AH 
ASTRONOMY. 
ioo years. He made alfo the fil'd ftar of Aries 26°diftant 
from tiie equinoctial point, and the obliquity of the eclip¬ 
tic only if 28'. He conftrudted a planetarium, repre¬ 
senting' the ceLeltial motions according; to the Ptolemaic 
hypothelis ; and lie publiflied a tranilation ©f Ptolemy’s 
geograpliy, with a commentary, in which he firft proposed 
the°method of finding the longitude at fea by obferving 
the Moon’s difiance from the fixed liars ; now fo luccefs- 
fully pradtifed for that purpofe. 
Nicolaus Copernicus was the next who made any confi- 
derable figure in aftronomy, by whom, as we have feen, 
the old Pythagorean fyftem of the world was reftored, 
which had been let aiide.from the time of Ptolemy. About 
the year 1507 Copernicus conceived doubts of this fyftem, 
and entertained notions about the true one, which he gra¬ 
dually improved by a feries of aftronomical observations, 
and the contemplation of former authors. By thefe lie 
formed new tables, and completed his work in the year 
1530, containing thefe, and his renovation of the true 
fyftem of the univerfe. But the work was only printed in 
1543, under the care of Schoncr and Ofiander, by the 
title of Revolutiones Orbium Casleftium ; and the author 
only received a copy of the work a few hours before his 
death, which happened on the 23d of May, 1343. 
After the death of Copernicus, practical aftronomy was 
greatly improved by many other perfons, as Schoner, 
Nonius, Appian, Gemma Frifius, Rothman, Byrgius, the 
landgrave of Helfe, &c. Schoner reformed and explained 
the calendar, improved the methods of making celeftial 
obfervations, and publiflied a treatife on colniography ; 
but lie died four years after Copernicus. Nonius wrote 
leveral works on mathematics, aftronomy, and navigation, 
and invented fome ufeful and more accurate inftruments 
than formerly; one of thefe was the aftronomical quad¬ 
rant, on which lie divided the degrees into minutes by a 
number of concentric circles ; the firft of which was di¬ 
vided id ninety equal parts or degrees, the fecond into 
eighty-nine, the third into eighty-eight, and fo on to lorty- 
fix ; fo that, the index of the quadrant always falling upon 
or near one of the divilions, the minutes would be known 
by an eafy computation. The chief work of Appian, The 
Caefarean Aftronomy, was publiiliedat Ingoldftadt in 1540; 
in which he teaches how to obferve the places of the ftars 
and planets by the aftrolabe; to refolve aftronomical prob¬ 
lems by certain inftruments ; to predict eclipfes, and to 
delcribe the figures of them ; and the method of dividing 
and ufing an aftronomical quadrant. Gemma Frilius wrote 
a commentary on Appian’s Cofmography, accompanied 
with many obfervations of eclipfes : he alfo invented the 
aftronomical ring, and feveral other inftruments, ufeful in 
taking obfervations at fea : and he was the firft who re¬ 
commended a time-keeper for determining the longitude 
at fea. Rheticus gave up his profeftbrfliip of mathema¬ 
tics at Wirtemberg, that lie might attend the aftronomical 
lectures of Copernicus ; and, for improving aftronomical 
calculations, he began a very extenlive work, being a ta¬ 
ble of fines, tangents, and fecants, to a very large radius, 
and to every ten feconds or jt of a minute ; which was 
completed by his pupil Valentine Otho, and publiftied in 
* 59 +- 
About the year 1561, William IV. landgrave of Hefte 
Cafifel, applied himfelf to the itudy of aftronomy, having 
furniftied himfelf with the beft inftruments that could then 
be conftrufted ; witli thefe he made a great number of ob¬ 
fervations, which were publiftied by Snellius in 1618, and 
which were preferred by Hevelius to thole of Tycho Brahe. 
Tycho Brahe began his obfervations about the fame time 
with the landgrave of Helfe ; and he obferved the great 
conjunction of Jupiter and Saturn : but, finding the ufual 
inftruments very inaccurate, he conftrudted others much 
larger and more exadt, with which he applied himfelf di¬ 
ligently to obferve the celeftial phenomena. In 1371 he 
difcovered a new ftar in the chair of Cafliopeia; which 
induced him, like Hipparchus on a fimilar occafion, to 
make anew catalogue of the ftars. In the year 1376, by 
favour of the king of Denmark, he built his new obfer- 
vatory, called Uraniburg, on the fmall ifiand Huerifta, 
oppofite to Copenhagen, and which lie very amply fur- 
nilhed with many large inftruments. Tycho employed his 
time at Uraniburg to the beft advantage, till the death of 
the king, when, falling into diferedit, he -was obliged t® 
remove to Holftein, and he afterwards found means of in¬ 
troducing hitnfelf’to the emperor Rodolpli, with whom 
he continued at Prague till the time of his death in 1601. 
We have already obferved that Tycho was the inventor 
of a fyftem of aftronomy, which he vainly endeavoured 
to eftablilh inftead of the Copernican or true fyftem. His 
works, however, which are very numerous, Ihew that he 
Was a man of great abilities ; and his difeov-eries, toge¬ 
ther with thofe of Purbach and Regiomontanus, were col- 
lefted and publiftied together in 1621, by Longomontanus, 
the favourite difciple of Tycho. 
While Tycho relided at Prague with the emperor, he 
prevailed on Kepler to leave the univerfity of Glatz, and 
to come to him, which he did with his family and library 
in 1600: but, Tycho dying in 1601, Kepler enjoyed all 
his life the title of mathematician to the emperor, who or¬ 
dered him to finifti the Tables of Tycho Brahe, which he 
did accordingly, and publiftied then in 1627 under the ti¬ 
tle of Rodolphine. He died about the year 1630 at Ra- 
tilbon, where he was foliciting the arrears of his penlion. 
From his own obfervations, and thofe of Tycho, Kepler 
difcovered feveral of the true laws of nature, by which 
the motions of the celeftial bodies are regulated. He 
found that all the planets revolved about the Sun, not m 
circular, but in elliptical, orbits, having the Sun in one 
of the foci of the ellipfe; that their motions were not 
equable, but varying, quicker or flower as they were nearer 
to the Sun or farther from him ; but that this motion was 
fo regulated, that the areas deferibed by the variable line 
drawn from the planet to the Sun are equal in equal times, 
and always proportional to the times of deferibing them. 
He alfo difcovered, by trials, that the cubes of the dif- 
tances of the planets from the Sun were in the fame pro¬ 
portion as the fquares of their periodical times of revo¬ 
lution. By obfervations alfo on comets, he concluded, 
that they are freely carried about among the orbits of the 
planets, in paths that are nearly rectilinear, but which he 
could not then determine. 
In Kepler’s time there are many other good proficients 
in practical afironomy ; as Edward Wright, baron Napier, 
John Bayer, &c. Wright made feveral good meridional 
obfervations of the Sun, with a quadrant of fix feet radius, 
in the years 1594, 159s, and 1596; from which he greatly 
improved the theory of the Sun’s motion, and computed 
more accurately his declination than anyperfon had done 
before. In 1399 he publiftied an excellent work entitled 
“ Certain Errors in Navigation difcovered and detected, !> 
containing a method which has commonly, though erro- 
neoufly, been aferibed to Mercator. To Napier we owe 
fome excellent theorems and improvements in fpherics, 
beiides the ever-memorable invention of logarithms, one 
of the molt ufeful ever made in the art of numbering, and 
of the greateft ufe in all the other mathematical fcien*es. 
Bayer, a German, publiftied his Uranometria, being a 
complete celeftial atlas, or the figures of all the conftella- 
tions vilible in Europe, with the ftars marked on them, 
and the ftars alfo accompanied by names, or the letters of 
the Greek alphabet; a contrivance by which the ftars may 
eafily be referred to with diftinCtnefs and precifion. About 
the fame time alfo practical aftronomy was cultivated by 
Mercator, Maurolycus, Maginus, Homelius, Scliultet, Ste- 
vin, Galileo, Thomas and Leonard Digges, John Dee, Ro¬ 
bert Flood, Harriot, &c. 
The beginning of the feventeenth century was particu¬ 
larly diftinguiihed by the invention of telefcopes, and the 
application of them to aftronomical obfervations; an in¬ 
vention to which we owe the molt brilliant difeoveries, and 
all the accuracy to which the practical part of the fcienc® 
is now brought. The more diftinguiihed early obferva- 
1 tions 
