chapter 7 The Earth-Moon-Sun System section 2 Time and Seasons What You ll Learn how to calculate time and date in different time zones how to distinguish rotation and revolution what causes seasons Before You Read Have you ever had or heard of jet lag? People get tired when they travel long distances to different time zones. Why do you think people get jet lag? Study Coach Summarize As you read each part of this section, write a short summary of its main points. B Ask Questions Make the following Foldable to help you organize the information about how Earth s movements affect time and the seasons. Time Seasons Read to Learn Measuring Time on Earth The position of the Sun in the sky can help you determine the approximate time of day. When the Sun reaches its highest point in the sky overhead, it is around noon. People have long used the movements of the Earth, the Moon, and the Sun to measure time. Around 3,000 B.C., the Babylonians developed a time-keeping method. They saw that the Sun seemed to take a circular path around Earth. So, based on their number system of 60, they divided this circle into 360 parts. The parts are called degrees ( is the degree symbol). How does Earth s turning affect time measurement? Earth makes one complete turn on its axis in about 24 hours. This movement is used to keep track of time. The turning Earth makes the Sun appear to move from east to west across the sky. The time from noon one day until noon the next day is 24 hours. Because Earth spins 360 in 24 hours, it spins 15 every hour. This led people to set up time zones. A time zone is a 15 -wide area of Earth where the time is the same. Ideally, time zones should all be equal in size. However, in places around cities or country borders, the time zone line is adjusted for convenience. 104 CHAPTER 7 The Earth-Moon-Sun System
What is the date line? There is one problem with dropping back one hour for each time zone. Eventually, you would come around to your starting point and it would be 24 hours earlier. It cannot be two different days at the same spot. To solve this problem, a day is added to the time at a place called the International Date Line. If it is Monday to the east of the line, then it is Tuesday to the west. For convenience, the International Date Line is drawn through a largely empty region of the Pacific Ocean, but directly opposite the Prime Meridian. The Prime Meridian is the imaginary line that passes through Greenwich, England. Time based on these lines is called Coordinated Universal Time (UTC). In some areas, time is modified in summer to have more hours of evening sunlight. This modification of time is called Daylight Savings Time. How is Earth s rotation used to measure days? Earth turning on its axis allows people to measure the hours of the day. Rotation is the spinning of Earth on its axis, which is an imaginary line drawn through Earth from its rotational north pole to its rotational south pole, as shown in the figure below. As Earth rotates, the Sun appears to rise in the east and set in the west. 1. Infer If it is Saturday west of the date line, what day is it east of the date line? Axis Rotation The rotation of Earth causes this apparent movement of the Sun. The period from noon one day until noon the next day is called a solar day. A solar day is a bit longer than the time it takes Earth to rotate on its axis. This difference is due to Earth s changing orbit around the Sun, which makes the Sun appear to move slightly east each day. If, instead of using the Sun, you measured a day by the time a certain star rises above the horizon until it rises again, the day would be a bit shorter. This is called a sidereal day. It is a more accurate measure of the time it takes Earth to rotate on its axis. Picture This 2. Draw and Label Draw a vertical axis through Earth. Label the angular difference between Earth s current axis and vertical. Reading Essentials 105
3. Identify By what motion of Earth do people measure years? 4. Predict How might Earth s seasons be different if Earth s axis were not tilted 23.5 from vertical? How does Earth s revolution measure years? The motion of Earth around the Sun allows people to measure years. Revolution is the motion of Earth in its orbit around the Sun. As Earth revolves around the Sun, the Sun appears to move through the sky compared with the seemingly fixed position of the stars. The time it takes the Sun to make one complete trip through the sky in relation to the stars is the same amount of time it takes Earth to complete one trip around the Sun, or one sidereal year. The apparent path of the Sun during this year is called the ecliptic. Actually, the ecliptic is the plane of Earth s orbit around the Sun. The 12 constellations (star patterns) through which we see the Sun moving through this year is called the zodiac. Why do seasons change? Recall that Earth s orbit around the Sun is an ellipse. Because of this, Earth is closer to the Sun at one time than it is at other times. However, the seasons on Earth are not caused just by the shape of Earth s path around the Sun. Seasonal changes are caused by three factors. These factors are the Earth s rotation, its revolution, and the tilt of its axis. Seasons change because the number of daylight hours varies and because sunlight strikes Earth s surface at different angles at different times of the year. Earth s axis is tilted 23.5 from a line drawn perpendicular to the plane of its orbit, or ecliptic. Because of this tilt, Earth s north geographic pole points toward Polaris (the North Star) throughout the year. This tilt helps cause Earth s seasons. How does the angle of sunlight change? In the summer, the Sun is high in the sky and sunlight hits Earth s surface at a high angle. As the year progresses, the Sun sits lower in the sky and the angle of sunlight is also lower. Sunlight striking Earth at a high angle, close to 90, is intense. Sunlight striking at this higher angle heats the surface more than sunlight striking at a lower angle. Since Earth remains tilted in the same direction as it revolves, different hemispheres are tilted toward the Sun at different times of the year. The figure at the top of the next page shows the angles of light that hit different parts of Earth. 106 CHAPTER 7 The Earth-Moon-Sun System
Tropic of Cancer Equator Tropic of Capricorn Arctic Circle N High angle More intense covers less area Picture This 5. Analyze Which part of Earth consistently gets relatively intense and direct sunlight? Antarctic Circle S Low angle Less intense covers more area Why are summer days longer? During the summer, the Sun is above the horizon for more hours each day than at other times of year. As the year progresses, the Sun is above the horizon for fewer hours. Around December 21 in the northern hemisphere, the Sun is above the horizon for the fewest hours. In the northern hemisphere, the Sun is above the horizon for the most hours around June 21. Summers, then, are warmer than winters because in summer the sunlight is more intense and the days are longer. What are equinoxes and solstices? Because Earth s axis is tilted, the Sun s position relative to the equator constantly changes. For most of the year, the Sun is north or south of the equator. But twice a year the Sun is directly over the equator. This is called an equinox. An equinox occurs when the Sun is directly over the equator and the number of daylight hours equals the number of nighttime hours. During the equinox, neither the northern nor southern hemisphere is tilted toward the Sun. In the northern hemisphere, the spring equinox is on March 20 or 21, and the fall equinox is on September 22 or 23. The opposite is true for the southern hemisphere. The solstice is the point at which the Sun reaches its greatest distance north or south of the equator. In the northern hemisphere, the summer solstice occurs on June 21 or 22 and the winter solstice is on December 21 or 22. In the southern hemisphere, the solstices are reversed. At summer solstice, there are more hours of daylight than during any other day of the year. During the winter solstice, there are more nighttime hours than during any other day of the year. 6. Identify What is an equinox? Reading Essentials 107
After You Read Mini Glossary ecliptic: the yearly path of Earth around the Sun equinox: occurs when the Sun is directly over the equator and the number of daylight hours equals the number of nighttime hours revolution: the motion of Earth in its orbit around the Sun rotation: the spinning of Earth on its axis, which is an imaginary line drawn through Earth from its rotational north pole to its rotational south pole solstice: the point at which the Sun reaches its greatest distance north or south of the equator time zone: a 15 -wide area of Earth where the time is the same 1. Explain how the tilt of Earth s axis affects the equinox and the solstice. 2. Fill in the blanks. One rotation equals Earth s tilted axis One revolution equals More intense heating during the Unequal heating of surface by Sun causes Day and night are equal during the 3. You summarized the parts of this section. How did summarizing help you understand the information in this section? Visit gpescience.com to access your textbook, interactive games, and projects to help you learn more about Earth and the Sun. End of Section 108 CHAPTER 7 The Earth-Moon-Sun System