DAY. 
Jneis, called February the 13th, at 21 hours. 
An astronomical day is somewhat greater 
tlian a complete revolution of the lieavens, 
which forms a sidereal day. For it the 
sun cross the meridian at the same instant 
with a star, the day following it will come 
to the meridian somewhat later than the 
star’, in consequence of its motion eastward, 
which causes it to leave the star ; and after 
a whole year has elapsed it toU have crossed 
the meridian just one time less than the 
star. A sidereal day is less than the solar 
day, for it is measured by 360“ ; whereas 
the mean solar day is measured by 360“ 
.'>9' 8" nearly. 
If an astronomical day be =: 1, then a 
sidereal day is = 0.997269722 ; or the dif- 
terence between the measures of a mean 
solar day and a sidereal day, inz. 59' 8", re- 
dviced to time, at the rate of 24 hours to 
360“, gives 3' 36"; from which we learn that 
a star Which was on the meridian w'ith the 
sail on one noon, will rettirn to that meri- 
dian, 3' 66 ", previously to the next noon : 
therefore, a clock which measures mean 
days by 24 hours, will give 23*' 56“ 4', for 
the length of a sidereal day. 
Astronomical, or solar days, as they are 
also called, are not equal. Two causes con- 
spire to produce their inequality, namely, 
the unequal velocity of the sun in his orbit, 
and the obliquity of the ecliptic. The 
effect of the first cause is sensible. At the 
summer solstice, when the sun's motion is 
slowest, the astronomical day approaches 
nearer the sidereal, than at the winter sol- 
stice when his motion is most rapid, "lo 
conceive the effect of the second cause it is 
necessarv to recollect that the excess of the 
astronomical day above the sidereal is ow- 
ing to the motion of the sun, referred to the 
e(]uator. The sun describes every day a 
small arch of the ecliptic. Through the ex- 
tremities of this arch suppose two meridian 
great circles drawn, the arc of the equator, 
which they intercept, is the sun’s motion 
for that day referred to the equator ; and 
the time which that arc takes to pass the 
meridian is equal to the excess of the a.stro- 
nomical day above the sidereal. See Time, 
equation of. 
The nychthemeron is divided into twenty- 
four parts, called hours, which are of two 
suits, equal and unequal, or temporary. 
Different nations begin their day at a dif- 
ferent hour: thus the Egyptians began their 
day at midnight, from whom Hippocrates 
introduced that way of reckoning into as: 
trqnomy, and Copernicus and others have 
followed him : but the greatest part of as- 
tronomers reckon the day begun at noon, 
and so count 24 hours till the noon of thfe 
next day; and not twice 12 according td 
the vulgar computation. The method of 
beginning the day at midnight prevails also 
in Great Britain, France, Spain, and most 
parts of Europe. The Babylonians began 
their day at sun-rising, reckoning the hour 
immediately before its rising again the 
24th hour of the day, from whence the hours 
reckoned in this way are called the Babylo- 
nic. In several parts of Germany they be- 
gin their day at sun-setting, and reckon on 
till it sets next day, calling that the 24th 
hour ; these are generally termed Italian 
hours. The Jews also began their day at 
sun-setting ; but then they divided it into 
twice 12 hours as we do, reckoning 12 for 
the day, be it long or short, and 12 for the 
night ; so that their hours continually vary- 
ing with the day and night, the hours of the 
day were longer than that of the night for 
one half year, and the contrary the other ; 
from whence their hours arc called tempo- 
rary: those at the time of the equinoxes 
became equal, because then those of the 
day and night are so. The Romans also 
reckoned their hours after this manner; as 
do tlie Turks at this day. This kind of 
hours are called planetary, because the 
seven planets were anciently looked upon 
as presiding over the affairs of the world, 
and to take it by turns each of these hours, 
according to the following order: Saturn 
first, then Jupiter, Mars, the Sun, Venus, 
Mercury, and last of all the Moon : hence 
they denominated each day of the week 
from that planet whose turn it was to pre- 
side the first hour. Thus assigning the first 
hour of Saturday to Saturn, the second will 
fall to Jupiter, the third to Mars, and so 
the twenty-second will fall to Saturn again, 
and therefore the twenty-third to Jupiter, 
and the last to Mars : so that on the first 
hour of the next day it will fall to the Sun 
to preside ; and by the like manner of rec- 
koning the first hour of the next w'ill fall to 
the Moon ; of the next, to Mars ; of the 
next, to Mercury, of the next, to Venus ; 
hence the days of the week came to be dis- 
tinguished by the Latin names of Dies Sa- 
turni, Solis, Luna;, Martis, Mercuni, Jnris, 
and Veneris; and among us, by the names 
of Saturday, Sunday, Monday, !ke. 
Day, in a legal sense, relates to the day 
of appearance of parties, or the continuance 
of suits where a day is given, &c. See 
Essoin. 
