Physical Science 8th grade (1).pdf

The moon earths single rocky moon is about one

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The moon Earth’s single rocky moon is about one-quarter the diameter of Earth. At a distance of 385,000 kilometers, the moon is about 30 Earth- diameters away from the planet, completing one orbit every 29 days. The seasons Earth’s orbit is within 2 percent of a perfect circle. The seasons are caused by the 23-degree tilt of Earth’s axis of rotation relative to its orbit. When Earth is on one side of the sun, the northern hemisphere receives a greater intensity of sunlight because the sun passes nearly straight overhead once per day, making it summer. Six months later, on the opposite side of Earth’s orbit, the northern hemisphere tilts away from the sun. This spreads the sunlight over a larger surface area. The lower intensity of sunlight each day makes for winter. Figure 15.11: Earth is the only planet not named after a Roman god. Its name comes from Old English “oerthe,” meaning land or country. Earth at a glance Type: Rocky Moons: one Distance from sun: 1 AU Diameter: 12,800 km Surface gravity: 9.8 N/kg Avg. surface temp.:10°C Atmosphere: dense, N 2 , O 2 Length of day: 24 hours Length of year: 365.25 days
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324 U NIT 6 A STRONOMY Figure 15.12: Mars was named after the Roman god of war. (ESA photo) Type: Rocky Moons: 2 Distance from sun: 1.5 AU Diameter: 0.53 of Earth Surface gravity: 38% of Earth Avg. surface temp.: -50°C Atmosphere: thin, CO 2 Length of day: 24.6 hours Length of year: 687 Earth days Shortest flight to Earth: 2.7 AU Travel time from. Earth: 3 1/2 mo. Mars Mars The fourth planet out from the sun, Mars appears as a reddish point of light in the night sky. Mars is a relatively small rocky planet with a mass only 11 percent the mass of Earth. Mars has two tiny, irregular-shaped moons named Deimos and Phobos. Both are much smaller than Earth’s moon and are more like asteroids. The surface of Mars The surface of Mars has deserts, huge valleys, craters, and volcanic mountains even larger than those on Earth. However, Mars’s “air” is mostly carbon dioxide and less than 1 percent the density of Earth’s atmosphere. Like Earth, Mars has polar ice caps, but they are made of a combination of water and frozen carbon dioxide. Because of the thin atmosphere and the planet’s distance from the sun, temperatures are below 0°C most of the time. Because it is tilted like Earth, Mars also has seasons. A day on Mars (24.6 hours) is similar in length to an Earth day. But Mars’s larger orbit makes a Martian year (687 days) almost twice as long as an Earth year. Mars was different in the past Mars is cold and dry today, but there is strong evidence that Mars was much wetter and had a thicker atmosphere in the past. Aerial photos of the Martian surface show erosion and patterns of riverbeds similar to those formed by flowing water on Earth. Even today, there is evidence of water beneath the Martian surface. Several robot space probes have landed on Mars searching for life but the results have been inconclusive. As Earth’s nearest match in climate, Mars will probably be the first planet in the solar system to be explored by humans.
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325 15.2 T HE P LANETS C HAPTER
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