Page:Popular Science Monthly Volume 29.djvu/537

Rh astronomical units, while all the time depending upon them for their ascertainment. And the recent changes of using "standard time" are in the same direction. Of course, every place, not just north or south of another, has a different noon. To prevent the confusion resulting from so many "times," our railroads have adopted as noon the mean times of certain standard meridians. These are taken just one hour apart, so that if the new time were universally adopted, the minute and second hands of all correct clocks would be the same over the whole United States, and the hour-hands would differ by one, two, or three hours. In England they have used Greenwich time over the island for many years, and our system is connected with theirs by using for our standard meridians those which are an even number of hours from Greenwich. In Philadelphia, for instance, which is situated on a standard meridian, the time is just five hours later; so that tidings of an event, happening at noon in London, if telegraphed immediately, will reach Philadelphia a few minutes after seven o'clock in the morning.

The objections to adopting this standard time, in some places, based on the inconveniences of having noon at some other time than when the sun is on the meridian, very much resemble those made in France when the Government substituted mean noon for apparent. In practice we never know when the sun is on the meridian, and if it gets there at 12.30 instead of 12, no one is the worse off, and the methods of living are readily adaptable to it.

Time being thus dependent on the facts of astronomy, its ascertainment is a part of the work of an astronomical observatory. The instrument used for the purpose is a transit-instrument. It consists of a telescope which is mounted, not to be pointed to any part of the sky, but to swing only in the plane of the meridian. It will point horizontally, north or south, to the zenith, and to intermediate points. A star in the east or west can not be seen by it. When it crosses the meridian, if the telescope is elevated to the proper angle, it will cross the field of view. To determine exactly what part of the field the meridian crosses, a spider-thread is stretched in the tube just in front of the eye-piece, which by a very accurate adjustment must be made to coincide exactly with the meridian. Just as the star crosses this thread, or, to speak more accurately, just as the particular meridian of the place passes under the star, the time must be recorded. As there is a possibility of an error in this, several spider-lines are inserted parallel to this central one, and symmetrically placed on either side.

The telescope is connected with an axis pointing east and west, working on the tops of two pillars set far enough apart to allow the telescope to swing between them.

Let us now go through the operation of "taking a transit." The observer, by means of graduated circles, points his telescope to the