December March June September An understanding of seasons can begin with observations. What is it like here in December? In June? What is it like in other cities? In other countries? Teachers can use satellite photos like these, or look at newspapers with temperatures for various cities around the world throughout the year. Photos from http://www.nasa.gov/vision/earth/features/blue_marble.html Using data from the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Terra satellite, scientists and data visualizers stitched together a full year’s worth of monthly observations of the land surface, coastal oceans, sea ice, and clouds into a seamless, photo-like mosaic of every square kilometer (.386 square mile) of our planet. Changes in ice are most obvious for the northern hemisphere; changes in vegetation can be seen in Africa and South America. A separate animation can be downloaded and played here: http://library01.gsfc.nasa.gov/svs/html/SVS000435.html http://www.nasa.gov/vision/earth/features/blue_marble.html

Why is it hot in the tropics and cold at the poles?

What Causes Earth’s Seasons? Earth’s axis is tilted 23.5o It always points in the same direction-- (toward Polaris, the North Star) We orbit our Sun once a year (revolution) Polaris Polaris Polaris More information is at http://www.lpi.usra.edu/education/skytellers/seasons/about.shtml This image shows the reason Earth experiences seasons.  Points we discuss using this image are: 1) Earth’s orbit around the Sun is only slightly elliptical 2) Earth’s path around the  Sun brings us closer to the Sun in January.  Many students think we have seasons because Earth is sometimes closer and sometimes farther from the Sun.  This is correct, however, we actually are closer to the Sun in January in the Northern Hemisphere! 3) Earth’s seasons are caused by Earth’s tilt on its axis (~23 degrees).  Earth’s axis essentially is fixed  - it always points to the same place in the sky (on the celestial sphere) – towards Polaris. As we orbit the Sun each year, first one polar region is tilted toward the Sun, and then the other is tilted toward the Sun. When the north polar region is tilted toward the Sun (summer) the south polar region is tilted away (winter). Notes: Earth’s tilt does change over very long time periods, but for the most part, it moves between 22 and 23 degrees. Earth’s axis also wobble a bit, but over time periods of thousands of years, pointing toward different stars. Polaris http://www.lpi.usra.edu/education/skytellers/seasons/about.shtml

The tilt of Earth’s axis causes one hemisphere (northern vs The tilt of Earth’s axis causes one hemisphere (northern vs. southern) to receive more DIRECT sunlight

Direct light causes more heating.

Solar Illumination

1 KW/m2 1 KW/m2 1 m2 2 m2 In winter a bundle of light is spread over a bigger area than in summer due to the tilt of the North Pole away from the Sun.

The Earth is curved and this causes the different angles of incidence.

Direct vs Indirect Rays

So what is the ALTITUDE of the Sun at NOON throughout the year? Total annual change: 47O Summer Solstice 23.5O Equinox 23.5O Winter Solstice sunrise sunset NE E SE SW W NW

Earth’s Orbit and the Seasons

Seasons on the Earth

Important Latitudes!!

Equinoxes and Solstices The Summer Solstice (June 21): The date when the Sun is directly striking its most northern latitude (the Tropic of Cancer). The Vernal (Spring) Equinox (March 21): The date when the Sun crosses the equator when going from south of the equator to north of the equator.

The Autumnal (Fall) Equuinox (September 23): When the Sun crosses the equator when going from north to south. The Winter Solstice (December 21): The date when the Sun’s vertical ray is striking its furthest southern latitude (the Tropic of Capricorn).

The Seasons

Summer solstice: The Sun will be directly overhead at the Tropic of Cancer (23.5oN). The Sun does not set for people within the Arctic circle (67.5oN). The Sun does not rise for people within the Antarctic Circle (23.5oS).

Earth on June 21

Winter Solstice: The Sun will appear directly overhead to someone at the Tropic of Capricorn (23.5o). The Sun does not set for people within the Antarctic Circle The Sun does not rise for people within the Arctic Circle

Earth on December 21

Autumnal equinox: The Sun will appear directly overhead to someone on the equator (0o). The North Pole begins 6 solid months of night. The South Pole begins 6 solid months of day.

Vernal equinox: The Sun will appear directly overhead to someone at the equator (0o). The South Pole begins 6 solid months of day. The North Pole begins 6 solid months of night.

The Seasons as Viewed From North Pole

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 our Sun (94 million miles) in summer –July 4 More information at http://www.physicalgeography.net/fundamentals/6h.html

Why doesn’t distance matter? Variation of Earth-Sun distance is small — about 3%; this small variation is overwhelmed by the effects of axis tilt.

July

Daylight Hours Across the Globe Time 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 Once the reason for seasons has been demonstrated, the concepts can be mastered with further activities. For instance, the participants may be able to predict the location of various cities (far north, north, equatorial, south, or far south) from the pattern of daylight hours throughout the year. From SkyTellers activity Seasons Across the Continents http://www.lpi.usra.edu/education/skytellers/seasons/activities/light.shtml

Seasonal changes are more extreme at high latitudes Path of the Sun on the summer solstice at the Arctic Circle

Analemma: If we plotted the position of the sun at noon for an entire year, it would make this shape!!

Sun’s altitude also changes with seasons Sun’s position at noon in summer: higher altitude means more direct sunlight. Sun’s position at noon in winter: lower altitude means less direct sunlight.

Effects of the Seasons Changes in solar intensity Changes in solar radiation Changes in day length Changes in temperature All of these changes are most extreme at high latitudes and minimized at the equator.