Man's physical universe

xanabras

miles.

THE SUN 87

One such prominence, as these tongues are called, was observed

to shoot out of the chromosphere at the rate of 60,000 miles per hour.

Although the volume of the sun is more than a million times that of

the earth, its relative mass is much less, because the density ^ of the

earth is about four times that of the sun.

Stinspots Are the Cooler Portions of the Sun's Surface.

The surface of the sun is mottled with dark spots, ranging from 500

to 150,000 miles in diameter. These sunspots represent relatively cool

areas, whose temperature is about 4000° C. as compared with the

6000° C. which is the temperature

of the hottest outer

portions.

It is thought that the sunspots

are areas of lowered

temperature resulting from

tremendous solar cyclones,

which increase to a maximum

and then for some unknown

reason decrease periodically

every eleven years. Studies

of annual tree rings show that

they vary in a similar elevenyear

cycle. When these solar

disturbances are at their maximum,

magnetic disturbances

occur on the earth that interfere

seriously with telegraphic,

telephonic, and radio communication.

The aurorae are

also more brilliant at such

Fig. 19. Solar disk with many spots of

unusual size, November 30, 1929. (Photograph

from the Yerkes Observatory, reprinted

by permission of the Chicago University

Press.)

times. These disturbances on the earth may be due to increased

streams of electrons and ions or greater ultraviolet radiations coming

from the sun, as a result of changing solar conditions.

George E. Hale proved that sunSpots behave like huge magnets.

The direction of the whirl of the sunspots in each succeeding cycle is

reversed, thus reversing the magnetic fields.

A great magnetic storm occurred in April, 1938, in which energy was

expended at the rate of two billion kilowatts for a two-hour interval,

• Density is the mass of a unit volume of a substance. For the present you may consider

that mass means about what is commonly meant by weight. A more exact usage of these

terms will be given later. Density may therefore be considered to be weight of a unit

volume.

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