We begin our workshop with a discussion of some of the misconceptions about seasons. Squirrel image taken at Kennedy Space Center http://nix.ksc.nasa.gov/info;jsessionid=as5nrgqin0a7k?id=KSC-99PC-0137&orgid=5 Blue Heron also taken at Kennedy Space Center http://mediaarchive.ksc.nasa.gov/detail.cfm?mediaid=27144
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://svs.gsfc.nasa.gov/vis/a000000/a000400/a000435/index.html SeaWiFS false color data showing seasonal change in the oceans and on land for the entire globe. The data is seasonally averaged, and shows the sequence: fall, winter, spring, summer, fall, winter, spring (for the Northern Hemisphere).
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
During this section, we demonstrate physically, using a planetarium or the horizon or the walls of the classroom, the location of the Sun’s path across the sky for each of the seasons, and ask the participants to predict how high the Sun rises in the sky and where it will set.
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.
At this point, we have participants use styrofoam balls with sticks and a bright lamp to model the seasons on the Earth, with the axis of the “Earth” tilted toward a “North Star” that has been placed high in the corner of the room. For part of our orbit the northern half of Earth is tilted toward the Sun. This is summer in the northern hemisphere; there are longer periods of daylight, the Sun is higher in the sky, and the Sun's rays strike the surface more directly, giving us warmer temperatures. The north pole is in constant daylight! When the northern half of Earth is tilted toward the Sun, the southern hemisphere is tilted away. People in the southern hemisphere experience the shorter day lengths and colder temperatures of winter. During winter in the northern hemisphere, our northern axis continues to point to the North Star, but, because we have moved in our orbit around the Sun, our northern hemisphere now points away from our Sun. The north pole is completely dark and other places in the northern hemisphere experience the shorter day lengths and colder temperatures of winter as the Sun traces a lower arc across the southern sky and the Sun's rays strike the surface at a lower angle. When it is winter in the northern half of Earth, the southern hemisphere, tilted toward our Sun, has summer. During fall and spring, some locations on Earth experience similar, milder, conditions. Earth has moved to a position in its orbit where its axis is more or less perpendicular to the incoming rays of the Sun. The durations of daylight and darkness are more equally distributed across all latitudes of the globe. Solstices occur when Earth's axis is pointed directly toward our Sun. This happens twice a year during Earth's orbit. Near June 21 the north pole is tilted 23.5 degrees toward our Sun and the northern hemisphere experiences summer solstice, the longest day of the northern hemisphere year. On that same day, the southern hemisphere is tilted 23.5 degrees away from our Sun and the southern regions of Earth experience the shortest day of the year — the winter solstice. The second solstice occurs on December 21 or 22 when the north pole is tilting 23.5 degrees away from our Sun and the south pole is inclined toward it. This is the shortest day of the year in the northern hemisphere — the northern hemisphere winter solstice. Twice each year, during the equinoxes (“equal nights”), Earth's axis is not pointed toward our Sun, but is perpendicular to the incoming rays. During the equinoxes every location on our Earth (except the extreme poles) experiences 12 hours of daylight and 12 hours of darkness. The vernal or spring equinox occurs in the northern hemisphere on March 21 or 22 (the fall equinox of the southern hemisphere). September 22 or 23 marks the northern hemisphere autumnal or fall equinox. As Earth orbits our Sun, the position of its axis relative to the Sun changes. This results in a change in the observed height of our Sun above the horizon. For any given location on Earth, our Sun is observed to trace a higher path above the horizon in the summer, and a lower path in the winter. During spring and fall, it traces an intermediate path. This means that our Sun takes a greater amount of time tocross the sky in the summer and a shorter amount of time in the winter. This effect is greater as you move toward the poles; people living near the equator experience only small changes in daylight during the year. The change is more extreme toward the poles. From the National Maritime Museum During the northern hemisphere summer solstice, Earth is tilted such that the Sun's rays strike perpendicular to the surface at the Tropic of Cancer (23.5 degrees north latitude, corresponding to the tilt of Earth's axis). At (solar) noon, our Sun is directly overhead in this location (and at a decreasing height above the horizon north and south of the Tropic of Cancer). At locations north, our Sun will be at its highest position above the horizon and will take the greatest amount of time to cross the sky. All northern locations have more than 12 hours of daylight. All locations south experience less than 12 hours of daylight. Locations above the Arctic Circle (north of 66.5 degrees latitude; 90 degrees minus the tilt of Earth's axis) receive 24 hours of sunlight. Locations below the Antarctic Circle (66.5 degrees south latitude) experience 24 hours of darkness. During the northern hemisphere winter solstice, the Sun's incoming rays are perpendicular to the Tropic of Capricorn at 23.5 degrees south latitude. The Sun's path is the lowest above the horizon in locations north of the equator, and these regions experience the shortest day of the year. Between the winter and summer solstices, daylight increases as Earth continues its orbit around our Sun. During the equinoxes, sunlight strikes perpendicular to the surface at Earth's equator. All locations on Earth, regardless of latitude, experience 12 hours of daylight and 12 hours of darkness. The spring equinox marks the change from 24 hours of darkness to 24 hours of daylight at Earth's poles . In these extreme locations, our Sun moves above the horizon at the spring equinox and does not go below the horizon until the fall equinox.
More information at http://www.physicalgeography.net/fundamentals/6h.html
After seasons on Earth have been mastered, the subject can be further extended by examining the seasons on other planets. Mars image from http://photojournal.jpl.nasa.gov/catalog/PIA01589 and taken by the Hubble Space Telescope Uranus image from http://photojournal.jpl.nasa.gov/catalog/PIA02963 and taken by the Hubble Space Telescope
Mars and Uranus are two extreme cases that students may wish to consider. Mars has an eccentric orbit that brings it closer and further from the Sun, making one hemisphere’s seasonal changes more extreme than the other hemisphere. Uranus is tilted over completely on its side—what will that do to its day and night? Its seasons?
When Mars’ south pole is tilted toward the Sun, Mars is many millions of miles closer to the Sun; when Mars’ south pole is tilted away from the Sun, it is much further from the Sun. So the south pole experiences more extreme changes in the seasons than the north pole does. The image of Mars was taken by the Hubble Space Telescope and can be found at http://photojournal.jpl.nasa.gov/jpegMod/PIA03154_modest.jpg; it was edited for this diagram by the staff of LPI. More information on Mars’ seasons is at http://barsoom.msss.com/http/ps/seasons/seasons.html
SeasonsWhat do your students think causes the seasons? Images at http://nix.ksc.nasa.gov/info Created by the Lunar and Planetary Institute For Educational Use OnlyLPI is not responsible for the ways in which this powerpoint may be used or altered.
Let’s Look at Some Data!• Temperatures around the World• Daylight hours
Average Daily Temperatures (°F) in Tourist Cities (from http://www.infoplease.com/ipa/A0004587.html )) January April July October Cape Town (South Africa) 69 66 60 65 Caracas (Venezuela) 75 81 78 79 London (United Kingdom) 44 56 73 58 Mexico City (Mexico) 66 77 73 70 Montreal (Canada) 22 51 79 56 Moscow (Russia) 21 47 76 46 Nairobi (Kenya) 77 75 69 77 Paris (France) 42 60 76 59 San José (Costa Rica) 75 79 77 77 Seoul (Korea) 33 62 84 67 Singapore 86 89 87 88 Stockholm (Sweden) 31 45 70 48 Sydney (Australia) 79 73 62 72 Tokyo (Japan) 48 64 84 70
True color images December MarchJune SeptemberImages at http://www.nasa.gov/vision/earth/features/blue_marble.html
Daylight Hours Across the Globe Time is indicated as number of hours (h) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecMiami 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 hBrisbane 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 hNairobi 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 hPunta Arenas 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 hNome 5h 7h 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 hSingapore 12 h 12 h 12 h 12 h 13 h 13 h 12 h 12 h 12 h 12 h 12 h 12 hCape Town 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 hSeattle 9h 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 hVostok 24 h 24 h 19 h 14.5 h 0h 0h 0h 0h 7h 15 h 24 h 24 h http://www.lpi.usra.edu/education/skytellers/seasons/activities/light.shtml
Let’s Start with some Observations• Using free downloaded computer program, Stellarium: http://www.stellarium.org/ – Stellarium is planetarium software that shows exactly what you see when you look up at the stars. Its easy to use, and free.• Let’s look at sunrise & sunset, and Sun’s height in the sky.
Height of Sun for USAWinter: The Sun rises in the southeast, stayslow in the sky, and sets in the southwest.Spring: The Sun rises due east, moves higherin the sky than in winter, and sets due west.Summer: The Sun rises in the northeast,travels high (near zenith), and sets in thenorthwest.Fall: The Sun rises due east, travels to amedium-height in the sky, and sets due west.
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 Image at http://www.lpi.usra.edu/education/skytellers/seasons/about.shtml
Northern Hemisphere Summer More daylight hours, more direct sunlight Image at http://www.lpi.usra.edu/education/skytellers/seasons/about.shtml
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
Seasons on Other Planets • In some cases, the changing distances from the Sun will affect the seasons. • In others, the axial tilt will make a huge difference!Image at http://photojournal.jpl.nasa.gov/catalog/PIA02963 Image at http://photojournal.jpl.nasa.gov/catalog/PIA01589
Seasons on PlanetsPlanet Axial Tilt Eccent. Orbit Perihelion AphelionMercury 0° 0.21 88 days 28 mill. Miles 43 mill. milesVenus 177° 0.01 224 daysEarth 23° 0.02 365 days 91 mill. miles 94 mill. milesMars 25° 0.09 686 days 129 mill. miles 155 mill. milesJupiter 3° 0.05 12 yearsSaturn 27° 0.06 30 yearsUranus 98° 0.05 84 yearsNeptune 30° 0.01 165 years
Mars’ Orbit and Seasons At vernal equinox, Mars is 145 million miles from the Sun At winter solstice, Mars is 128 million miles from the Sun At summer solstice, Mars is 153 million miles from the Sun At autumnal equinox, Mars is 134 million miles from the SunOriginal images from http://photojournal.jpl.nasa.gov/index.html