soon as the world was made a fit habitation for man, the measurement of time became necessary on many accounts; our pleasures as well as our interests, require that this object should be accomplished; but it is only an acquaintance with astronomy that can furnish the means of doing it correctly. For time has always been measured and defined by the motions of the heavenly bodies, and particularly by the sun, as being the most regular and constant in his apparent revolutions.* The principal divisions of time are the year and the day, which are measured by the annual and diurnal revolution of the sun. The day, or the time in which the sun appears to go round the earth, has been divided into twenty-four equal parts, which are called hours, and these again subdivided into minutes, &c. This division is, however, merely arbitrary; there being no astronomical appearance to warrant or regulate such a division of the day, more than a division into twenty-two, forty-eight, or any other number of equal parts. The length of the tropical year, or the time the sun is in going from any point of the ecliptic to the same again, is 365 days, 5 hours, 48 minutes, 49 seconds. But the sidereal year, or the time which intervenes between the conjunction of the sun and any fixed star and his next conjunction with the same star, is 365 days, 6 hours, 9 minutes, 11 seconds. The difference between these two periods, which amounts to 20′ 22′′, is occasioned by the recession of the equinoxes, or the falling back of the equinoctial points 50 seconds of a degree every year. This retrograde motion of the equinoctial points is caused by the joint attraction of the sun and moon upon earth, in consequence of its spheroidical figure.t the This Time is distinguished according to the manner of measuring the day, into apparent, mean, and sidereal. Apparent time, which is also called true, solar, and astronomical time, is derived from observations made on the sun. Mean, or mean solar time, sometimes called equated time, is a mean or average of apparent time, which is not always equal; for the intervals between two successive transits of the sun over the meridian are not always the same. is owing to the eccentricity of the earth's orbit, and its obliquity to the plane of the equinoctial. If the earth's orbit were an exact circle, and coincident with the equinoctial, the sun would always return to the meridian of any place at equal intervals of time, and apparent and mean solar time would be the same. But as this is not the case, mean time is deduced from apparent by adding or subtracting the difference between them, which is usually called the equation of time. Mean solar days are all equal, being twenty-four hours each; but apparent solar days are sometimes more than twenty-four hours, and * As it may contribute to perspicuity in treating of this important subject, we shall consider the apparent motions of the sun as real. †These variations are computed and inserted in a table, which is called a Table of the Equation of Time: sometimes less. A sidereal day is the interval between two successive transits of a star over the same meridian, and is always of the same length; for all the fixed stars make their revolutions in equal times, owing to the uniformity of the earth's diurnal rotation about its axis. The sidereal day is however shorter than the mean solar day by 3′ 561". This difference arises from the sun's apparent annual motion from west to east, by which he leaves the star as it were be-hind him. Thus if the sun and a star be observed on any day to pass the meridian at the same instant, the next day, when the star passes the meridian, the sun will have advanced nearly a degree to the eastward; and, as the earth's diurnal rotation on its axis is from west to east, the star will come to the meridian before the sun, and in the course of a year the star will have gained a whole day on the sun, that is, it will have passed the meridian 366 times while the sun will only have passed it 365 times. Now as the sun appears to perform the whole of the ecliptic in 365 days, 5 hours, 48 minutes, 49 seconds, he describes 59′ 8.3", or nearly one degree of it per day, at a mean rate; and this space reduced to time is exactly 3′ 561", the excess of a mean solar day above a sidereal day.* The equation of time, or the difference between mean and apparent time, as already mentioned, arises from two causes; namely, the obliquity of the ecliptic to the plane of the equinoctial, and the eccentricity of the earth's orbit. There are, however, four days in the year when the equation of time is nothing, or when the mean and apparent time coincide; these days are, at present, the 15th of April, the 15th* of June, the 1st of September, and the 24th of December. From the first of these days to the second, the apparent time is before the mean; from the second to the third, the mean time is before the apparent; from the third to the fourth, the apparent is before the mean ;* and from the last of those days to the first, the mean is again before the apparent, and so on alternately.† If the revolution of the sun consisted of an entire number of days, for instance 365, the year would naturally be made to do the same, and there would be no difficulty in the formation of the calendar, or in adjusting the reckoning in years and in days to one another. All the years would thus contain precisely the same number of days, and would also begin and end with the sun in the same point of the ecliptic. But the sun's revolution includes a fraction of a day, and therefore a year and a revolution of the sun cannot be precisely completed at the same moment. However, as this fraction makes a whole day in four revolutions, one day is added every four years, in order to make this number of years equal to the same number of revolutions. The year to which this day is added therefore contains 366 days. This is the arrangement of what is called the Julian Calendar, and *This excess is sometimes called the acceleration of the fixed stars. + Clocks and watches ought to be regulated by mean time, as none of them can shew apparent time, because they are all constructed on the principle of uniform and equable motion. the year thus computed, is termed the Julian year, from Julius Cæsar, by whom it was introduced at Rome.* But as the real length of the year is 365 days, 5 hours, 49 minutes, nearly, the manner of rec.. koning adopted by Julius Cæsar was not sufficiently exact to preserve the seasons in the same time of the year; for in four years the difference between the year thus regulated and the true solar year amounted to about 44 minutes, and in 132 years to one entire day. The Julian year must, therefore, have begun one day earlier than the solar year at the end of this period. Consequently, the continuance of this erroneous mode of reckoning would have made the seasons change their places altogether in the course of twenty-four thousand years. At the time of the Council of Nice, in the year 325 of the Christian era, the Julian calendar was introduced into the church; and at that time the vernal equinox fell on the 21st of March; but on account of the imperfection of the mode of reckoning just noticed, the reckoning fell constantly behind the true time: so that in the year 1582, the Julian year had fallen nearly ten days behind the sun; and the equinox, instead of falling on the 21st of March, fell on the 11th of March. The defects of the calendar were discovered long before the year 1582; but all attempts made to reform it proved in vain. At last, Pope Gregory, who was desirous of rendering his pontificate illustrious by bringing about a reformation, which his predecessors had failed to accomplish, invited all the astronomers in Christendom to give their opinions on this important affair. This invitation had the effect of bringing forth many ingenious plans, but the one which he ordered to be adopted was afforded by an astronomer of Verona, named Lilius. The first step was to allow for the loss of the ten days; which was done by counting the 5th of October, 1582, the 15th of that month. By this means, the vernal equinox was again brought to the 21st of March, as it was at the time of the Council of Nice. And to prevent the like inconvenience in future, it was decreed that the last year of every century, not divisible by four, should be accounted a common year, which, according to the Julian reckoning, should be leap year; but that those hundreds which were divisible by four, such as 1600, 2000, 2400, &c. should still be accounted leap year. Although this correction be sufficiently exact to keep the seasons to the same time of the year, yet it does not altogether correspond with the real length of the year, for the time that the Julian year exceeds the true, will amount to 3 days in 390 years. If, therefore, at the end of 390 years, three days were expunged, the equinox would very nearly keep to the same day of the month; but by suppressing 3 days *The intercalary day, or the day which was added every fourth year, was accounted the 24th of February, and called by the Romans the 6th of the Kalends of March; on this account there were every fourth year two 6ths of the Kalends of March, and therefore they called this year Bissextile. With us it is called Leap Year. : only in 400 years, as in the Gregorian account, a small deviation will take place in the course of twelve or sixteen centuries, but so trifling as scarcely to deserve notice. As this reformation of the calendar was brought about under the auspices of Pope Gregory, it is called the Gregorian Calendar, and sometimes the New Style, to distinguish it from the Julian account. This new calendar was immediately adopted in all Catholic countries; but it was not adopted in this country till the year 1752. In Russia, Prussia, and some other countries, the Julian account is still used. OF THE TIDES. The ebbs of tides, and their mysterious flow, DRYDEN. The Tides have been always found to follow, periodically, the course of the sun and moon; and hence it has been suspected, in all ages, that the tides were, some way or other, produced by these bodies. The celebrated Kepler was the first person who formed any conjectures respecting their true cause. But what Kepler only hinted, has been completely developed and demonstrated by Sir Isaac Newton. After his great discovery of the law of gravitation, he found it an easy matter to account for the whole phenomena of the tides: for, according to this law of nature, all the particles of matter which compose the universe, however remote from one another, have a continual tendency to approach each other, with a force directly proportional to the quantity of matter they contain, and inversely proportional to the square of their distance asunder. It is therefore evident, from this, that the earth will be attracted both by the sun and moon. But although the attraction of the sun greatly exceeds that of the moon, yet the sun being nearly four hundred times more distant from the earth than the moon, the difference of his attraction upon different parts of the earth is not nearly so great as that of the moon; and therefore the moon is the principal cause of the tides. Attractive pow'r! whose mighty sway If all parts of the earth were equally attracted by the moon, it would always retain its spherical form, and there would be no tides at all. But the action of the moon being unequal on different parts of the earth, those parts being most attracted that are nearest the moon, and those at the greatest distance least, the spherical figure must suffer some change from the moon's action. Now as the waters of the ocean directly under the moon are nearer to her than the central parts of the earth, they will be more attracted by her r than the central parts. For the same reason, the central parts will Thus it appears, if the earth were entirely covered by the ocean, as represented by the the circle bdec in the following figure, |