Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 3, ASS-BOO

Part IT.
30
ASTRONOMY.
Apparent The intercalated day was alfo called the 6th before the
Motions of calehds of March. So that every fourth year there
the Heaven-were two Jays denominated the 6th before the calends
ly Bodies. TYJard,, Hence that year was called biffextile. In
v ~ Britain it is denominated leap year. After the death
of Julius Caefar there was a degree ot confufion re-
fpe&ing the intercalations, from the ignorance of the
priefts. Auguflus correfled the miftake, and after
that time the Julian period went on without any inter¬
ruption .
It is obvious that the Julian year, though a great
improvement upon the ancient Roman, was ftill imper-
fedl. It went on the fuppofition that the revolution of
the fun occupied precifely 365 days and 6 hours,, which
is about ix minutes more than the truth. 1 his error
> in the interval which elapfed between the reformation
of Julius Caefar and the year 1582, had accumulated
till it amounted to 10 days; of courfe the year began
10 days later than it ought to have begun ; and the
fame error had taken place refpefting the feafons and
the equinoftial points. Various attempts had been
made to corredt this«error ; at laft it was corrected by
Pope Gregory XIII. The Gregorian calendar com¬
menced in the year 1582; the changes which he intro¬
duced were two in number. He ordered, that after
the 4th of O&ober 1582, ten days (hould be omitted,
fo that the day which fucceeded the 4th was reckoned
not the 5th but the 15th of the month. This correct¬
ed the error which bad crept into the Julian year. To
prevent any fuch error from accumulating again, he
ordered that the fecular years 1700, 1800, 1900, {hould
not be biflextile but common years; that the fecular
year 2000 {hould be biflextile, the next three fecular
years common, the fourth again biflextile, and fo on,
as in the following table.
64
and by
Gregory
XIII.
1600 biflextile.
1700 common.
1800 ib.
1900 ib.
2000 biflextile.
2100 common.
2200 ib.
2300 ib.
2400 biflextile.
2500 common.
2600 common.
2700 ib.
2800 biflextile.
2900 common.
3000 ib.
' In fhort thefe fecular years only are biflextile whofe
number, omitting the cyphers, is divifible by 4.
The Gregorian calendar is fufficiently exaft for the
purpofes of common life, though it does not correfpond
precifely with the revolution of the fun. The error
will amount to a day in 3600 years, fo that in the year
5200 it will be neceflary to omit the additional day
which ought to be added according to the rule laid
down above.
The Gregorian calendar was immediately adopted
by all the Roman Catholic kingdoms in Europe, but
the Proteftant ftates refufed at firft to accede to it. It
was adopted by moft of them on the continent about
the beginning of the 18th century ; but in England
the change did not take place till 1752. From that
year 11 days were omitted; the omiflion of the addi¬
tional day in 1700 having made the difference between
the Julian and Gregorian calendar amount to 11 days.
The Julian calendar is called the oldJlyle, the Grego¬
rian, the new Jlyle. At prefent the difference between
them is 12 days, in confequence of the omiffion of the
additional day in 1800.
Sect. III. Of the Nature of the Sun. Apparent
^ Motions of
The fmallnefs of the fun’s parallax is a demonftra-tbeHeaven-
tion of its immenfe fize. We are certain that at the ^ Bodies.,
dillance at which the fun appears to us under an angle
of o°.53424, the earth would be feen under an angle
not exceeding o°.oo9. Now, as the fun is obvioully
a fpherical body as well as the earth ; and as fpheres
are to each other as the cubes of their diameters, it
follows from this, that the fun is at leaf! 200,000 times
bigger than the earth. By the exadteft obfervations it
has been afcertained, that the diameter of the fun is
nearly 883,000 miles.
Dark fpots are very frequently obferved upon the
furface of the fun. Thefe were entirely unknown be¬
fore the invention of telefcopes, though they are fome-
times of fufficient magnitude to be difcerned by the
naked eye, only looking through a fmoked glafs to pre¬
vent the brightnefs of the luminary from deftroying the
fight. The fpots are faid to have been firft difcovered Solar fpots
in the year 1611 ; and the honour of the difcovery is when firft
difputed betwixt Galileo and Scheiner, a German Jefuitdlfcoverttl*
at Ingolftadt. But whatever merit Scheiner might have
in the priority of the diicovery, it is certain that Gali¬
leo far exceeded him in accuracy, though the work of
Scheiner has confiderable merit, as containing obferva¬
tions feledled from above 3000, made by himfelf. Since
his time the fubjedt has been carefully ftudied by all
the aftronomers in Europe. <j6
There is great variety in the magnitudes of the Dr Long’s
folar fpots ; the difference is chiefly in fuperficial ex- account of
tent of length and breadth ; their depth or thicknefsthem-
is very fmall ; feme have been fo large, as by compu¬
tation to be capable of covering the continents of Afia
and Africa ; nay, the whole furface of the earth, or
even five times its furface. The diameter of a fpot,
when near the middle of the dilk, is meafured by com¬
paring the time it takes in pafling over a crofs hair in
a telefcope, with the time wherein the whole dilk of
the fun paffes over the fame hair ; it may alfo be mea¬
fured by the micrometer ; and by either of thefe me¬
thods we may judge how many times the diameter of
the fpot is contained in the diameter of the fun. Spots
are fubjedt to increafe and diminution of magnitude,
and feldom continue long in the fame ftate. Ihey are
of various fliapes ; moft of them having a deep black
nucleus furrounded by a dulky cloud, whereof the in¬
ner parts near the black are a little brighter than the
outlkirts. They change their ftiapes, fomething in the
manner that our clouds do; though not often fo fud-
denly : thus, what is of a certain figure to-day, lhall
to-morrow, or perhaps in a few hours, be of a different
one ; what is now but one fpot, {hall in a little time
be broken into two or three ; and fometimes two or
three fpots {hall coalefce, and be united into one. Dr
Long, many years fince, while he was viewing the
image of the fun through a telefcope call upon white
paper, faw one roundiih fpot, by eftimatios not much
lefs than the diameter of our earth, break into two,
which receded from one another with prodigious velo¬
city. This obfervation was Angular at the time ; for
though feveral writers had taken notice of this after it
was done, none of them had been making any obfervation
at. the time it was adiually doing.