Page:Popular Science Monthly Volume 88.djvu/460

 How to Ascertain Your Latitude and

Longitude

By Hereward Carrington

��THERE is a very simple way by means of which the novice, un- trained in astronomical observation, can determine his latitude, without the aid of complicated and expensive apparatus.

If you were situated on the equator, the north star would be directly north of you. This star must be learned and iden- tified, so that it can be picked out any- where, at a moment's notice. This is all the astronomy you need know — as the location of this star will give the latitude.

When half-way to the north pole the north star is midway between the zenith and the northern horizon. At the pole it is directly overhead. In all other places its "angle" varies, being for example, 30° at New Orleans, 40° in Philadelphia, and so on. The altitude of the north star is the latitude of a place north of the equa- tor. All that is necessary then, to deter-

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��mine the latitude, is to measure the angle of the north star and thus determine the altitude of the celestial pole. This will give the latitude.

Take a pair of ordinary compasses. Open them, and place one point in a level window sill, holding the arm upright. Now point the other arm of the compass at the north polar star. The angle thus formed by the pair of compasses will be fairly accurate, provided the pointing has been done carefully and the other arm is held at right angles to the sill.

When the compasses have been ad- justed, as explained, proceed to measure the angle formed by the arms of the compass. This will indicate your lati- tude. For every degree of curvature of the earth, the north star rises one degree from the horizon. It is thus an easy mat- ter to see your latitude, from the number of degrees made by the angle of your compass.

Another way to discover the latitude of any given place — and a method much more often used — is by means of the sun. Observations of the sun are depended upon by vessels at sea.

The first thing to do is to ascertain what is known as your true north-south line. To do this you must know your longitude and have the correct time. Next, measure the altitude of the sun at apparent noon — that is, when its shadow is north. Place a curved piece of card- hoard in the window, as shown in the diagram, with the blind drawn down to the wood of the upper window. The angle made by the shadow will then in- dicate the altitude of the sun with suffi- cient accuracy.

Next, consult what is known as the "Analemma" (see diagram). If you live in the northern hemisphere, you must subtract from the declination of the sun (which the analemma gives you) the sun's declination. Subtract this result from 90°, and the remainder is your lati- tude.

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