Seasons Page 520 A. What Causes Seasons? 1. Seasons are caused by the tilt of the earth s axis as it moves around the sun. 2. Seasons happen because the Earth is tilted on its axis at a 23.5 angle. 3. The tilt affects how much solar energy the Earth receives as it moves around the sun. 1
B. The Earth and Sun 1. The Earth rotates on its axis once every 24 hours. 2. 1 rotation = 1 day. 3. The Earth revolves around the sun once every 365 days. 4. 1 revolution = 1 year. Why does the Earth Tilt? No one knows why the Earth's axis is tilted by 23.5 degrees. Some astronomers think that about 5 billion years ago, when the Earth was still very young, it was struck by a Marssized planet. This colossal impact could have tipped our planet over. Whatever the reason, it's a good thing - if the Earth did not tilt, countries near the poles would be cold and dark all year round. If it tilted too much, the seasons would be very extreme like on the planet Uranus. Here the winter lasts for 42 years in total darkness! 2
C. Our 4 Seasons 1. Spring 2. Summer 3. Winter 4. Fall DO ALL AREAS OF EARTH EXPERIENCE ALL 4 SEASONS???? D. Summer in the Northern Hemisphere 1. During our summer months, the Northern Hemisphere has warmer temperatures and longer days because it tilts toward the sun. 2. When it is summer in the Northern Hemisphere it is winter in the southern hemisphere because it is tilted away from the sun. 3
E. Winter in the Northern Hemisphere 1. During our winter months, the Northern Hemisphere is tilted away from the sun, temperatures are lower and days are shorter. 2. In the Southern Hemisphere it is summer because it tilts toward the sun. 4
F. Seasons and The Equator 1. The equator receives generally the same amount of energy from the sun all year round. 2. These areas have few seasonal changes. 3. If you live near the equator, the Sun doesn't shift up and down in the sky as much. This means that the temperature doesn't vary as much. So countries near the equator only have two seasons - rainy and dry. 5
G. Seasons and Weather 1. Different seasons bring different types of weather. 2. This weather is caused by the heating or cooling of the Earth s atmosphere. H. Solstice 1. The winter solstice occurs on December 21 or 22 and marks the beginning of winter (this is the shortest day of the year). 2. The summer solstice occurs on June 21 and marks the beginning of summer (this is the longest day of the year). 6
I. Equinox 1. Equinoxes are days in which day and night are of equal duration. 2. 12 hours of daylight and 12 hours of darkness 3. There are two each year: 4. The vernal equinox occurs in late March (this is the beginning of spring in the Northern Hemisphere and the beginning of fall in the Southern Hemisphere). 5. The autumnal equinox occurs in late September (this is the beginning of fall in the Northern Hemisphere and the beginning of spring in the Southern Hemisphere). 7
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SEASONS QUIZ 1. Name the four seasons. 2. Are the Earth's seasons caused by the differences in the distance from the Sun throughout the year or the tilt of the Earth's axis? 3. What is the tilt of the Earth's axis (in degrees)? 4. During which season do the Sun's rays hit the Northern Hemisphere at the most direct angle? 5. During which season are the days the shortest? 6. What is the name of the shortest day of the year (and the beginning of winter)? 7. What is the name of the longest day of the year (and the beginning of summer)? 8. What is the name of a day in which the day and night are of equal duration? 9. How many times each year do we have days in which the day and night are of equal duration? 10. What are the names of each of these days? 11. How many days does it take for the Earth to rotate once on its axis? 12. How long does it take for Earth to revolve around the sun one time? ANSWERS 1. Name the four seasons. summer, autumn (fall), winter, spring 2. Are the Earth's seasons caused by the differences in the distance from the Sun throughout the year or the tilt of the Earth's axis? The tilt of the Earth's axis 3. What is the tilt of the Earth's axis (in degrees)? 23.5 degrees 4. During which season do the Sun's rays hit the Earth at the most direct angle? summer 5. During which season are the days the shortest? winter 6. What is the name of the shortest day of the year (and the beginning of winter)? winter solstice 7. What is the name of the longest day of the year (and the beginning of summer)? summer solstice 8. What is the name of a day in which the day and night are of equal duration? an equinox 9. How many times each year do we have days in which the day and night are of equal duration? 2 times 10. What are the names of each of these days? the vernal equinox and the autumnal equinox 9
Earth Viewer http://www.fourmilab.ch/earthview/vplanet. html Animations: http://www.educypedia.be/education/climat eanimations.htm 10