The purpose of observing the transit, or passing of a star is to see how much his wonderfully accurate clock has lost or gained during the past 24 hours. So his clock is right at hand.
The chronograph is another accurate clockwork which revolves a cylinder about the size and shape of a phonograph cylinder, and around which is wrapped a sheet of white paper. This cylinder makes one revolution every minute. A fountain pen marks a spiral line on the paper when the cylinder is revolving but at every second the pen is thrown out of position and this makes a notch in the line.
After starting the chronograph the astronomer takes an electric push button, which is connected with and controls the lever which holds the pen of the chronograph that makes the notches, and takes up his position with his eye at the end of the transit instrument.
The instant he sees the star on one of the lines in the telescope he presses the button and this closes the electric circuit and makes a big notch in the line traced on the paper of the chronometer.
From the position of the big notches which he caused to be made on the paper as the star crossed the ruled lines, and the little notches made regularly every second, he can dope out just how much his clock is in error—that is, how much it is too fast or too slow.
The next thing he does is to change this absolutely correct star time into mean solar time, when it is ready to be sent all over the United States east of the Rocky Mountains by telegraph. The Pacific Coast folks get their correct time from a Government observatory at Mare Island in San Francisco Bay.
How Time Is Sent by Telegraph.—The wires of the Western Union Telegraph Company run into the United States Naval Observatory at Washington, and for a few minutes each day all of this company’s wires are controlled by the Government.
About five minutes before 12 o’clock noon, standard time at Washington, each day the wires all over the country east of the Rockies are cleared and all business and other messages are cut off for the time signals.
At five minutes of 12 sharp the United States Naval Observatory begins to send the beats of every second of the wonderfully accurate observatory clock, which are ticked out on telegraph sounders in all the cities and towns.
But no, not every beat, for the 29th second of each minute, the last 5 seconds of each of the first 4 minutes, and then the last 10 seconds of the last minute are not sent. The first click of the sounder after the 10 second rest is the noon signal, and all local clocks are set by it.