205. The Earth’s Orbit being elliptical, and the Sun constantly keeping
in it’s lower Focus, which is 1,377,000 miles from the middle point of
the longer Axis, the Earth comes twice so much, or 2,754,000 miles
nearer the Sun at one time of the year than at another: for the Sun
appearing under a larger Angle in our winter than summer, proves that
the Earth is nearer the Sun in winter, (_see the Note on Art. 185_.) But
here, this natural question will arise, Why have we not the hottest
weather when the Earth is nearest the Sun? In answer it must be
observed, that the excentricity of the Earth’s Orbit, or 1 million 377
miles bears no greater proportion to the Earth’s mean distance from the
Sun than 17 does to 1000; and therefore, this small difference of
distance cannot occasion any great difference of heat or cold. But the
principal cause of this difference is, that in winter the Sun’s rays
fall so obliquely upon us, that any given number of them is spread over
a much greater portion of the Earth’s surface where we live; and
therefore each point must then have fewer rays than in summer. Moreover,
there comes a greater degree of cold in the long winter nights, than
there can return of heat in so short days; and on both these accounts
the cold must increase. But in summer the Sun’s rays fall more
perpendicularly upon us, and therefore come with greater force, and in
greater numbers on the same place; and by their long continuance, a much
greater degree of heat is imparted by day than can fly off by night.
[Sidenote: Fig. II.]
206. That a greater number of rays fall on the same place, when they
come perpendicularly, than when they come obliquely on it, will appear
by the Figure. For, let _AB_ be a certain number of the Sun’s rays
falling on _CD_ (which, let us suppose to be _London_) on the 22d of
_June_: but, on the 22d of _December_, the line _CD_, or _London_; has
the oblique position _Cd_ to the same rays; and therefore scarce a third
part of them falls upon it, or only those between _A_ and _e_; all the
rest _eB_ being expended on the space _dP_, which is more than double
the length of _CD_ or _Cd_. Besides, those parts which are once heated,
retain the heat for some time; which, with the additional heat daily
imparted, makes it continue to increase, though the Sun declines toward
the south: and this is the reason why _July_ is hotter than _June_,
although the Sun has withdrawn from the summer Tropic; as we find it is
generally hotter at three in the afternoon, when the Sun has gone toward
the west, than at noon when he is on the Meridian. Likewise, those
places which are well cooled require time to be heated again; for the
Sun’s rays do not heat even the surface of any body till they have been
some time upon it. And therefore we find _January_ for the most part
colder than _December_, although the Sun has withdrawn from the winter
Tropic, and begins to dart his beams more perpendicularly upon us, when
we have the position _CF_. An iron bar is not heated immediately upon
being put into the fire, nor grows cold till some time after it has been
taken out.