Unit 4 : Activity 4 : Apparent Motions of the Sun : Sidereal Time : Unit Exam

Unit 1: How to Point to a Star
Unit 2: Where on Earth Are You?
Unit 3: Earth's Rotation and the Sun's Apparent Motion
Unit 4: Yearly Changes in the Sky
Unit 5: Seasons and Climate
Unit 6: Sundials
Unit 7: Navigation
Unit 8: Ancient Astronomy
Unit 9: Constellations





Activity 4

--Complete this activity prior to proceeding to Unit 4 --

For this exercise, a flashlight or laser pointer will be helpful.  Put it on its side on a table or shelf or kitchen counter, aimed towards your globe so that the beam would pass through the center of the globe if the globe were transparent:

The spot of light on the globe then represents the point where the sun would be directly overhead, in the observer's zenith.

The earth's axis is tilted 23.5 degrees compared with the orbit of the earth (or the direction to the sun).  Most globes are mounted to show this tilt.  Your unmounted globe can be arranged to show the tilt:

The earth's axis is the solid line; the blue line is the equator.

Start with the tilt oriented as shown above - with the direction to the sun as close to the North Pole as it can get.  Look at your globe.  What latitude is being illuminated?  It should be 23.5N.  This line is labeled "Tropic of Cancer" on many globes.  At this latitude, the sun is in the zenith on June 21 (give or take a day), the day of the summer solstice. 

Six months later, the earth will have moved around the sun.  This puts it in the arrangement shown below:

Now, where is the sun in the zenith?  That is where the bright spot is located.  You should find that it is in the zenith for 23.5S, the "Tropic of Capricorn".

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