EARTH AND NATURAL HISTORY ORIENTATION 



able to breathe the Martian atmosphere for very 

 brief periods of time; with warm clothing and a sim- 

 ple breathing apparatus he could probably live on 

 Mars. 



THE MOON 



Although it cannot sustain complex life, some of 

 the moon's physical properties are of interest. Its 

 distance from the earth varies between 222,000 and 

 253,000 miles during a single revolution about the 

 earth. This revolution requires 27 days, 7 hours, 43 

 minutes, and 1 1.5 seconds. Because the' same surface 

 of the moon always faces the earth, the rotation speed 

 is equal to the revolution speed. The moon has a 

 diameter of 2162 miles and a mass that is 1/100 that 

 of the earth; its surface gravity is 16/100 that of the 

 earth. There is essentially no atmosphere on the 

 moon, so the surface temperature fluctuates greatly; 

 in sunlight it is about 214°F. and in shade about 

 — 243°F. The escape velocity is 1.5 miles per second 

 — considerably less than that at the earth's surface 

 (see Table 1.1). The phases of the moon result from 

 our view of sunlight striking the surface of the moon. 



EARTH AND SUN 



Most of the physical aspects of our planet, con- 

 sidered on a planetary scale, are influenced by the 

 sun. Primary influences of the sun on the earth in- 

 clude the seasons, solar radiation, earth's magnetism, 

 and radiation belts. 



EARTH'S SEASONS 



Our seasons are due to a simple association be- 

 tween the sun and the earth (Figure 1.4). Specifi- 

 cally, there are seasons because the axis of the earth 

 is tilted. This constant direction of tilt causes the 

 sun's rays in our latitude to be more direct and more 

 effective in warming the earth during the summer, 

 and oblique to our part of the earth at other seasons 

 of the year (see Figure 2.2, p. 12). Solar rays in our 

 latitude are least direct during the winter. 



SOLAR RADIATION 



The sun is a gaseous body whose surface has a tem- 

 perature of about 11,000°F. Above the surface, two 



Figure 1.4 Earth's seasons. The constant orientation of Earth's axis 

 in space during its movement obout the sun causes an annual cycle of 

 change in the angle of solar rays striking the planet, a, vernal equinox, 

 March 21; b, summer solstice, June 21; c, autumnal equinox, Septem- 

 ber 23; d, winter solstice, December 22. 



layers compose the solar atmosphere. It is the atmos- 

 phere of the sun that produces solar radiations. 



Although severe solar storms are unusual, period- 

 ically (about every eleven years) especially wild 

 storms occur. At such times, so-called fiery promi- 

 nences of 30,000° F. leap from the surface in the form 

 of arches, hedges, jets, or loops. Concurrently, rela- 

 tively low-temperatured (7000°F.) sunspots appear. 

 The sunspots are surrounded by masses of flaming 

 hydrogen (the \ 8,000° F. plages), other violent disturb- 

 ances (the 30,000° F flares), and certain localized 

 areas with temperatures of one million degrees. The 

 chief significance of these solar storms to earth life is 

 that they release X rays and various charged particles 

 in the form of solar radiation. 



Normal solar radiation from the surface and atmos- 

 phere of the sun has various effects upon the earth. 

 The sun's surface gives rise to visible light, infrared 

 rays, and a complex of ultraviolet rays. When visible 

 light is intercepted by water droplets, rainbows ap- 

 pear in the sky. Infrared rays produce earth heat and 

 storm clouds. Ultraviolet rays create the upper two 

 layers of our three-layered ionosphere. (The iono- 

 sphere is 40 to 220 miles up and is the last completely 

 atmospheric zone in the atmosphere; see Chapter 2). 

 Ultraviolet light, which causes sunburn, would be 

 fatal to man and many other animals if most rays 

 were not absorbed by the atmospheric ozone (a par- 

 ticular combination of three oxygen atoms) about 16 



