33 



observatory to be built for him on the hill in Greenwich 

 Park. The foundation stone of the Observatory was laid 

 August 10, 1675, and Flamsteed retained his office until his 

 death, which occurred on the last day of 1719. 



This period of a little more than 44 years may be divided 

 into two parts, during the earlier of which Flamsteed worked 

 practically single-handed. Nevertheless, in 13 years he had 

 made 20,000 observations and had revised the whole of the 

 theories and tables of the heavenly bodies then in use. In 

 1688 he was joined at Greenwich for the second time by 

 ABRAHAM SHARP (1653-1742), a most skilful instrument- 

 maker as well as an excellent astronomer and computer. 

 Sharp furnished Flamsteed with a new mural arc, 140 in 

 range and seven feet in radius, with which he commenced 

 observation on December 12, 1679, and used as his chief 

 instrument for the remainder of his life. During these 30 

 years he determined the places of some 3,000 stars, and pre- 

 pared a great catalogue, the Historia Ccelestis, which won 

 from Airy the high praise : " This work may shame any other 

 collection of observations in this or any other country." 

 Toward the other side of his great task the determination 

 of the movements of the moon he revised and improved 

 by far the best lunar theory then existing, viz., that due to 

 JEREMIAH HORROX (1619-1641), the wonderfully gifted 

 curate of Hoole, whose premature death in his twenty-second 

 year was so great a loss to science. Incidentally to these, 

 the two chief items of the official programme, Flamsteed had 

 determined the latitude of his Observatory, the obliquity 

 of the ecliptic, the position of the equinoctial points, the time 

 in which the sun rotates on its axis and the position of that 

 axis, and had observed an apparent movement of the stars 

 in the course of the year, the explanation of which had to 

 wait till it was afforded by Bradley, the third Astronomer 

 Royal. 



Flamsteed was succeeded in 1720 by EDMUND HALLEY 

 (1656-1742), then in the 64th year of his age. Halley's 

 chief services to astronomy were rendered before he came to 

 Greenwich. The greatest claim which he has upon our 

 gratitude is that he succeeded in inducing ISAAC NEWTON 

 (1642-1727) to publish his Principia, and undertook to 

 guarantee the expense of publication. But his own services 

 to astronomy were of high importance. He computed the 

 orbits of a number of comets and, in so doing, discovered 

 that one of them which now bears his name had returned 

 on two or three occasions, and he predicted that it would 



o 



