Cause of Seasons
Compiled by Mark Francek at Carleton College (SERC) and Central Michigan University
Find animations dealing with the cause of the seasons and how the seasons impact the sun's path across the sky, the subsolar point, and the circle of illumination.
Elliptical vs. Circular Orbit, Analemma.com (more info)
Find the contrast between a highly exaggerated earth elliptical orbit and circular orbit depicted in .mov format. It should be mentioned to students that in reality the earth's elliptical orbit around the sun would hardly be noticeable if viewed from this distance. Taken alone, the video could unfortunately perpetuate the misconception that earth sun distance is responsible for the seasons. Still, the video is useful for pointing out that the earth's speed around the sun is not constant, with the earth moving fastest in January and slowest in July. This phenomenon helps explain why summer is longer in the Northern Hemisphere and for the analemma. The animation can be paused and rewound to emphasize important points.
Seasonal Views of Earth from Space, Charles Burrows (more info)
Find a Flash animation showing how the circle of illumination varies by month from three different perspectives: north pole, south pole, and from the sun. The progression of the circle of illumination can be tied to individual months which rapidly scroll by on the bottom right hand portion of the window. The position of light/dark can be frozen for any month by stopping or rewinding the animation.
Seasonal Sunlight, Exploring Earth (more info)
Actual satellite imagery spliced together to create a movie illustrating seasonal changes in sunlight. Taken from a geostationary satellite, the Flash movie can be stopped at key points in the video to discuss solstices and equinoxes. It should be pointed out that the earth's axis remains fixed in its orbit around the sun and that the apparent wobble is due to changes in the circle of illumination. This can best be verified by looking at the position of Australia which remains constant throughout the movie. The animation can be rewound or paused to stress important points.
Orbit, Declination, and Sunlight on Ground, University of Illinois (more info)
Two things make this Flash animation unique. First, the two insets, one picturing the sun's seasonal variation in declination above the equator and other the variation of how much sunlight could actually reach the ground. Second, the animation is from the perspective of the southern hemisphere, a view not often taken by North American students. The animation can easily be rewound or paused to stress important points.
Sunlight and the Seasons, Charles Burrows (more info)
This simple Flash animation allows the user to view where the direct rays of the sun are striking during solstice and the equinox. Note how the circle of illumination, the lit portion of the animation, changes for each date. The yellow line located on the right of the animation represents the subsolar point. Upper and lower arrows depict the sun's oblique rays.
Path of the Sun at Different Latitudes, Charles Burrows (more info)
There is a lot of information packed into this Flash animation. Find the path of the sun above the horizon (on the celestial sphere) for four locations (both poles, equator, and 41 degrees north) during equinox and solstices. An important point that can be extrapolated from this animation is that although each pole has perpetual day during its respective summer solstice, the sun never exceeds around 24 degrees above the horizon. Given the sun's oblique rays, there is enhanced atmospheric scattering and absorption resulting in lower temperatures. In addition, ice shelves and glaciers promote high albedos, further lowering temperatures. Thus, it's a refrigerator even during perpetual day. The animation can easily be rewound or paused to stress important points.
Subsolar Point Variation, NASA (more info)
This animated GIF depicts the seasonal variation in the subsolar point, reaching its northern most point around June 21 at the Tropic of Cancer and the Tropic of Capricorn around Dec. 21. Again, remind students that the earth doesn't wobble during its orbit around the sun.