Water on the Earth’s Surface
As the Earth and the other planets of the solar system cooled, the Sun’s energy gradually replaced the heat of planet formation in maintaining their surface temperatures. The Earth developed a nearly circular orbital path at a mean distance of 149 million kilometers (149 X 106 km), or 93 million miles (93 X 106 mi) from the Sun. (If you are unfamiliar with scientific notation to express very large numbers, see appendix a.)
Moving along this orbit the Earth is 152.2 x 106 km (94.5 x 106 mi) from the Sun in June and 148.5 x 106 кm (92.2 X 106 mi) away in December, as shown in figure 1.5. At these distances from the Sun, the Earth’s orbit keeps the annual heating and cooling cycle within moderate limits. The Earth’s mean surface temperature is about 16°C, which allows water to exist as a gas, as a liquid, and as a solid.
Fig. 1.5. The Earth’s yearly season
The rotation of the Earth on its axis is also important in moderating temperature extremes. The Earth completes one rotation, turning from west to east, in twenty-four hours. If the Earth rotated more slowly, the side of the Earth toward the Sun would be exposed to the Sun’s energy for a longer period than it is at present and would become very hot, while the side in darkness would lose heat and become very cold. Temperature changes from day to night would be large. By contrast, a shorter period of rotation would decrease the present variation from day to night.
The Earth’s solar orbit, its rotation, and its blanket of atmospheric gases produce surface temperatures that allow the existence of liquid water. The atmosphere covering the Earth’s surface acts as a protective shield between the Earth and the Sun. Without it, solar heating would evaporate water at a much higher rate. Compare the Earth’s distance to the Sun, its period of rotation, and the time required for the Earth to complete one orbit of the Sun with those of other planets, as shown in table 1.1. Note the surface temperature of the Earth as compared with those of other planets.
The amount of water on the Earth’s surface can be expressed in several ways. For example, the oceans cover 361 million square kilometers (361 X 106 km2), or 139 million square miles (139 X 106 mi2). Because these numbers are so large, they do not convey a clear idea of size; therefore, an easier concept is to remember that 71% of the Earth’s surface is covered by the oceans, and only 29% of the surface area is land above sea level.
The volume of water in the oceans is enormous: 1.37 billion cubic kilometers (1.37 X 109 km3, or 0.328 X 109 mi3). Another way to express the oceanic volume is to think of a smooth sphere with exactly the same surface area as the Earth (510 X 106 km2, or 197 X 106 mi2), uni' formly covered with the water from the Earth’s oceans.
The ocean water would be 2686 m (8800 ft) deep, a depth of about 1.7 miles. If the water from all other sources in the world were added, the depth would rise 56 m to 2742 m (9000 ft). When water volumes are considered as depths over a smooth sphere, they are referred to as sphere depths. The ocean sphere depth is 2686 m, and the total water sphere depth of all the Earth’s water is 2742 m. This information is summarized in the last column of table 1.3.
Date added: 2023-11-08; views: 258;