The Earth, Moon and Sun Though the sun is an average of 93 million miles away from the Earth, we have seen how it has a tremendous affect on life here on planet earth. The massive gravitational pull of the sun keeps the earth and the planets of the solar system revolving around it. The earth itself rotates around on its own axis as well, driving the cycle of day and night. The sun's radiant energy warms both the earth and atmosphere, driving the water cycle and creating the weather on our planet. We will study how the sun interacts with the earth creating the day, night, the seasons and year. As you know, the sun rises in the east and sets in the west. Of course the sun is not revolving around the earth, even though it appears to do so. In the past people did perceive the earth as the center of the universe. Claudius Ptolemy formalized this explanation in the second century A.D. The Ptolemy system or the geocentric system placed the earth at the center of the universe. The celestial bodies were thought to revolve around us. During the Renaissance, Nicolaus Copernicus saw through this deception. Later Galileo advocated for the Copernicus model and supported a world system called heliocentricity, meaning the sun is "at the center" of our galaxy. By careful observation of the moon, stars, planets and our sun, astronomers were able to deduce that we on earth revolved around our sun. The stars and our sun appear to revolve around the earth due to the earth's rotation. The earth rotates in a clock-wise direction. The eastern portion of the United States encounters sunlight prior to those states farther west. The sun appears to rise over the horizon as that portion of the earth turns toward the sun. Thus, our day and night is governed by the rotation of the earth. The earth rotates around its axis, spinning at over 1,000 miles per hour at the equator and down to zero miles per hour at the poles. The earth is also tilted on its axis; the angle created by this tilt gives the world its changing seasons. The angle of the sun and the amount the sun shines over a geographic area varies due to this angle. Winters in the northern hemisphere are always colder than the summers. The earth is not lined up perpendicular to the sun; it actually tilts at 23.5 degrees from the vertical. This creates the temperature changes we experience season to season. One theory about how the earth came to be tilted has to do with the earth's moon. A Mars-sized planet collided with the earth early after it formed. It is believed that this collision not only produced the moon but also the degree of tilt seen on the earth today. For one day the sun shines directly down on the 23.5 degree latitude. On June 21 the sun shines down on the latitude called the Tropic of Cancer. This is called the spring solstice. On this day all the locales in the northern hemisphere receive the longest amount of sunlight and the most direct sunlight This is the beginning of summer. You will notice there is a lag between the longest day of the year and the warmest day of the year. Normally July has the highest average temperatures. Just as noon is not the warmest part of a day, and sunset is not the coldest. The warmest portion of the day occurs around 3-4 in the afternoon. The radiant light is absorbed and then reradiated into the atmosphere, accounting for the time lag. It is similar with the summer and winter temperatures. The coldest portion of the year in the northern hemisphere is not on December 21 but is in January and February. The coldest portion of a day is just prior to sunrise, due to the earth losing its heat to the atmosphere all night. The opposite seasonal changes are experienced in the southern hemisphere. The beginning of summer in the southern hemisphere occurs on December 21 when the sun is directly over the Tropic of Capricorn During the six months between the summer solstice and the winter solstice, the sun moves north or south relative to the earth. The angle in which the sun's light hits the earth, therefore, varies as it moves. The steeper the angle, the more radiant energy reaches a geographic location. The length of day is also affected, which means that there are more hours of sunlight during the summer in the northern hemisphere and less hours of sunlight in the winter months. Shorter days and less direct light lead to cooler average temperatures as you move northward. The earth rotates on an angle at 1,000 miles per hour but it also revolves around the sun at a staggering 670,000 miles per hour. It takes approximately 365.25 days to revolve around the sun, or one calendar year. In review, the sun dictates the length of our daylight, alternating day and night every 24 hours. The angle of the earth to the sun dictates the amount of energy received by different geographic areas; thus, controlling our changing seasons. And lastly, the earth revolves around the sun once every year.
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