222. Thus it is plain, that an absolute turn of the Earth on it’s Axis
(which is always completed when the same Meridian comes to be parallel
to it’s situation at any time of the day before) never brings the same
Meridian round from the Sun to the Sun again; but that the Earth
requires as much more than one turn on it’s Axis to finish a natural
day, as it has gone forward in that time; which, at a mean state is a
365th part of a Circle. Hence, in 365 days the Earth turns 366 times
round it’s Axis; and therefore, as a turn of the Earth on it’s Axis
compleats a sidereal day, there must be one sidereal day more in a year
than the number of solar days, be the number what it will, on the Earth,
or any other Planet. One turn being lost with respect to the number of
solar days in a year, by the Planet’s going round the Sun; just as it
would be lost to a traveller, who, in going round the Earth, would lose
one day by following the apparent diurnal motion of the Sun: and
consequently, would reckon one day less at his return (let him take what
time he would to go round the Earth) than those who remained all the
while at the place from which he set out. So, if there were two Earths
revolving equably on their Axes, and if one remained at _A_ until the
other travelled round the Sun from _A_ to _A_ again, _that_ Earth which
kept it’s place at _A_ would have it’s solar and sidereal days always of
the same length; and so, would have one solar day more than the other at
it’s return. Hence, if the Earth turned but once round it’s Axis in a
year, and if _that_ turn was made the same way as the Earth goes round
the Sun, there would be continual day on one side of the Earth, and
continual night on the other.
[Sidenote: To know by the Stars whether a Clock goes true or not.]
223. The first part of the preceding Table shews how much of the
celestial Equator passes over the Meridian in any given part of a mean
solar day, and is to be understood the same way as the Table in the
220th article. The latter part, intitled, _Accelerations of the fixed
Stars_, affords us an easy method of knowing whether or no our clocks
and watches go true: For if, through a small hole in a window-shutter,
or in a thin plate of metal fixed to a window, we observe at what time
any Star disappears behind a chimney, or corner of a house, at a little
distance; and if the same Star disappears the next night 3 minutes 56
seconds sooner by the clock or watch; and on the second night, 7 minutes
52 seconds sooner; the third night 11 minutes 48 seconds sooner; and so
on, every night, as in the Table, which shews this difference for 30
natural days, it is an infallible Sign that the machine goes true;
otherwise it does not go true; and must be regulated accordingly: and as
the disappearing of a Star is instantaneous, we may depend on this
information to half a second. [Illustration: Pl. VI.
_J. Ferguson inv. et delin._ _J. Mynde Sc._]