Now to find the latitude by taking the Sun, or as sailors sometimes call it, shooting the Sun, in order to learn the position of the ship at sea, the sextant is held in both hands firmly, and the horizon which is sighted through the telescope is brought into view through the clear part of the horizon glass E.
The arm C, carrying the index mirror D, is now moved over the scale A until the light from the Sun just as it crosses the meridian at noon is reflected by its polished surface into the silvered part of the horizon glass E, and this reflects the sunlight into the telescope right in a line with the line of sight to the horizon. This forms an angle of the two beams of light just as an angle is formed of the two sticks of wood in the pail experiment and the number of degrees the end of the arm C points to on the scale B is the latitude in degrees or the distance of the ship north or south of the equator.
Fig. 99.—Sextant in Use. Shooting the Sun.
How Longitude is Found.—To find the longitude at sea, that is, the position of a ship east or west of a given place, is just as simple a matter as finding the latitude or its position north or south of the equator. Two instruments are used to find longitude, and these are the sextant, which has just been described, and a very accurate clock, called a chronometer (pronounced chro-nom´-e-ter).
As you know, imaginary lines running from the north pole to the south pole are called meridians of longitude. Now the Earth has been divided into 24 of these lines, the zero meridian, from which distances east and west are measured, running through Greenwich, England.
There are 24 of these standard meridians and hence they are 15 degrees apart—since there are 360 degrees in a circle—and they are 1 hour apart—since there are 24 hours in a day, and therefore 15 degrees equal 1 hour. ([See Chapter X], The Time o’ Day.)
Now, since it is 12 o’clock noon when the Sun passes over any one of these standard meridians, it will be 11 o’clock A. M., 15 degrees west of it, and 1 o’clock P. M., 15 degrees east of it, and consequently there will be an hour’s difference in the time, either fast or slow, for every 15 degrees, depending on whether you count east or west from the noon meridian.
The purpose of a sextant in finding longitude on shipboard is to know when it is exactly noon by the Sun, and in this way the local time is found. The purpose of a chronometer is to carry exact Greenwich time, and the difference between the local time found each day by taking the Sun, and Greenwich time shown by the chronometer gives the distance in degrees the ship has traveled from Greenwich.
By knowing the latitude and longitude of a ship the distance in miles north and south and east and west from any port can be figured out without much trouble.