the Athenian, upwards of 1900 years ago, is now removed above 30 degrees from that point; fo that Aries is now where Taurus was, Taurus where Genini was, &c. The difcovery of this motion is due to Hipparchus of Rhodes, one of the most celebrated aftronomers of ancient times.

Hence the conftellations on the zodiac. of a celeftial globe, do not agree in figure and character, the figns or conftellations of the zodiac being to the east of thofe figns, or arcs of the ecliptic, which are called by the fame names: for in order to avoid confufion, aftronomers thought proper to let the feveral portions of the ecliptic, where those conftellations were firft obferved to be, retain their old names, confequently the vernal equinox is ftill confidered as the first point of Aries.

The fpaces formerly occupied by the zodiacal conftellations, retaining their ancient names, are called anaftra, or without their former stars; whereas the fpaces they now poffefs are called Atellata.

This flow motion of the ftars forward, is really caufed by a like flow motion of the equinoctial points backwards; and this is owing to the revolution of the axis of the equator about the axis of the ecliptic; the pole of the equator defcribing in the heavens a circle about the pole of the ecliptic.

By this preceffion of the equinoctial points from east to west, they meet the fun every year 50 feconds of longitude before a complete revolution has been made. The time, in which the fun appears to revolve from tropic to tropic, is called a tropical year; this with the time he has yet further to go to complete the revolution, namely, 50 feconds, is called the fiderial year. Sir Ifaac Newton attributes this motion to the fpheroidal figure of the earth, deducing from this L 3

figure

figure the revolution of the poles of the world round those of the ecliptic.

This motion carries the ftars about 1 degree, 20 minutes, 23 feconds, in 100 years; fo that the total revolution of the fixed ftars eastward, back to the equinoctial points again, will be completed in 25972 years.

LECTURE

LECTURE XLII.

OF SOLAR AND SIDERIAL DAYS, OF MEAN TIME, THE EQUATION OF TIME, &c.

THE

"HE rotation of the earth about it's axis being uniform, it neceffarily follows, that the apparent diurnal revolution of the ftars about the earth must be alfo uniform, that is, made in equal times; they therefore will form a very proper measure to denote time. But then as they turn fucceffively with a conftant motion, one must be felected, by whose revolutions time may be meafured; we muft alfo fix a term from whence to commence our reckoning.

The fun being the moft confpicuous object, was fixed upon by the aftronomers of early ages, as the most proper measure for the parts of time. But when more accurate obfervations were made, the fun's motion was found not to be uniform, and confequently the time measured thereby would be neither regular nor equal; they were therefore obliged to find out a mean or regular time for the bafis of their calculations.

An aftronomical or folar day is divided into 24 hours, reckoning them in numeral fucceffion, from 1 to 24. The first twelve hours are fometimes diftinguished by the mark P M for afternoon, the other twelve are diftinguished by A M for before noon. Aftronomers generally reckon through the 24 hours from noon to noon; and what is by the common way of reckonig called morning hours,

L 4

hours, is by them reckoned in fucceffion from noon to midnight. Thus 5 o'clock in the morning of April the 10th, is by aftronomers called April 9, 17 hours.

If the fun had no other apparent motion but that of it's diurnal revolution, it would every day describe the fame parallel, and be accompanied by the fame stars. But it has alfo an apparent annual motion, by which it feems to be carried through the zodiac every year, from weft to caft, that is, in a direction contrary to that of it's diurnal revolution..

Hence, if on any day the fun and a ftar pafs the meridian at the fame inftant, on the next day when the ftar returns to the meridian, the fun will have departed towards the weft, as much fpace as in that interval it has paffed over by it's annual motion, and will therefore arrive at the meridian some moments after the star; the day following it will be still later, so that at the end of fix months, it paffes 12 hours after the ftar, which has therefore gained 12 hours on the fun; and at the end of the year, the ftar will have paffed 366 times over the meridian, where the fun has only paffed 365 times.

In this view we have confidered the fun's apparent motion; the refult is the fame, if you confider the earth's real motion. If indeed the earth had no real motion, and confequently the fun no apparent motion, the length of a natural day would be about 23 hours 56 minutes, for in that time a revolution of the earth is completed, as appears by an easy obfervation; for any fixed ftar that is on the meridian at a given hour of night, will after 23 hours 56 minutes, be on the meridian again the night following. This interval of time is called a fiderial day.

Thus

Thus you fee that there is a diftinction between a solar day and a fiderial day.

A folar or aftronomical day is the fpace of time that intervenes between the fun's departing from any one meridian, and it's return to the fame again. The fiderial day is the fpace of time that elapfes between the departure of a ftar from a given meridian, and it's return to the fame again.

I fhall now endeavour to fhew you, why thefe days differ in length; that is, why the fun takes up more time to complete one revolution than a ftar.

motion.

This difference arifes from the fun's annual The fun does not continue always in the fame place in the heaven, as the fixed ftars do: but if it is feen at M, fig. 2, pl. 4, one day, near the fixed ftar R, it will have fhifted it's place the next day, and will be near to fome other fixed star L. This motion of the fun is from west to east, and one entire revolution is completed in a year. Suppofe, therefore, that the fun, when it is at M, near to the fixed ftar R, appears in the fouth of any particular place S; and then imagine the earth to turn once round upon it's axis from west to caft, or in the direction S T V W, fo that the place may be returned to the fame fituation; after this rotation is completed, the ftar R will be in the fouth of the place as before; but the fun having, in the mean time, moved caft wards, and being near to the ftar L, or to the east of R, will not be in the fouth of the place S, but to the eastward of it upon this account, the place S muft move on a little farther, and must come to T before it will be even with the fun again, or before the fun will appear exactly in the fouth.

This may be illuftrated by an instance. The two hands of a watch are close together, or even