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q discuss the variation in day length over a year for different latitudes Day Length = the time when the sun appears above the horizon, from sunrise to sunset There is still daylight after sunset. This period of time is called twilight. Length of a day depends on an area’s latitude (= the distance from the equator) and the tilt of the earth’s axis. The rotation of the earth, along with the latitudes, causes some interesting things: a) At a given latitude, the duration of the longest day in the northern hemisphere (on or about June 21) is the same as the duration of the longest day in the southern hemisphere (on or about December 21); b) Also, the duration of the shortest day in the northern hemisphere (on or about December 21) is the same as the duration of the shortest day in the southern hemisphere (on or about June 21); c) The Arctic Circle is the southernmost latitude where 24-hour daylight can occur at least on one day of a year (northern summer solstice on or about June 21); d) The Antarctic Circle is the northernmost latitude where 24-hour daylight can occur at least on one day of a year (southern summer solstice on or about December 21). (Above) Pictures of the sun taken 1 hour apart. This and the following images were from a scientific expedition in Antarctica, at a base located approximately 75o South. (Above)Although DdU is a little south of the Antarctic circle, the sun is always visible at noon, even at the darkest of the winter. The reason is because of the refraction of light on the low atmosphere layers: you can actually see it while it's somewhat below the horizon. This picture was taken on June 21st with 9 exposures set 11 minutes apart if I remember right. One hour of sunlight is all we get in winter. Enough to go for a walk, but little else. I created my animated HR bar starting from this picture. •Each pole has only one sunrise and one sunset per year, around the time of the equinoxes. Each pole’s sunrise is nearly coincidental with the other’s sunset. Evidence That Shows The Earth Rotates About Its Axis And Revolves Around The Sun 1) Foucault’s Pendulum a) 1851- Foucault suspended a 60-m pendulum weighing about 25kg from the domed ceiling of the Pantheon in Paris. b) He started the pendulum swinging evenly by drawing it to one side with a cord and then burning the cord to start the pendulum swinging. c) The direction of swing of the pendulum was recorded on a ring of sand placed on a table beneath its point of suspension. d) At the end of each swing a pointed stylus attached to the bottom of the bob cut a notch in the sand. e) After a few moments it became apparent that the plane of oscillation (swing) of the pendulum was slowly changing with respect to the ring of sand, and hence with respect to the Earth. Foucault’s Pendulum in the Pantheon in Paris The pendulum bob (above) weighed 28 kg and was brass-coated lead. It was suspended from a wire 67 meters long! This replica in the Pantheon in Paris has been permanently swinging since 1995. These drawings show the pattern that emerged when the pendulum was set in motion. (left) The blue arcs show the pendulum swing; the green lines show the pattern left in the sand. Each pass was 11° different from the previous pass. http://www.youtube.com/watch? v=6oYvffE6iGY&feature=related 2) The Earth isn't round. It bulges around the equator, just as you would expect if it rotated. This was known when Foucault made his pendulum. http://earthsky.org/brightest-stars/altair-the-bluish-jewel-of-the-eagle Altair needs only 10 hours to spin once on its axis, in contrast to roughly a month for our sun. This mighty star spins on its axis more rapidly than Earth! 3) Coriolis Effect 1) Because each and every part of the Earth rotates full circle in 24 hours, different parts of the face of the Earth are actually moving through space at different speeds. 2) A point on the equator of the Earth moves about 24,000 miles in 24 hours, or about 1000 miles per hour. 3) On the other hand, a piece of ground 4 feet from the North Pole moves only 24 feet in 24 hours, for a slow speed of 1 foot per hour! Here’s an animation: http://www.classzone.com/books/earth_science/terc/content/visualizations /es1904/es1904page01.cfm Or two: http://www.bing.com/videos/search?q=coriolis+animation&qpvt=coriolis+an imation&FORM=VDRE#view=detail&mid=F1A36B32B7AC44BE6B5DF1A36B3 2B7AC44BE6B5D Or three: http://www.bing.com/videos/search?q=coriolis+animation&qpvt=coriolis+an imation&FORM=VDRE#view=detail&mid=9C8146F0AA0E2892C6199C8146F0 AA0E2892C619 FUNKY NOTE: The Coriolis Effect was largely a curiosity until World War I, when the Germans built a generation of long-range cannons and found that they could not hit anything if they aimed directly at it; they had to aim to the left. (If the target was exactly east or west of the gun's position, the gun would be aimed directly at it.) If you understand this discussion, you should be able to convince yourself that a cannonball fired southward (toward the equator) from some point in the Northern Hemisphere will be deflected to its right (west). Effects such as these that come about because we live on a rotating frame of reference are referred to as Coriolis effects. The Motion Of Stars And Planets Are Caused By Rotation And Revolution Of The Earth 1) The earth’s axis is in line with one Northern Hemisphere star: Polaris. 2) Polaris is part of the Little Dipper (Ursa Minor) 3) It is found by drawing a line from the outer lip of the Big Dipper (part of Ursa Major). 4) Polaris is located 10 off to the side of the north celestial pole, so it makes a very small arc as the earth rotates 5) All stars and planets rotate around Polaris Polaris Left: Star rotation shown by a long exposure. The star in the middle is Polaris. http://www.nakedeyeplanets.com Above: A shooting star and a passing satellite caught by a long exposure photography.