Why do we have seasons?

GEMS 2, Part 3

Do changes in the Sun/Earth distance cause the seasons? How do you know?

What would you tell someone who said: “When the Earth is closer to the Sun, it is Summer.”

Does the tilt of the Earth make it closer to the sun?NoEarth travels around the sun in a nearly circular orbitPatterns of temperature change and seasons are

opposite in the different hemispheres. Winter in the Northern Hemisphere is summer in the

Southern Hemisphere. No, the tilt of the Earth does not significantly make the

tilted side closer.

Warm up

Does the sun always set at the same time every day?No, it varies greatly.

Is the number of hours of daylight always the same?No, it again varies greatly.

At what time of year does the sun set the latest and rise the earliest?Summer

At what time of year are the days the shortest?Winter

So, what causes the seasons?

On any given day, do you think the hours of daylight are the same all over the world?

We are going to look at some data for cities that are all at 38 degrees N latitude. San Francisco, CaliforniaCharleston. West VirginiaSt. Louis, MissouriSeoul, South KoreaTientsin, ChinaAthens, GreeceCordoba, SpainWe will now look at data from a location at 38 degrees N

latitude. The data table shows sunrise/set times for different moths and the length of daylight on the 21st of each month.

Looking at data

Space Science Sequence 6–8 © 2008 The Regents of the University of California Permission granted to

purchaser to photocopy for classroom use.

Space Science Sequence 6–8 © 2008 The Regents of the University of California Permission granted to

purchaser to photocopy for classroom use.

We will divide into groups of 5 or 4.Each student will take a card with a different

latitude in the envelope. All students will graph their own data.If you are in a group of 4, the first person

done will then graph the latitude that is not done yet. Be sure to use a different colored pencil.

Mrs. O will demonstrate how to graph data. Pay attention!

Graphing data

Compare your graphs as a team. What patterns or interesting features do you

notice?Make notes in the margins of your graphs

about anything that you notice. Things to keep in mind: How does the amount of light and dark

compare in different places?Are there months when the daylight is the

same as the night?If that is the case, are there the same months for

any locations on Earth?

Patterns?

Places far North and South can have 24 hours The Northern and Southern Hemispheres are highly

symmetricalLocations at or near the equator have little to no

variations in seasons and amount of lightEquinox: When the amount of light (day) and dark (night)

are about the sameSpring/Vernal: On or about March 21st

Fall/Autumnal: On or about September 21stSolstice: When the day or night is the longest of the year

Summer: On or about June 21st in the Northern HemisphereWinter: On or about Dec 21st in the Northern Hemisphere

Summarize what we learned

For any given latitude, spring and summer days have more hours of daylight while fall and winter nights are longer.

In the Northern and Southern Hemispheres, day-length changes and seasons are opposite of one another during the year. When it is summer in the North, it is winter in the South.

Day-length changes are more dramatic farther away from the Equator.

What we observed today will better help us understand why we have seasons. The longer the amount of daylight the warmer that area of the Earth becomes.

Wrap up