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## Quadrilaterals with every vertex on a circle and opposite angles that are supplementary.

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Sunrise at Stonehenge

Each year in the middle of June, people gather at Stonehenge in England. On the summer solstice, the sunrise's first rays supposedly shine right into the center of the monument. This Stone Age structure seems to have been built to align with astronomical phenomena marking important days of the year, such as solstices and equinoxes. How did ancient people construct such a complex structure? Perhaps by using squares inscribed inside circles!

#### An Ancient Mystery

For centuries, it was debated how a Stone Age culture could build a monument as complex as Stonehenge. Some people believed that its creators must have made complex calculations and utilized long-forgotten technology. Others claimed that magic, or even aliens, must have aligned the gigantic stones. Archaeologists have other theories. Recently, one historian proved that all the shapes in the design of the monument can be constructed with a single length of rope—all it takes is a basic knowledge of geometry.

For instance, Stonehenge consists of a 56-sided regular polygon inscribed in a circle. Archaeologists have proven that the ancient builders could have constructed this shape by inscribing congruent squares inside the main circle. The vertices of the polygon line up with eclipses of the moon, which occur every 19 years. No one knows how Stonehenge was intended to be used or what purpose it was supposed to serve. Some historians believe it was a temple. Others think it served as a giant calendar that tracked the sun and moon. One thing is clear, however: the ancient people of England understood basic geometry.

#### Explore More

Using stone circles may have been one way ancient cultures kept track of their calendars. However, throughout history there have been almost as many calendar systems as there have been cultures and civilizations. Check out the links below to learn about a few other ancient timekeeping methods.

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