Seasons

What do your students think causes the seasons?

http://nix.ksc.nasa.gov/info

By the Lunar and Planetary Institute

For use in teacher workshops

01/06/11 The Seasons 2

Rotation vs. Revolution

• Rotation is the spin of an object about its axis.– The Earth rotates once a day (once every 24

hours).• Revolution is the orbit of one object around another.

– The Earth revolves around the Sun every 365.26 days.

Solar Heating

The sun or any light, heats a surface mosteffectively when it is shining directly onto

the surface.

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Day Night Cycle

True color images

June

December

March

September

http://www.nasa.gov/vision/earth/features/blue_marble.html

What Causes Earth’s Seasons?

• Earth’s axis is tilted 23.5 degrees – it always points in the same direction (Polaris, the North Star) as we orbit our Sun once a year

• This tilt causes the hemispheres to alternate in the amount of our Sun’s light and heat they receive through the year

http://www.lpi.usra.edu/education/skytellers/seasons/about.shtml

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More daylight hours, more direct sunlight

Northern Hemisphere Summer

http://www.lpi.usra.edu/education/skytellers/seasons/about.shtml

Seasons vs. hemispheres

• Northern and southern hemisphere have reversed seasons.

Seasons

In summer, the sun is at a smaller angle,more effective heating.

In winter, the sun is at a larger angle, lesseffective heating.

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Seasons Misconception

• Many people carry the misconception that the seasons are due to the distance of the Earth from the Sun. However, consider the following facts:– The Earth’s orbit around the Sun is nearly a

perfect circle. The Earth is slightly closer to the Sun in January and farther from the Sun in July.

• Perihelion (closest to the Sun) is around January 3 when Earth is about 91,405,436 miles from the Sun.

• Aphelion (farthest from the Sun) is around July 4 when Earth is about 94,511,989 miles from the Sun.

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Seasons Misconception

– While it is winter in the Northern hemisphere it is summer in the Southern hemisphere. If the seasons were due to our distance from the Sun both hemispheres would have the same seasons at the same time.

Earth’s orbit is almost a perfect circle

• Earth is CLOSEST to our Sun (91 million miles) in winter—January 3

• Earth is farthest from on our Sun (94 million miles) in summer –July 4

Equinox

• Sun rises due east• Sun sets due west• Exactly 12 hours of day, 12 hours of night• Once in spring• Once in fall• When the sun is directly over the equator.

Solstice

• Summer solstice:• Longest daylight off the year• Sun at noon is highest in the sky

• Winter solstice:• Shortest daylight off the year• Sun at noon is lowest in the sky

Daylight Hours Across the GlobeTime is indicated as number of hours (h)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 10.5 h 11 h 12 h 12.5 h 13 h 14 h 14.5 h 14 h 12.5 h 12.5 h 11 h 11.5 h

14.5 h 13.5 h 13.5 h 12.5 h 11 h 11 h 11 h 11.5 h 11.5 h 12.5 h 14 h 14.5 h

12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h

17.5 h 15.5 h 13.5 h 11.5 h 10 h 8.5 h 8.5 h 9.5 h 11.5 h 13 h 15 h 16.5 h

5 h 7 h 10 h 13.5 h 17.5 h 21.5 h 22 h 18 h 15.5 h 11.5 h 8.5 h 5.5 h

12 h 12 h 12 h 12 h 13 h 13 h 12 h 12 h 12 h 12 h 12 h 12 h

15 h 14 h 13.5 h 12.5 h 11.5 h 10 h 10.5 h 11 h 11.5 h 12.5 h 14 h 14 h

9 h 10 h 11 h 13.5 h 15 h 17 h 16.5 h 15.5 h 13.5 h 11.5 h 10.5 h 9.5 h

24 h 24 h 19 h 14.5 h 0 h 0 h 0 h 0 h 7 h 15 h 24 h 24 h

Miami

Brisbane

Nairobi

Punta Arenas

Nome

Singapore

Cape Town

Seattle

Vostok

http://www.lpi.usra.edu/education/skytellers/seasons/activities/light.shtml