Page:Collier's New Encyclopedia v. 09.djvu/465

LEFT TIME 403 TIME 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 to 23 -{- 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. M., 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 sidei*eal 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. Time Sigytal. — Many observatories send out time signals either daily, hourly, or sometimes continuously every second, or every other second, to various parts of the country for the purpose of giving accurate time to all sorts of industries. They are sent over the telegraph lines, the wires being permanently run into the observatories for the purpose, and the signals^ are generally sent automatically by a distributing clock which is kept as near the exact times as possible. An electric current passes through the clock and is broken or closed regularly by a toothed wheel on the second hand arbor of the clock. Perhaps the best known set of time signals is that sent out by the Naval Observatory at Wash- ington. It is as follows: three or four minutes before noon, whenever the telegraph companies switch in the loops to the observatory, the clock begins to send out make-circuit signals every sec- ond over the various lines, the minutes being indicated by leaving out the sec- onds 55, 56, 57, 58, and 59 in each, and the half minutes by leaving out the 29th second of each. The click following such a one-second gap then always indicates the beginning of a half-minute, and the first following a gap of five seconds indi- cates the beginning of a minute, except at the exact noon. Just before this there is a gap of 10 seconds, and then exactly at noon the circuit closes and remains closed for just a whole second, the be- ginning of the mark indicating exact noon. The closing for a whole second is in order to make sure that that particu- lar mark goes through all the telegraph lines, for the particular signal is made to do a great many things at different places, such as the dropping of time balls and it is more important that this partic- ular second be distinctly sent than any of the others. After the break at the close of the noon signal the telegraph companies quickly switch out the loops to the observatory, and the lines imme- iately resume their normal work. In the city of Washington this particular noon signal drops a time ball on the top of the State, War, and Navy Department building, and it also automatically cor- rects, by setting forward or back ex- actly to hours minutes seconds, all the clocks in the department build- ings of the government, no matter how much they may have gained or lost since the preceding noon. Local, Universal and Standard Time. — Local mean time is that indicated by the transits of the sun at any particular meridian, and of course this differs for places of different longitudes on the sur- face of the earth. In fact, the differ- ence of longitude between two places is simply the difference of their local times, and the accurate determination of this difference is one of the most common kinds of astronomical work, especially in the principal observatory of any country. The operation consists simply in making the most accurate determination possible of the clock-corrections at each observa- tory and then comparing the clocks by telegraphic time signals with each