ASTRONOMY. 
454 
The method of afcertaimng time by equal altitudes is 
univerfally ufed by practical aftronomers, becaufe it de¬ 
pends neither on an accurate knowledge of the latitude, 
nor on that of the declination ; for theie elements are only 
necellfary in taking out the equation of declination, and 
any probable error therein will not fenfibly affect that equa¬ 
tion; neither does it depend on the exact quantity of the 
altitude, provided only it be the fame in both obfervations.' 
To reduce Degrees of the Eqjtator into Time, and 
Time into Degrees of the Equator. 
To reduce degrees into time, multiply by 4, obferving 
that minutes, when multiplied by 4, produce feconds, and 
divided by 4 produce degrees; and fo on. 
Reduce 23 0 26' into time. 
_ 4 _ 
ill. 33'. 44" . 
To reduce time into degrees, multiply by 10 in a fimilar 
manner, and increafe the produce one-half; or otherwife 
divide the time by 4. 
Reduce ill. 33' 44". into degrees. 
ih._ 
33 '- 
44". 
IO 
Otherwife. 
2 7 
30 
4)ih. 33'. 44". 
Half 
7 
48 
40 
Degrees 23 26' 
Degrees 
23 
26 
O 
Of Reduction from ojje Meridian to another. 
As all the heavenly bodies rife, culminate, and fet, foon- 
er to thofe who are towards the eaft, and later to thofe who 
are towards the weft, and as all tables and calculations in 
England for aftronomy and navigation are adapted to the 
meridian of Greenwich, it is neceftary for thofe who may 
be under any other meridian to be able to find the time 
at Greenwich correfponding to that pointed out by their 
own clocks and watches. Without this reduction, no cal¬ 
culations ca'n be made from fuch tables, or from the va¬ 
rious articles contained in the Nautical Almanac, relating 
to the longitude, right afeenfion, and declination, of the 
Sun, the equation of time, Moon’s motion, &c. fo as to 
adjuft them to any other meridian than that for which they 
were made. 
To find, the Time at Greenwich correfponding with any given 
Time under another Meridian. Find the difference of lon¬ 
gitude between the two meridians, or how much the given 
place or meridian is to the eaft or weft of Greenwich, and 
reduce this difference to time, by the foregoing rule. If 
the given place be eaft of Greenwich, fubtraft the diffe¬ 
rence of meridians from your time; if it be weft, add the 
difference to your time. The refult will give the time it 
then is at Greenwich. The time being given at Green¬ 
wich, the correfponding time under any other meridian is 
found by reverfing this rule. 
Ex. What is the time at Greenwich, when it is 8h. 17'. 
iq". at Jerufaletn, 35 0 30'eaft of Greenwich, or in time 
ah- 2i'- 20". Anfwer, 5I1. 35'. 59". 
What is the time at Jerufalem, when it is 2ih. 49'. 17". 
on the 9th of September, at Greenwich ? Anfwer, 24IU 
30'. 37". or oh. 20'. 37". on the 10th of September. 
To know the time at Paris, Genoa, &c. when it is any 
given time where you are, take the difference between 
your meridian and that of Paris, &c. and then proceed as 
in the foregoing rule. 
To find at what Time, by a Clock keeping mean Time, any 
fixed Star will be on the Meridian on any given Day. The 
rmht afeenfion of the ftars being reckoned on the equator, 
they pafs the meridian fucceffively in times proportional 
to their refpedlive diffances therefrom. The diftance of 
a ftar from the meridian is therefore nothing more than its 
difference, in right afeenfion, from the Sun reduced to time; 
from whence it is plain, tl% to find the time when any ftar 
comes to the meridian,, you muft fubtrail the right afirenfion 
of the Sun at noon from that of the ftar, and the difference 
is the time required. This fimple calculation would be 
futficient in general for finding the time when a (far tran- 
fits the meridian, if it always preferved the fame diffe¬ 
rence in right afeenfion from the Sun : but the Sun, by its 
diurnal acceleration, approaches the ftar infenfibly; and 
will confequently pafs the meridian fooner, by a quantity 
proportional to this acceleration, and its diftance from the 
Sun. It is therefore necefTary to fubtradl from the quan¬ 
tity firft found, another fmall quantity, that may beafeer* 
tained. Hence the following rule: 
Take the difference between the Sun’s and planet’s mo¬ 
tion, in right afeenfion, in twenty-four hours, if the pla¬ 
net is progreflive, or their fum if retrograde; then fay, 
As twenty-four hours, diminiflied by this fum or difference 
when the planet’s motion is greater than the Sun’s, or in- 
crealed by it when lefs : twenty-feur hours :: the diffe¬ 
rence between the Sun’s and planet’s right afeenfion at 
noon : the time required. 
For a ftar. Take the increafe of the Sun’s right afeen- 
fion in twenty-four hours and add it to twenty-four hours; 
then fay, As this fum : twenty-four hours :: the difference 
between the Sun’s and ftar’s right afeenfion : the time re¬ 
quired. In the latter part of both thefe rules, the Sun’s 
right afeenfion is to be taken from the planet’s or ftar’s 
right afeenfion ; and, if their right afeenfions fliould be lefs 
than the Sun’s, they muft be increafed by twenty-four 
hours before you fubtradf. 
Ex. On July 1, 1767, the Sun’s right afeenfion, when on 
the meridian of Greenwich, was 6h. 40'. 25". and on July 
the 2d, it was 6h. 44'. 33". alfo the Moon’s right afeenfion 
was 159° 2'; and ou July 2d it was 169° 39'. Required 
the time of the Moon’s paffage over the meridian ? 
Sun’s R. A. July 1, 
6h. 
40'. 
2 5 "‘ 
July 2, 
6 
44 
33 
Daily increafe 
O 
4 
8 
Moon’s R. A. 139 0 2' 
IO 
316 
8 
169 39 
11 
18 
36 
Daily increafe 
0 
4 2 
28 
Moon’s motion in twenty-four hours 
42 
28 
Sun’s - 
- 
4 
8 
Difference 
- 
38 
20 
Sun’s R. A. at noon 
6 
40 
2 5 
Moon’s R. A. at noon 
IO 
3 6 
8 
Difference 
3 
55 
43 
As 24b. — oh.j8'. 20". = 23I1.21'. 40". : 24 :: 3I1. 55'. 
43". : 4I1. 2'. 9". the time required. 
At what time will the ftar Ardturus come to the meri¬ 
dian of Greenwich on the firft of September, 1787? 
Sun’s R. A. Sept. 1, ioh.4i'. 59". Star’sR. A. ih 46' o" 
Sept. 2, 10 43 37 10 41 39 
Increafe in 24hours o 3 38 Diff. 3 2 4 1 
As 24I1. 3'. 38''. : 24I1. :: 3I1. 24'. 1". : 3I1. 23'. 31". 
the time required. 
To find the Altitude of the Sun , or any other celefiial Body. 
The quadrant being adjufted, as in all cafes previous to 
its ufe, the celeftial body muft be viewed through the 
fights, and the plumb-line will fhew its altitude on the 
graduated limb of the inftrument. If the obfervation be 
made on the Sun, the dark glafs muft be ufed to defend 
the eye, or the luminous fpot formed by the fmall hole 
muft be made to fall on the centre of the crofs immediate¬ 
ly beneath the eye-hole. 
The Sun having no vifible point to mark out its centre, 
you muft take me altitude either of the upper or lower 
limb. If the lower limb be obferved, you muft add the 
Sun’s femidiameter thereto, in order to find the altitude 
