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Things Not Generally Known: Solar

Solar Time. Solar time is that used for all the ordinary purposes of life, and is measured by the daily motion of the sun. A solar day is the interval of time between two successive transits of the sun over the same meridian, and the hour angle of the sun is called solar time. This is the most natural and direct measure of time. But the intervals between the successive returns of the sun to the same meridian are not exactly equal, owing to the varying motion of the earth round the sun, and to the obliquity of the ecliptic. The intervals between the sun's transits over the meridian being unequal, it is impossible to regulate a time-piece so that it shall follow the sun. To avoid the irregularity which would arise from using the true sun as a measure of time, a fictitious sun, called a mean sun, is supposed to move in the equator with a uniform velocity. This mean sun is supposed to keep, on the average, as near the real sun as is consistent with perfect uniformity of motion; it is sometimes in advance of it, and sometimes behind it, the greatest difference being about sixteen minutes. Mean solar time, which is perfectly equable in its increase, is measured by the motion of this mean sun. The clocks in ordinary use and the chronometers used by navigators are regulated to mean solar time. Mean solar time is generally called mean time simply. True or apparent solar time is measured by the motion of the real sun. The difference between apparent and mean time is called the equation of time; by means of it we change apparent to mean time, or the reverse. Sidereal Time. Sidereal time is measured by the daily motion of the stars; or, as it is used by astronomers, by the daily motion of that point in the equator from which the true right ascensions of the stars are counted. This point is the vernal equinox, and its hour angle is called sidereal time. Clocks regulated to sidereal time are called sidereal clocks.

A Sidereal Day is the interval of time between the transit of the vernal equinox over any meridian and its next succeeding return to the same meridian. It is about 3 m.

56 s. shorter than the mean solar day; 3651/4 solar days, or a year, being divided into 36614 sidereal days. It is divided into 24 hours. The sidereal hours are counted from o to 24, commencing with the passage of the vernal equinox over the upper meridian, and ending with its return to the same meridian. About March 21 of each year, the sidereal clock agrees with the mean time or ordinary clock, and it gains on it about 3 m. 56 s. per day, so that at the end of a year it will have gained an entire day, and will again agree with it. The Civil Day commences at midnight, and comprises twenty-four hours from one midnight to the next following. The hours are counted from 0 to 12 from midnight to noon, after which they are again reckoned from 0 to 12 from noon to midnight.

Thus the day is divided into two periods of 12 hours each, the first of which is marked A. M., the last is marked P. M. The Astronomical Day commences at noon on the civil day of the same date. It also comprises twenty-four hours: but they are reckoned from o to 24 hours, and from the noon of one day to that of the next following. The astronomical as well as the civil time may be either apparent or mean, according as it is reckoned from apparent noon or from mean noon. The civil day begins twelve hours before the astronomical day; therefore, the first period of the civil day answers to the last part of the preceding astronomical day, and the last period of the civil day corresponds to the first part of the same astronomical day. The rule then for the transformation of civil time into astronomical time is this: if the civil time is marked A. M., take one from the day and add twelve to the hours, and the result is the required astronomical time; if the civil time is marked P. M., take away the designation P. M. and the astronomical time is obtained without change. To change astronomical to civil time, we simply write P. M. after it, if it is less than twelve hours. If greater than twelve hours, we subtract twelve hours from it, add one to the days, and write A. M. If the longitude from Greenwich be expressed in time, and when west added to the local time, or when east subtracted from the local time, the result is the corresponding Greenwich time. If the local mean time is used, the result is the Greenwich mean time. The rule is the same whether we use mean or sidereal time. For general convenience, the time changing continually during a passage, apparent time is kept on board ship at sea. This fact must be remembered, if looking out for the meridian passage of stars to determine latitude. Lunar Day. The average duration of a lunar day, or the interval that elapses between two successive transits of the moon over the meridian of the same place, is 24 h.

54 m.; the average period of the moon's revolution round the earth is 27 d. 7 h.

43 m. 11.5 s.; while the interval between new moon and new moon is 29 d. 12 h. 44 m. 2.9 s. At any time of the year, add twelve hours to the time of the sun's setting, and from the sum subtract the time of rising, for the length of the day. Subtract the time of setting from twelve hours, and to the remainder add the time of rising next morning, for the length of the night. These rules are equally true for apparent time. To ascertain at what hours the sun rises and sets at a given place, for any given day, rectify the terrestrial globe for the latitude of the place: find from the wooden horizon the sun's place in the ecliptic for the given day, and bring it to the meridian. Set the index to 12 (noon), and turn the globe till the sun's place comes to the eastern edge of the horizon, the index will show the hour of rising. Then turn the globe till the sun's place comes to the western edge of the horizon, and the index will show the time of setting. At London and Berlin the longest day has 16½ hours; at Stockholm and Upsala, 16½ ; at Hamburg, Dantzic and Stettin, 17, and the shortest 7; at St. Petersburg and Tobolsk the longest has 19, and the shortest 5 hours; at Bornea, in Finland, the longest day has 21/2, and the shortest 22, At Wanderbus, in Norway, the day lasts from the 21st of May to the 22d of July, without interruption; and at Spitzbergen the longest day is 32 months.

Year. The earth completes her revolution round the sun in 365 d. 9 h. 9 m. 9.6 mean solar time or in 366 d. 6 h. 9 m. 9.6 s. reckoned in sidereal time. This is called the sidereal year. But the year in which mankind in general are most interested, is the tropical year of 365 d 5 h. 48 m. 49.7 s. This year, on which the return of the seasons depends, is the interval between two successive arrivals of the sun at the vernal equinox, or first points of Aries, and differs from the sidereal year by reason of the motion of the equinoctial points, known as the precession of the equinoxes. The tropical year is a compound phenomenon, depending chiefly and directly on the annual revolution of the earth round the sun, but subordinately and indirectly on its rotation round its own axis. The Gregorian Calendar, now generally used among civilized nations, depends upon this tropical year, and may be thus briefly described: Every year whose number is not devisible by 4 without a remainder consists of 365 days; every year which is divisible by 4, but is not divisible by 100, of 366 days; every year divisible by 100, but not by 400, consists of 365; and every year divisible by 400, consists of 366 days. For example:

1873, not being divisible by 4, consists of 365 days.

1876, being divisible by 4, but not by 100, consists of 366 days.

1800 and 1900, being divisible by 100, but not divisible by 400, consist of 365 days each.

2000, being divisible by 400, consists of 366 days.

The error of the Gregorian Calendar amounts to only 0.944 days in 4000 years. 60 seconds. 60 minutes. 24 hours. 7 days.