1. Introduction to Astronomy March 24, 2009

2. Days and Years The ancient Egyptians were among the first people to study the stars. The ancient Egyptians were among the first people to study the stars. The study of the moon, stars and other objects in space is called astronomy. The study of the moon, stars and other objects in space is called astronomy. Ancient astronomers also studied the movements of the sun and the moon as they appeared to travel across the sky. Ancient astronomers also studied the movements of the sun and the moon as they appeared to travel across the sky.

3. Days and Years It seemed to earlier astronomers that the Earth was standing still and the sun and the moon were moving. It seemed to earlier astronomers that the Earth was standing still and the sun and the moon were moving. Actually, the sun and moon seem to move across the sky each day mainly because Earth is rotating on its axis. Actually, the sun and moon seem to move across the sky each day mainly because Earth is rotating on its axis.

4. Rotation The imaginary line that passes through Earth’s center and the North and South Poles is called Earth’s axis. The imaginary line that passes through Earth’s center and the North and South Poles is called Earth’s axis. The spinning of the Earth on this axis is called rotation. A point on the equator rotates at about 1,600 kilometers per hour. Even most commercial jet planes can’t fly this fast! The spinning of the Earth on this axis is called rotation. A point on the equator rotates at about 1,600 kilometers per hour. Even most commercial jet planes can’t fly this fast!

5. Day and Night KNOW THIS! KNOW THIS! Earth’s rotation on its axis causes day and night. Earth’s rotation on its axis causes day and night. As Earth rotates eastward, the sun appears to move westward across the sky. As Earth rotates eastward, the sun appears to move westward across the sky. It is day on the side of Earth facing the sun. It is day on the side of Earth facing the sun. As Earth continues to turn to the east, the sun appears to set in the west, so it is night there. As Earth continues to turn to the east, the sun appears to set in the west, so it is night there.

6. Rotation To rotate around the axis, Earth takes about 24 hours to do so. To rotate around the axis, Earth takes about 24 hours to do so. As you know, each 24-hour time is a day. As you know, each 24-hour time is a day.

7. Revolution In addition to rotating on its axis, Earth travels around the sun. The movement of one object around another object is called revolution. In addition to rotating on its axis, Earth travels around the sun. The movement of one object around another object is called revolution. One complete revolution around the sun is called a year. One complete revolution around the sun is called a year. Earth’s path as it revolves around the sun is called its orbit. Earth’s path as it revolves around the sun is called its orbit.

8. Check Point Why do the sun and moon seem to move each day? Why do the sun and moon seem to move each day?

9. Calendars The Egyptian astronomers counted the number of days between each first appearance of the star Sirius in the morning sky. The Egyptian astronomers counted the number of days between each first appearance of the star Sirius in the morning sky. In this way, they found that there were about 365 days in each year. By dividing the year into 365 days, the ancient Egyptians had created one of the first calendars. In this way, they found that there were about 365 days in each year. By dividing the year into 365 days, the ancient Egyptians had created one of the first calendars.

10. Earth’s Orbit Earth’s orbit around the sun actually takes slightly more than 365 days – about 365 ¼ days. Earth’s orbit around the sun actually takes slightly more than 365 days – about 365 ¼ days. Four years of about 365 ¼ days each can be approximated by taking 3 years of 365 days and a fourth year of 366 days. You know this fourth year is a leap year. Four years of about 365 ¼ days each can be approximated by taking 3 years of 365 days and a fourth year of 366 days. You know this fourth year is a leap year.

11. “Moonth” Dividing the year into smaller parts was difficult also. Early people used moon cycles as a sort of calendar. The time between one full moon and the next one is about 29.5 days. Dividing the year into smaller parts was difficult also. Early people used moon cycles as a sort of calendar. The time between one full moon and the next one is about 29.5 days. A year of 12 “moonths” only adds up to 354 days. A year of 12 “moonths” only adds up to 354 days.

12. “Moonth” The ancient Egyptians worked out a calendar that had 12 months of 30 days each, with an extra 5 days that were not part of any month. The ancient Egyptians worked out a calendar that had 12 months of 30 days each, with an extra 5 days that were not part of any month. The Romans borrowed this calendar and made changes to it. With more changes, it eventually became the calendar we know: 11 months having 30 or 31 days, plus one month (February) having 28 or 29 days. The Romans borrowed this calendar and made changes to it. With more changes, it eventually became the calendar we know: 11 months having 30 or 31 days, plus one month (February) having 28 or 29 days.

13. Seasons on Earth Most places outside of the tropics have four distinct seasons: winter, spring, summer, and autumn (fall). There are great differences in temperature from place to place. Most places outside of the tropics have four distinct seasons: winter, spring, summer, and autumn (fall). There are great differences in temperature from place to place.

14. Seasons on Earth Remember how sunlight hits the Earth. Remember how sunlight hits the Earth. Earth’s tilted axis is the cause of the seasons on this planet. Earth’s tilted axis is the cause of the seasons on this planet. Latitude: a measurement of distance from the equator, expressed in degrees north or south. Latitude: a measurement of distance from the equator, expressed in degrees north or south. The equator has latitude 0 degrees and the North Pole has latitude 90 degrees north. The equator has latitude 0 degrees and the North Pole has latitude 90 degrees north.

15. Equinox vs. Solstice Equinox: Means “equal night.” Equinox: Means “equal night.” During an equinox, the length of nighttime and daytime are about the same. During an equinox, the length of nighttime and daytime are about the same. The vernal equinox, or spring equinox, occurs around March 21, and marks the beginning of spring in the Northern Hemisphere. The vernal equinox, or spring equinox, occurs around March 21, and marks the beginning of spring in the Northern Hemisphere. The autumnal equinox occurs around September 23. It makes the beginning of fall in the Northern Hemisphere. The autumnal equinox occurs around September 23. It makes the beginning of fall in the Northern Hemisphere.

16. Solstice On two days each year, the noon sun is overhead at either 23.5 degrees south or 23.5 degrees north. Each of these days is known as a solstice. On two days each year, the noon sun is overhead at either 23.5 degrees south or 23.5 degrees north. Each of these days is known as a solstice. Summer Solstice: June 21 Winter Solstice: December 21

17. Review Explain the process that causes day and night. Explain the process that causes day and night. What two factors cause the cycle of seasons? What two factors cause the cycle of seasons? Compare rotation and revolution. Compare rotation and revolution. What do the words solstice and equinox mean? How are they related to the position of Earth’s axis. What do the words solstice and equinox mean? How are they related to the position of Earth’s axis.