Astronomy Ancient Egyptians were among the first people to study the stars. They relied on the stars to predict the annual flood. They relied on the stars to plant their crops Astronomy is the study of the moon, stars, and other objects in space.
Rotation Earth’s axis is an imaginary line that passes through the Earth’s center and the North and South Poles. The North end points toward Polaris The Earth spinning on its axis is called rotation. Earth’s rotation on its axis is what causes day and night.
Revolution In addition to rotation, the Earth also travels around the sun. The Earth’s movement around the sun is called a revolution. One complete revolution around the sun is called an orbit. Earth’s orbit around the sun -- notice that the orbit is not quite a circle -- it’s oval shape!
Calendars The Egyptians were one of the first civilizations to use a calendar. Egyptian astronomers would count the number of days between each first appearance of the star Sirius. They found that 365 passed between each sighting
Calendar -- cont’d Earth’s orbit around the sun takes about 365 1/4 days. We view this as 3 years of 365 days and 1 year of 366 days (leap year)
Seasons Most places have 4 distinct seasons. Winter Spring Summer Autumn (Fall) While there are 4 distinct seasons, the temperature will vary depending on where you live.
Seasons -- cont’d Earth has seasons because its axis is tilted as it moves around the sun. It’s warmer near the equator because sunlight hits Earth’s surface at a direct angle. The sunlight is spread out more in the other areas.
Earth in June In late June, the Earth’s axis is tilted toward the sun. The hemisphere that’s tilted toward the sun has more hours of daylight. The combination of direct sunlight and longer hours gives us summer in the Northern Hemisphere. At the same time, there is less sunlight and fewer hours, giving us winter in the Southern Hemisphere.
Earth in December People in the Southern Hemisphere receives the most sunlight giving them summer. The Northern Hemisphere will receive indirect sunlight giving us winter.
Solstice There are two days each year when the noon sun is overhead at either 23.5 degrees south or 23.5 degrees north. These days are called solstice. 23.5 degrees south is called winter solstice ( around 12/21 is shortest day of the year) and 23.5 degrees north is called summer solstice (around June 21 is longest day of the year).
Earth in March and September Halfway between the winter and summer solstices, neither hemisphere is tilted toward or away from the sun. This happens on only two days each year. The noon sun is directly overhead at the equator. These days are called equinox
Earth in March and September -- cont’d Vernal equinox is also known as spring equinox and it’s usually on March 21. The vernal equinox marks the beginning of spring in the Northern Hemisphere. Autumnal equinox is usually around September 23 and marks the beginning of fall in the Northern Hemisphere.
Review Questions Explain the process that causes day and night. What two factors cause the cycle of the seasons? Compare rotation and revolution. What is solstice? How does solstice relate to the position of the Earth’s axis? What is equinox? How does equinox relate to the position of the Earth’s axis?