EQUATION OF TIME is the difference between the time as indicated by a sun-dial, and that by a good clock. It is necessary because the sun, the chief agent in measuring time, does not upon all days of the year appear to move equally fast, inasmuch as an hour by a sun-dial, correctly indicating the sun's motion, is sometimes longer, sometimes shorter, than an hour by the clock, the hours of which are supposed to be perfectly equal, although the sun's are not. The Equation of Time shows how many minutes are to be added to, or subtracted from, sun-dial time in order to show clock time. The same table of equation will serve all over the world. [See following pages for Equation Table.] TRUE OR SOLAR TIME is that marked by the sun, and it is taken at the moment when he has attained his greatest height above the horizon,--such a moment being of course dependent upon the latitude of the place of observation. The solar time by which our nautical standard is fixed, is that of the meridian of Greenwich. SIDEREAL TIME is that measured by the fixed stars, which are at such an immense distance from the earth that the diurnal motion of the earth brings these stars to the meridian at sufficiently regular intervals. It is necessary, however, to remember when making observations for sidereal time that these must be made from fixed or twinkling stars, not from planets. Of the various Eras from which time has been dated, the following are the chief:-- A.M. _Anno Mundi._ The Year of the World, dating from the Creation, according to Jewish Calendar 5635 The Deluge, Era of, variously reckoned 2348 to 3155 B.C. The first Olympiad 776 B.C. A.U.C. or _Anno Urbis Conditæ_, the year of the building of Rome 753 B.C. The Hegira, or Flight of Mahomet from Mecca to Medina 622 A.D. The Birth of Christ in the year of the World 4004 The Jewish year 5635 commenced Sept. 12, 1874 A.D.