitude 
of places. 
The sun’s right ascension in time is useful 
to the practical astronomer in regular ob- 
servatories, who adjusts his clock by sidereal 
time. It serves also for converting appa- 
rent into sidereal time ; as e. g. that of an 
eclipse of Jupiter’s satellites, in order to 
know at what time it may be expected to 
happen by his clocks. For this purpose, the 
sun’s right ascension at the preceding noon, 
together with the increase of right ascension 
from noon, must be added to the apparent 
time of the phenomenon set down in the 
ephemeris. The sun’s right ascension in time 
serves also for computing the apparent time 
of a known star’s passing the meridian: thus, 
subtract the sun’s right ascension in time at 
noon from the star’s right ascension in time, 
the remainder is the apparent time of the 
star’s passing the meridian nearly; from 
which the proportional part of the daily in- 
crease of the sun’s right ascension from this 
apparent time from noon being subtracted, 
leaves the correct time of the star’s passing 
the meridian.The sun’s right ascension in time 
is also useful for computing the time of the 
moon and planets passing the meridian. The 
practical method of finding the right ascen- 
sion of a body from that of a fixed star, by 
a clock adjusted to sideral time, is this : 
let the clock begin its motion from 0 h O' 0" 
at the instant the first point of Aries is on 
the meridian; then, when any star comes to 
the meridian, the clock would shew the ap- 
parent right ascension of the star, the right 
ascension being estimated in time at the 
rate of 15° an hour, provided the clock was 
subject to no error, because it would then 
shew at any time how far the first point of 
Aries was from the meridian. But as the 
Slock is liable to err, we must be able at 
ASC 
any time to ascertain its error, or the dif- 
ference between the right ascension shewn 
by the clock and the right ascension of that 
point of the equator which is at that time on 
the meridian. To do this we must, when a 
star, whose right ascension is known, passes 
the meridian, compare its apparent right as- 
cension with the right ascension shewn by 
the clock, and the difference will shew the 
error of the clock. E. g. Let the apparent 
right ascension of Aldebaran be 4 h 23 50 
at the time when its transit over the meri- 
dian is observed by the clock ; and suppose 
the time shewn by the clock to be 4 h 23 52 , 
then there is an error of 2" in the clock, as 
it gives the right ascension of the star 2" 
more than it ought. If the clock be com- 
pared with several stars, and the mean error 
taken, we shall have more accurately the 
error at the mean time of all the observa- 
tions. These observations, being repeated 
every day, will give the rate ot the clock’s 
going, or shew how much it gains or loses. 
The error of the clock and the rate of its 
going being thus ascertained, if the time of 
the transit of any body be observed, and the 
error of the clock at the time be applied, 
we shall have the right ascension of the 
body. This is the method by which the 
right ascension of the sun, moon, and pla- 
nets are regularly found in observatories. 
To find the right ascensions mechanically 
by the globe, see Globes, the use of. The 
arch of right ascension is that portion of the 
equator intercepted between the beginning 
of Aries and the point of the equator which 
is in the meridian: or it is the number of 
degrees contained in it. This coincides 
with the right ascension itself. The right 
ascension is the same in all parts of the 
globe. We sometimes also say, the right 
ascension of a point of the ecliptic, or any 
other point of the heavens. The right as- 
cension of the mid-heaven is often used by 
astronomers, particularly in calculating eclip- 
ses by the nonagesimal degree ; and it de- 
notes the right ascension of that point of the 
equator which is in the meridian, and is 
equal to the sum of the sun’s right ascension 
and the horary angle or true time reduced 
to degrees, or to the sum of the mean longi- 
tude of the sun and mean time. 
ASCENSIONAL difference, the differ- 
ence between the right and oblique ascen- 
sion in any point of the heavens; or it is the 
space of time that the sun rises or sets be- 
fore or after six o’clock. 
The ascensional difference may be found 
by this proportion, viz. As the radius is to 
