3. The interval of time from the sun's centre leaving any meridian to its returning to the same again, is called a true solar day; and is counted twenty-four hours without interruption; that is, in numerical succession from 1 to 24.

This is usually called an astronomical day, because astronomers reckon their day from noon: It is also called a natural day, because it is of the same length in all latitudes. True solar time is that which is shown by a true sun-dial.

4. A mean solar day is the time elapsed from 12 o'clock at noon on any day, to 12 o'clock at noon on the next day, as shown by a perfectly well regulated clock or watch.

The time shown by a well regulated clock, or watch, and a true sun-dial, is never the same but on or about the 15th of April, the 15th of June, the 1st of September, and the 24th of December. The clock, if it goes equally and true; will be faster than the sun from the 24th of December till the 15th of April; from that time till the 15th of June the sun will be faster than the clock; from the 15th of June till 1st of September the clock will be again faster than the sun; and from thence to the 24th of December the sun will be faster than the clock.

The difference between the true solar noon, as shown by a true sun-dial, and the mean solar noon as shown by an equally going clock, is the greatest about the 3d of November; the time shown by the clock being then 16 minutes and 15.9 second slower than the time shown by the sun-dial

Though the difference between the true and mean solar noons about the 3d of November, is 16 minutes and a quarter nearly, we are not however to infer that the difference between the mean solar day and true solar day, is equal to the same; for, in fact, they are nearly equal at that time, as may be readily seen from the Nautical Almanac for the present year.

The difference between mean and apparent time, usually called the equation of time, depends upon two causes, the obliquity of the ecliptic with respect to the equator, and the unequal motion of the earth in an elliptical orbit. The effects of both these causes shall be fully considered in a subsequent part of this work.

5. The civil day is from midnight to midnight again, the first twelve hours are the morning hours, and the last twelve the afternoon hours.

The astronomical day begins at the noon of the civil day; for instance, May 13th, at 4 o'clock in the afternoon, according to the civil account, will be the same as the astronomical account; but supposing it was 4 o'clock in the morning of May 13th, according to the civil account, it would be May 12th, 16 hours by the astronomical way of reckoning.

In civil life, according to Laplace, the day is the interval of time which elapses between the rising and setting of the sun, and is variable according to the different latitudes of places: the night is the time which the sun remains below the horizon, and varies in like manner.

6. The time in which any star appears to re volve from the meridian to the meridian again; or, which amounts to the same thing, the time in which the earth makes one complete revolution on its axis, is called a sidereal day, which is twenty-three hours, fifty-six minutes, four and one-tenth seconds of mean solar time.

A sidereal day is, therefore, less than a mean solar day, by 3 minutes 55.9 seconds. This difference is occasioned by the immense distance of the fixed stars; for the earth's orbit, when compared with this distance, is but a point; and therefore any meridian will revolve from a fixed star to that star again, in exactly the same time as if the earth had only a diurnal motion, and was to remain for ever in the same part of its orbit.

But this is not the case with respect to the sun; for as the earth, at a mean daily motion, advances 59′ 8.2" eastward in its orbit, and that its diurnal motion is also eastward, it is evident that the same meridian can never be brought round from the sun, to the sun again, by one entire revolution of the earth upon its axis, but that it will require as much more of another revolution as is equivalent to the space which the earth has advanced in its orbit during that time So that three hundred and sixty-six terrestrial revolutions would be exactly equal to three hundred and sixty-five diurnāl revolutions, if the equinoctial points were at rest in the heavens.

7. A clock that is so regulated as to move through twenty-four hours in the course of a sidereal day, is said to be regulated to sidereal time.

Astronomers have found, by comparing a certain number of solar and sidereal days, that a mean solar day is 24 hours, 3 minutes, and 56.55 seconds of sidereal time, so that the excess of a mean solar day above a sidereal day, is 3 minutes, 56.55 seconds in sidereal time.

8. An hour is a certain determined part of a day, and is equal or unequal. An equal hour is the 24th part of a mean solar day, as shown by well regulated clocks; unequal hours are those measured by the returns of the sun to the meridian, or those shown by a correct sun-dial.

Hours are divided into 60 equal parts, called minutes, a minute into 60 equal parts called seconds, a second into 60 equal parts called thirds, &c.

9. A sidereal hour is the 24th part of a sidereal day, and is therefore less than an hour of mean solar time.

For, a mean solar day is to a sidereal day, as 24 hours is to 23h. 56m. 4.1 seconds in mean solar time; or, as 24h. 3m. 56.555 seconds is to 24 hours in sidereal time: And, consequently, if the length of a solar hour be taken equal to unity or 1, a sidereal hour will be equal to .99727 in mean solar time; or, if a sidereal hour be taken equal to unity, or 1, an hour of mean solar time will be equal to 1.0027379 in sidereal time. Hence, by multiplying any given portion of sidereal time by .99727, we shall have the corresponding mean solar time; and, on the contrary, to reduce mean solar time, to sidereal time, we must multiply by 1.0027379.

10. A year, in the general extent of the word, is a period or space of time, measured by the revolution of some celestial body in its orbit.

As year denoted originally a revolution, and was not limited to that of the sun; accordingly, we find by the oldest accounts, that people have, at different times, expressed other revolutions by it, particularly that of the moon; and

consequently that the years of some accounts are to be reckoned only months, and sometimes periods of 2, or 3, or 4 months. This will assist us greatly in understanding the accounts that certain nations give of their own antiquity, and perhaps also of the age of men. We read expressly, in several of the old Greek writers, that the Egyptian year, at one period, was only a month; and we are farther told that at other periods it was 3 months, or 4 months.

The Egyptians boasted, almost 2000 years ago, of having accounts of events 48000 years distance. A great deal must be allowed to fallacy on the above account; but besides this, the Egyptians had, in the time of the Greeks, the same ambition which the Chinese have at present, and wanted to pass themselves on that people, as these do upon us, for the oldest inhabitants of the earth. They had also recourse to the same means, and both the present and the early impostors have pretended to ancient observations of the heavenly bodies, and recounted eclipses in particular, to vouch for the truth of their accounts.

Since the time in which the solar year, or period of the earth's revolution round the sun, has been received, we may account with certainty; but for those remote ages, in which we do not precisely know what is meant by the term year, it is impossible to form any satisfactory conjecture of the duration of time in the accounts.

11. The returns of the sun to the same equinox mark the years, in the same manner as its returns to the meridian mark the days. The solar year is either astronomical or civil.

12. The astronomical solar year is that which is determined precisely by astronomical observations; and is of two kinds, tropical and sidereal, or astral.

13. It is found by observation that the sun, in consequence of its annual motion in the ecliptic, employs three hundred and sixty-five days, five hours, forty-eight minutes, and fifty-one seconds, in moving from one equinox to the same again.. This period of time is called the tropical year.

This is the only proper or natural year, because it always keeps the same seasons to the same months.

14. Observation also shows us that the sun em ploys 365 days, 6 hours, 9 minutes, and 11 seconds, in passing from any fixed star, till it returns to the same again. This period is called the sidereal year.

Hence, the sidereal year is 20 minutes and 20 seconds longer than a tropical year; and it likewise follows that the equinoctial points must have a motion along the ecliptic in a direction contrary to the order of the signs, amounting to 50" 1, in a year: for, as the sun describes the whole ecliptic, or 360° in a year, 365d. 5h. 48m. 51sec.: 360°:: id: 59' 8" 2, the daily mean motion of the earth, or the apparent mean motion of the sun in a day; and therefore 1d: 59′ 8′′:: 20′ 20′′: 50′′ 1. This retrograde motion of the equinoctial points is called the recession of the equinoctial points.

15. That form of year which a nation has adopted for computing their time by, is called a civil year. The American civil year is a period of 365 days, 6 hours, which is either common or bissextile. The common civil year is that consisting of 365 days; having seven months of 31 days each; four of 30 days, and one of 28 days: the bissextile, usually called the leap year, consists of 366 days, having one day extraordinary, called the intercalary, or bissextile day; and takes place every 4th year. In this year February contains 29 days.

What is time?

QUESTIONS.

How is time measured?

What is a true solar day?

What is a mean solar day?

What is a civil day?

What is a sidereal day, and what is its duration

in mean solar time?

What is an hour?

What is a sidereal hour?