Section 13.3 Earth’s Seasons (pg )

What Causes the Seasons? The combination of two factors results in the different seasons:

1) Earth’s tilted axis of rotation  Earth rotates (spins) about its internal axis once every 24 hours (approximately)  Rotation causes day and night  This axis is tilted 23.5 degrees from the vertical. (diagram including sun rays)

2) Earth’s revolution (orbit)  Revolution : elliptical path around the sun taking days (one year)

Combine these Factors:  Sunlight strikes the same part of the Earth at different angles depending on where the earth is in its orbit.  Tilted toward the sun = more vertical sunlight = more energy = warmer  Tilted away from sun = less vertical sunlight = less energy = colder Flashlight demo activity

Flashlight Activity  Take a piece of graph paper and hold it vertically  Turn on your phone flashlight and hold it about 1 inch from the paper.  How many squares are lit up?  Now tilt the graph paper to approximately 23.5 degrees (roughly ¼ of the way to horizontal)  How many squares are lit up now?  Which results in the most energy per square?  Which would have the “warmest” squares?

**Note  when the Northern hemisphere (N.H.) is tilted toward the sun, the southern hemisphere (S.H.) is tilted away.  Results in opposite seasons above and below the equator.  Ex: Summer in N.H. is winter in S.H. Bill Nye explains the seasons

Practice problem  Billy lives in gander at 49 o N. It is summer. What season is it where Sally lives in Buenos Aries 35 o S? a)Summer b)Winter c)Fall d)Spring

Solstices and Equinoxes Solstice : “sun stands still” in its path through the sky Equinox : “equal night” means day and night are equal in length Solstice and Equinox visualization

Summer solstice (June 21 st )  Max. tilt toward sun  Vertical sunlight striking Tropic of Cancer  Most vertical sunlight of the year (strongest sunlight)  Longest day Winter Solstice (Dec. 21 st )  Tilted away from sun  Vertical sunlight striking Tropic of Capricorn  Weakest sunlight (spread over larger area)  Shortest day. The following relates to the N.H. It is opposite for the S.H.

Autumnal Equinox (Sept. 23 rd )  First day of fall  Sun directly over equator  Sun spends = time above/below horizon Vernal Equinox (March 21 st )  First day of spring  “same as autumnal equinox”

Questions 1.What is the difference between a revolution and a rotation? 2.What would happen to the seasons if the Northern end of the earth’s axis was always tilted toward the sun? Explain why you think this would be good or bad. 3.Bill lives in Gander (49.8 o N). Wally lives in Sydney Australia (33.9 o S) a.On what day would each person have the most intense sunrays? b.On which days of the year would they have equal amounts of sunlight? c.Which day of the year marks the start of winter for each? 4.On the equinoxes, compare the amount of solar energy striking the equator with the amount striking an equal sized area at 50 O N