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Archeoastronomers have found three types of early observatories, simple markers, circles

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of stone and wood, and temples.

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Early on, markers were used to create sight lines to the horizon, so that during the equinox

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or solstice, the sun would appear to rise exactly on the sight line.

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Stonehenge in England was set up this way, as were a number of ancient Native American

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buildings such as the ones at Chaco Canyon in New Mexico and Hovenweep in Utah.

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England's Stonehenge is one of the earliest examples of an observatory in Europe.

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Stonehenge is a large calendar, capable of predicting the equinoxes and the solstice.

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Before Stonehenge, in 3000 BC, the ancient Egyptians had devised a solar calendar of

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365 days, the starting point of which hinged on the helical rising of the star Sirius,

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which also happened to coincide with the summer solstice and the annual flooding of the Nile.

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By being in touch with celestial phenomenon and their natural surroundings, the ancient

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Egyptians were able to predict events of great significance in their desert environment.

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At Abu Simbel, massively carved statues of Ramses the Great face east to greet the sun

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god Ray, the bringer of light.

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As the sun rises each day, the statues are illuminated again, perhaps a sign of rebirth

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for Ramses.

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But the most compelling is a passage to the temple's inner sanctuary, which is aligned

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so that on October 18th, the sun filters into the sanctuary, illuminating a statue of Ramses.

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While October 18th doesn't mean much to us in the Western world, this October date corresponds

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to the beginning of the Egyptian civil year and the celebration that occurred during the

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time in which Ramses lived.

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It was the Greeks, however, that created the first portable cosmological tool for keeping

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track of these motions, a stick.

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The Greeks actually called it a gnomon, and it was used to keep track of the shadow of

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the sun.

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Actually, it's a little bit more difficult than that because the shadow depends on your

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latitude.

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Again, if you are not near the equator, the shadow will be shortest during the summer

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solstice and longest during the winter solstice.

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For the spring equinox and fall equinox, the shadow will be halfway between the shadow

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lengths at the solstices.

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In the southern hemisphere, the shadows will be reversed, just as you all know the seasons

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are reversed.

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When it's summer in the United States, it's winter in Argentina.

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This all works pretty well if you're not at the equator.

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At the equator, the summer solstice sun casts a shadow in the southerly direction, and the

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winter solstice sun casts a shadow in the northerly direction.

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During the equinox, at the equator, the shadow disappears.

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Oh, and another thing that they were used for is sundials, and it looks to me like it's

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time to go back to Jennifer.

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Okay, guys, let's take a look at how a gnomon works and see the angle of the sun at certain

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times during the day.

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Students from Newcomb Elementary School in Newcomb, New Mexico, will preview this show's

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hands-on activity.

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Yá'át'ééh.

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Hello.

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We are students from Newcomb Elementary School.

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We are located on the Navajo Reservation in the Four Corners region of New Mexico.

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Tracking the passage of the sun in the sky continues to play a very important role in

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the life of our Navajo culture.

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Traditional Navajos still use this system of tracking the sun's shadows to tell time

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and to tell the changing of the seasons.

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For example, when my grandfather herd sheep, he does not wear a watch like this.

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He uses the sun's shadow to tell time.

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It also helps him to tell when to take the sheep back home in their corral.

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It also helps him to tell when to plant corn and watermelon on his farm.

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NASA Connect asked us to show you this program's hands-on activity.

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In this activity, the students will make sun shadow plots every half hour, marking the

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ends of the shadows made by the sun and a gnomon.

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You can download a copy of the educator guide from the NASA Connect website for directions

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and a list of materials.

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Turn a cardboard box upside down.

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Tape a large piece of paper to the cardboard box.

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Draw two lines that are perpendicular to each other, from top to bottom, and the other from

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left to right across the paper.

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Mark its center with a dot.

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And make a very small hole in the center of the box using the point of a scissors.

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Stick the gnomon through the dot and the hole in the cardboard.

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Secure it with tape so that 10 centimeters is sticking straight up out of the box.

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Use a protractor to make sure the gnomon is perpendicular to the box.

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On a clear, sunny day, find a large, flat area.

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Tape the box to the ground on all four sides.

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Starting as early in the morning as possible, mark the end of the gnomon's shadow every

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half hour until the end of the day.

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Next to the dot, label the time of the day it was marked.

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You will analyze the data you collect by measuring angles and length.

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Remove the gnomon and draw a straight line from each dot to the hole that the gnomon

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was placed in.

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Measure and record the angle between the horizontal line drawn through the center of the paper

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and each marked shadow.

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Then, measure and record the length of each shadow.

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Using geometry, find and label true north on your sun-shadow plot.

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Verify local solar noon using shadow length times and sunrise-sunset times.

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How do the lengths, positions, and angles of the shadows change?

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What do the changes tell you about the position of the sun throughout the day?

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Would the curve change if you used a different sized gnomon to cast the shadow?

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Don't forget to check out this cool web activity for this program.

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You can download it from the NASA Connect website.