CORRECTIONS A 
continues to be 24 hours ; if not, the dif- 
ference siiows how much it gains or loses 
in that time. A clock thus adjusted is said 
to be adjusted to sidereal time, and all the- 
sidereal days are equal. But all the solar 
days are not equal, that is, the intervals 
from the sun’s leaving the meridian till it 
returns to it again, are not all equal, so that 
if a clock be adjusted to go 24 hours in one 
interval, another interval will be perform- 
ed in more or less than 24 hours, and thus 
the sun and the clock will not agree ; that 
is, the clock will not continue to show 12 
when the sun comes to the meridian. It is 
found that the length of the solar day is 
equal to the time of the earth’s rotation 
about its axis, together with the time of de- 
scribing an angle equal to the increase of 
the sun’s right ascension in a true solar day. 
Now if the sun moved, or appeared to 
move, uniformly, and in the equator, this 
increase would be always the same in the 
same time, and therefore the solar days 
would be all equal ; but the sun moves, or 
appears to move, in the ecliptic, and there- 
fore, if its motion were uniform, equal arcs 
upon the ecliptic would not give equal arcs 
upon the equator. But the apparent mo- 
tion of the sun in the ecliptic is not uni- 
form, and hence also any arc upon the 
ecliptic, described in a given time, is sub- 
ject to a variation, and consequently that 
on, the equator is subject to a variation. 
The increase then of the sun’s right ascen- 
sion in a true solar day, varies from two 
causes : first, because the ecliptic, in which 
the sun appears to move, is inclined to the 
equator ; secondly, because his motion in 
the ecliptic is not uniforni, therefore the 
length of a true solar day is subject to a 
continual variation: consequently a clock 
which is adjusted to go 24 hours for any 
one true solar day, will not continue to 
show 12 when the sun comes to the meri- 
dian, because the intervals by the clock 
will continue equal, if the clock be sup- 
posed accurate, but the intervals of the 
sun’s apparent passage over the meridian 
are not equal. 
As the sun appears to move through 360'^ 
of right ascension in about 365j days, there- 
fore 363.25 ; 1 day :: 360“ : .59' 8" 2", the in- 
crease of' right ascension in one day, if tlie 
increase were uniform, or it would be the 
i ncrease in a mean solar day, that is, if the 
solar days were all equal ; for they would 
be all equal, if the sun’s right ascension in- 
creased uniformly. As the earth describes 
qn angle of 360“ 39' 8" 2' " about its axis in a 
SID ADDITIONS. 
mean solar day of 24 hours, and an angle 
of 360“ in a sidereal day, we say, as 
360“ .39' 8''2"' : 360“ 24>' : 23'' 56' 4'' the 
length of a sidereal day in mean solar time, 
or the time from the passage of a fixed 
star over the meridian till it returns to it 
attain. From these considerations it will 
be evident, that if a clock be adjusted to 
go 24 hours in a mean solar day, it will not 
continue to coincide with the sun, that is, 
to show twelve when the sun comes to the 
meridian, because the true solar days diifer 
in length from a mean solar day, but the 
sun will pass the meridian, sometimes be- 
fore 12, and sometimes after 12, and this 
difference is called the equation. A clock 
thus adjusted, is said to be adjusted to 
mean solar time. The time shown by the 
clock is called true or mean time ; and that 
shown by the sun is called apparent time ; 
thus, when the sun comes to the meridian, 
it is said to be 12 o’clock apparent time. 
Hence the time shown by the sun-dial is 
apparent time, and therefore a dial will dif- 
fer from a clock by how much the equation 
of time is on that dily. When, therefore, 
we, set a clock or watch by the dial, we 
must attend to what the equation of time 
is upon that day by a table, such as that 
given below, and allow for it : thus, if the 
equation be 4 minutes, as it is on new year’s 
day, and the watch or clock be faster than 
the srin ; then the watch or clock must be 
made to show 4 minutes past 12 when the 
dial shows 12 precisely. On the 30th of 
April, when the dial shows 12, the clock of 
watch, to be accurate, must want 3 minutes 
of that hour, and so of the rest. In calcu- 
lating tables of the equation of time, for 
every day in the year, the sun and clock 
are set together, when the sun is in his apo- 
gee, and then they investigate the differ- 
ence between the sun and the clock, for 
every day at noon, and insert them in a ta- 
ble, stating, by means of the signs and 
how much the clock is before or after 
the sun. The inclination of the equator to 
the ecliptic, upon which the equation of 
time partly depends, and the place of the 
sun’s apogee, when the clock and sun set 
off together, being both subject to vary, the 
equation of time ;or the same days of the 
year, will every year vary, and therefore it 
must, where great accuracy is required, be 
calculated for every year. Besides the 
time when the sun is in his apogee, there 
are three other times of the year when the 
clock and sun agree, or when mean and 
apparent time is the same, as will be seen 
i 
