8. Spring tides and neap tides.—The pull of the sun also makes tides, and when the sun and moon pull together (position A in Fig. B), tides are higher than common and are called spring tides. When they pull against each other (position B in Fig. B) the tides are lower than common and are called neap tides.
9. Cotidal lines.—In the open sea the high-tide wave runs around the world following the moon. When it strikes the shores its direction and speed are often changed by the shape of the land. The twelve o'clock line (XII NOON) is close to the entrance of New York Bay and of Long Island Sound. (Fig. C.) See how islands check its speed between one o'clock and four, especially in the bay at the eastern end of Long Island.
Notice that the East River, between Man hattan Island (compare Fig. 278) and Long Island, has tides running in from both the north and the south, and that the Sound tide (IV), gets there about two hours after the New York Bay tide (II P. M.).
10. Height of tides.—The height of the tide depends upon the shape of the bay.
Bays with narrow mouths have low tides and bays with wide mouths have higher tides. The Arabic figures (Fig. C) show the 1 o'clock tide at the end of Long Island Sound to be only 1.9 feet. How high is it near the west end of the sound? Low tide is called ebb tide.
In East River, the tide from Long Island Sound is so much higher than the tide from New York Bay, that water rushes through into New York Bay with a swift current (tidal race), so dangerous to ships that the early sailors called it Hell Gate.
We measure elevations on land from sea is half way between high and low tide.
11. Latitude and longitude.—Suppose you had a nice new baseball and knew that there was a nugget of gold hidden just beneath the cover in such a way that it could not be felt or seen. How would you tell any one just where it was? Now suppose you took a spot on the ball and called it north pole, and another exactly on the other side and called it south pole. Then suppose you drew a line from pole to pole and called it the first or prime meridian, and then drew another line clear around the world half way between the poles, and called it equator. Now you could locate your nugget by saying that it was a certain distance on the north pole (north) side of the equator and a certain distance on the left (west) or right (east) side of the prime meridian. (Fig. D.)
That is the way geographers locate places on the earth's surface. Instead of saying a south of the equator in degrees of latitude. For more exact locations each degree is divided into 60 minutes and each minute into 60 seconds.
Degrees differ greatly in length. You can quickly see that th of the distance around the earth at the equator, where it is large, will be much more than of the distance around the earth near the pole. For exact lengths of degrees of longitude at different distances from the equator, see the table at the bottom of the page.
12. The shape of the earth.—We say the earth is round like a ball; but it is not exactly round. Actually, it is slightly flat tened because it spins so fast. The other planets are also flattened a little for the same reason. The earth turns around an imaginary central line called the axis, of which one end is the North Pole and the other end is the South Pole. It is so nearly round that the polar diameter (distance through the earth from pole to pole) is 7899.4 miles, while the equatorial diameter is 7926.7.
13. Keeping time.—Fig. D also shows us something about how we count time. The earth rotates on its axis from west to east, and every time it goes around once we call it a day, divided into 24 hours. In Fig. D we see at the right the rays of the sun, which are always lighting up one-half of the earth and leaving the other half in shadow. In this figure the Greenwich meridian is the shadow line, or sunrise line. Therefore it is sunrise at Greenwich, and since the sun shines on both poles we know (Fig. F) that it must be March 21st or September 21st.
How fast does sunrise travel? In 24 hours it goes round the world, 360 degrees. In one hour it therefore goes 15 degrees. How long does it take to go one degree? 14. Standard time.—How shall we keep time on this whirling globe where the sunrise -place is so many miles north or south of the equator, or east or west of the prime merid ian, they use a fraction of the distance around the earth. For a long time mathe maticians have divided circles into 360 equal parts called degrees. Thus a quarter of a pie is 90 degrees along the edge of the crust, and a quarter of the way around the world is 90 degrees.