TIME. Time and space are the two great elements with which the astronomer in the observatory has to deal; conse quently a great part of astronomical work, and a great part of the equipment of every observatory, is devoted to the determination of time in various ways and for various purposes.
Measurement of Time.—This is accom plished by the joint work of the clock and watchmakers, and of the astronomer who makes observations of the transits of the heavenly bodies across the meri dian to determine the necessary daily cor rections to these instruments. For no piece of chronometric apparatus, even the best constructed astronomical clock, can begin to approach in regularity of run ning the motions of the heavenly bodies, especially the rotation time of the earth on its axis, on which most time determi nations depend. The principal pieces of apparatus used for keeping time are various forms of clocks and watches. Be fore the invention of these instruments the measurement of time was very rough, the most accurate being the method of the sundial; but as this was only good in the day time and when the sun was shining, various rougher devices were employed, such as running sand in the hour glass, the water clocks, or clepsy dra', or even a burning candle. The senseless division of the clay into two periods (of 12 hours each), which was probably originally brought about by the use of the sun's shadow in the day time and something or nothing else in the night, is one of the relics of barbarism which seems to be the hardest to get rid of. Even the division into 24 hours (in stead of decimals of the whole day, as it ought to be) is bad enough, but, on account of the radical change necessary in the construction of all clocks and watches in order to bring this about, it will probably be a long time in coming. But the small changes necessary in the dials in order that they may read up to 24 hours instead of 12 makes it surpris ing that there is so much opposition to the proposal of the railroad managers simply to print their time tables with the day divided in that way.
Different Kinds of Time.—Two differ ent kinds of time, with different actual lengths of the unit, are in common use— mean solar and sidereal. The mean solar day marks the average interval of the return of the sun to the same meridian. The time marked by the return of the actual sun is not uniform, and is called apparent time. The difference between
mean and apparent time is called the equation of time. Mean solar time is that in common use, but the mean solar day is used in two different ways; the one called the civil day, or civil time, be gins the day at midnight, and is the one used by the people at large. The other, used by astronomers, begins the day at mean noon, reckoning it from 0 to 23 -I hours. The day begins 12 hours later than the civil day, or 12 hours earlier, according to how the matter is viewed. Thus at 6 P. NI., Jan. 23, the astronomer calls it Jan. 23 days 6 hours, or Jan. 23.25 days, if he expresses it decimally. Likewise he calls Jan. 24 at 6 A. m. Jan. 23 days 18 hours, or Jan. 23.75 days. If we consider that 23.25 days and 23.75 days are still parts of the 23d day, then the astronomical day is half a day behind; but if they are to be regarded as parts of the 24th day, as they should be just as much as 1850 is regarded as the middle of the 19th and not the 18th century, then the astronomical day is 12 hours ahead of the civil. It is simply a question of interpretation. The astronomical time as above defined is generally called simply "mean time," to distinguish it from either civil time or sidereal time.
Sidereal time is of an entirely distinct length from mean solar, the sidereal day marking the successive transits of the vernal equinox over the meridian, and being very nearly equal to the rotation time of the earth on its axis. The sidereal day equals 23 hours 56 minutes 4.090 seconds of mean solar time, and there is one more sidereal day in the year than solar days. The sidereal day is not of absolute uniform length, as the precession of the equinoxes is not abso lutely uniform, but the variations in its length are out in decimal places of sec onds beyond those given above. Sidereal time is only used in observations by astronomers. All observations with transit instruments for the determination of the right ascensions of the heavenly bodies are recorded by sidereal clocks, and the observation made simply for the purpose of getting the corrections to a mean time clock for the purpose of time distribution are generally made on the stars, using a sidereal clock or chro nometer, and then the mean time clock is compared with the sidereal afterward to determine its correction.