The EarthPower and Light Compared to the rest of the universe, the Earth is very small. Our planet and seven others orbit the Sun, which is only one of about 200 billion stars in our galaxy. Our galaxy, the Milky Way, is part of the universe, which includes millions of other galaxies and their stars and planets. By comparison, the Earth is microscopic. Compared to a person, on the other hand, the Earth is enormous. It has a diameter of 7,926 miles ( 12,756 kilometers) at the equator, and it has a mass of about 6 ~ 1024 kilograms. The Earth orbits the Sun at a speed of about 66,638 miles per hour (29.79 kilometers per second). Don’t dwell on those numbers too long, though; to a lot of people, the Earth is inconceivably, mind-bogglingly big. And it’s just a fraction of the size of the Sun. From our perspective on Earth, the Sun looks very small. This is because it’s about 93 million miles away from us. The Sun’s diameter at its equator is about 100 times bigger than Earth’s, and about a million Earths could fit inside the Sun. The Sun is inconceivably, mind-bogglingly bigger. But without the Sun, the Earth could not exist. In a sense, the Earth is a giant machine, full of moving parts and complex systems. All those systems need power, and that power comes from the Sun. The Sun is an enormous nuclear power source--through complex reactions, it transforms hydrogen into helium, releasing light and heat. Because of these reactions, every square meter of our planet’s surface gets about 342 Watts of energy fi’om the Sun every year. This is about 1.7 x 1017 Watts total, or as much as 1.7 billion large power plants could generate. You can learn about how the Sun creates energy in How the Sun Works. When this energy reaches the Earth, it provides power for a variety of reactions, cycles and systems. It drives the circulation of the atmosphere and the oceans. It makes food for plants, which many people and animals eat. Life on Earth could not exist without the Sun, and the planet itself would not have developed without it. To a casual observer, the Sun’s most visible contributions to life are light, heat and weather. Night and Day Some of the Sun’s biggest impacts on our planet are also its most obvious. As the Earth spins on its axis, parts of the planet are in the Sun while others are in the shade. In other words, the Sun appears to rise and set. The parts of the world that are in daylight get warmer while the parts that are dark gradually lose the heat they absorbed during the day. You can get a sense of how much the Sun affects the Earth’s temperature by standing outside on a partly cloudy day. When the Sun is behind a cloud, you feel noticeably cooler than when it isn’t. The surface of our planet absorbs this heat from the Sun and emits it the same way that pavement continues to give off heat in the summer after the Sun goes down. Our atmosphere does the same thing--it absorbs the heat that the ground emits and sends some of it back to the Earth. The Earth’s relationship with the Sun also creates seasons. The Earth’s axis tips a little―about 23.5 degrees. One hemisphere points toward the Sun as the other points away. The hemisphere that points toward the Sun is wanner and gets more light―it’s summer there, and in the other hemisphere it’s winter. This effect is less dramatic near the equator than at me poles, since the equator receives about the same amount of sunlight all year. The poles, on the other hand, receive no sunlight at all during their winter months, which is part of the reason why they’re frozen. Most people are so used to the differences between night and day (or summer and winter) that they take them for granted. But thcse changes in light and temperature have an enormous impact on other systems on our planet. One is the circulation of air through our atmosphere. For example: The Sun shines brightly over the equator. The air gets very warm because the equator faces the Sun directly and because the ozone layer is thinner there. As the air warms, it begins to rise, creating a low pressure system. The higher it rises, the more the air cools. Water condenses as the air cools, creating clouds and rainfall. The air dries out as the rain falls. The result is warm, dry air, relatively high in our atmosphere. Because of the lower air pressure, air rushes toward the equator from the north and south. As it warms, it rises, pushing the dry air away to the north and the south. The dry air sinks as it cools, creating high-pressure areas and deserts to the north and south of the equator. This is just one piece of how the Sun circulates air around the world―ocean currents, weather patterns and other factors also play a part. But in general, air moves from high-pressure to low-pressure areas, much the way that high-pressure air rushes from the mouth of an inflated balloon when you let go. Heat also generally moves from the warmer equator to the cooler poles. Imagine a warm drink sitting on your desk--the air around the drink gets warmer as the drink gets colder. This happens on Earth on an enormous scale. The Coriolis Effect, a product of the Earth’s rotation, affects this system as well. It causes large weather systems, like hurricanes, to rotate. It helps create westward-running trade winds near the equator and eastward-running jet streams in the northern and southern hemispheres. These wind patterns move moisture and air from one place to another, creating weather patterns. (The Coriolis Effect works on a large scale--it doesn’t really affect the water draining from the sink like some people suppose. ) The Sun gets much of the credit for creating both wind and rain. When the Sun warms air in a specific location, that air rises, creating an area of low pressure. More air rushes in from sunounding areas to fill the void, creating wind. Without the Sun, there wouldn’t be wind. There also might not be breathable air at all. Water and Fire The Sun has a huge effect on our water. It warms the oceans around the tropics, and its absence cools the water around the poles. Because of this, ocean currents move large amounts of warm and cold water, drastically affecting the weather and climate around the world. The Sun also drives the water cycle, which moves about 18,757 cubic miles (495,000 cubic kilometers ) of water vapor through the atmosphere every year. If you’ve ever gotten out of a swimming pool on a hot day and realized a few minutes later that you were dry again, y.ou have firsthand experience with evaporation. If you’ve seen water form on the side of a cold drink, you’ve seen condensation in action. These are primary components of the water cycle, also called the hydrologic cycle, which exchanges moisture between bodies of water and land masses. The water cycle is responsible for clouds and rain as well as our supply of drinking water.What does our atmosphere do A．It absorbs the heat from the ground and sends it to the Sun.B．It absorbs the heat from the Sun and emits it back to the Earth.C．It absorbs the heat from the Sun and emits it the same way.D．It absorbs the heat the ground emits and sends some of it back to the Earth.