How Time Is Distributed.—In this country the correct standard time is sent out by the United States Naval Observatory at Washington to all cities east of the Rocky Mountains, by wire telegraph, and all over the Atlantic ocean and seaboard by wireless telegraph.
Fig. 166.—Standard Time at Different Cities.
When time is received over the wires from Washington it is distributed by local telegraph or by time balls to various jewelry stores and to private citizens who always want to be set right.
How an astronomer gets the correct time; how it is sent out over the wires and by wireless and how it is distributed to the common people is a mighty interesting piece of business. Briefly it is like this:
How Correct Time Is Obtained.—Every observatory has, besides its big telescopes, a transit instrument, a wonderfully accurate clock, and a clockwork device called a chronograph. The transit instrument is nothing more than a telescope with a thin piece of clear glass with a number of lines ruled on it with a diamond, about ⅛ inch apart, and this ruled glass is set between the eyepiece and the object glass, as shown in [Fig. 167].
Fig. 167.—Ruled Glass in Transit Instrument.
This telescope, or transit instrument—so called because it is used to observe the transit, or passage, of a star across a meridian—is set on an axis so that the telescope can be pointed to any place on the meridian but it cannot be moved east or west.
Sometime before a star is due to cross the meridian the astronomer sets his transit instrument so that the star will pass right across the line of sight of his telescope.