Observatory is a place where astronomical observations are made. In the early ages astronomy was chiefly a subject in which speculation held sway, and any observations of the heavens were of an extremely rough nature. About 300 B.C., however, an observatory was founded at Alexandria, and from this time onward astronomy became more truly a science, with a tendency towards founding its laws upon observed phenomena, though for many hundred years a large amount of unfounded theory still accompanied it. Hipparchus and Ptolemy made great strides in the science at the Alexandrian observatory, but after the 2nd century A.D. it does not seem to have been used. About the 9th century, observatories began to be built in Arabia and the East, and in the 15th century the first one in Europe was erected at Nuremberg. From this time observations became much more exact, and Tycho Brahe in the 16th century at his own observatory started a series of consecutive records of the positions of different heavenly bodies. The utility of such records became at once apparent, and hence arose many national and public observatories, among the first being those of Paris and Greenwich. Such institutions are now to be found in almost every country of the world, one of the last being the Lick observatory (q.v.) in the United States. An enormous change has, of course, taken place in the instruments employed. The sextant and mural quadrant of Tycho Brahe have been succeeded by the transit instrument and the meridian circle, while the invention of the telescope added enormously to the field of observation. Errors in reading the instruments were lessened by the use of micrometers, and time was more accurately measured when chronometers were improved. Not only are observations made on the sun, planets, and stars, but meteorological and magnetic phenomena are also recorded in many observatories. The work to be done is therefore enormous, and so it is often found advisable to have different observatories devoted to different branches of the work, the largest being generally those maintained for pure astronomical observations.