In the northern part of Persia, an observatory was erected by a descendant of the renowned warrior Genghis Khan, where some tables were constructed by Nasir-Eddin; and at Saniarcand, Ulugh Beg, a grandson of Timur, made, in 1433 A.D., many obser vations, and the most correct catalogue of .stars which, up to his time, had been published.
In the 13th c., A. was again introduced into western Europe, the first translation from the Almagest being made under the emperor Frederick IL of Germany, about 1230; and in 1252 an impulse was given to the science by the formation of astronomical tables under the auspices of Alfonso X. of Castile. An Englishman, named Holywood (Sacro bosco). in 1220 wrote a book of great repute in its day on the spheres, chiefly abridged from Ptolemy7 and among others who did much to promote a taste for A. were Purhach (1460), Regiomontanus (John Muller), who died in 1476, and Waltherus, a pupil of the latter, who made numerous observations of merit.
We now come to the illustrious name of Copernicus (b. 1473, d. 1543), to whom was reserved the grand honor and the danger—for there is ever danger in bringing old notions into disrepute by introducing new systems of truth—of exploding the Ptolemaic idea, and of promulgating a correct though imperfect theory of the universe. His sys tem is in some part a revival and systematic application of the opinions said to have been held by Pythagoras. It makes the sun the immovable center of the universe, around which all the planets revolve in concentric orbits, Mercury and Venus within the earth's orbit, and •all the other planets without it. In the Copernican theory, there were many of the old notions which have since been exploded. It is a current belief that Copernicus, afraid to state boldly such heterodox views of the universe as those he entertained, gave them forth in the form of an hypothesis. Humboldt, in his second volume of Cosmos (p. 345), denies that he did so. This distinguished authority says: "The language of Copernicus is powerful and free, and bursts forth from his inmost convictions, and thus sufficiently refutes the ancient opinion, that he has brought for ward the system which is immortalized by his name, as an hypothesis made for the con venience of calculating astronomers, or for one which has but a probable foundation."
The same author also refutes the popular notion that Copernicus died a few hours after receiving a printed copy of his book. He was broken down in body and mind when his work On the Revolutions of the heavenly Bodies was brought to him, but he did not die until " many days afterwards, on the 24th May, 1543." Among the contemporaries of Copernicus were Rheiuhold, who constructed the Prutenic tables; Recordp, who was the first to write on A. in English; and Nonius, Portuguese, who invented a method for dividing the circle. The study of A. was also much aided about this time by the liberality of the Landgrave of Hesse-Cassel, William IV.
Decidedly the most industrious observer and eminent practical astronomer from the time of the Arabs to the latter half of the 16th c. was Tycho Brai': (b. 1546, d. 1601). Considerable odium attaches to him on account of his repudiation of the Copernican system, but it should not be forgotten that in the time of Tycho that system was not supported by the conclusive evidence we are now in possession of. Tycho's system, which made the sun move round the earth, and all the other planets round the sun, they moving with it round the earth, explained all natural phenomena then observed equally well, while it must have appeared more probable than the crude and, at that era, undo monstrable theories of Copernicus. Tycho Bralie compiled a catalogue of 777 fixed stars, more perfect than any that had previously appeared. He made the first table of refractions, and discovered the variation and annual equation of the moon, the inequali ties of the motion of the nodes, and the inclination of the lunar orbit, and rejected the trepidation of the precession, which had hitherto injuriously affected all tables. He also made some interesting cometary investigations.
To his researches are mainly due the discovery by Kepler (b. 1571, d. 1630) of those famous laws which have rendered his name immortal. See KEPLER. To Kepler is due the credit of divesting the Copernican system of its absurdities. Kepler is also said to have had some notion of the law of gravitation.