The first thing you might not know about obelisks is what they are. If you have ever visited the Washington Monument, however, or walked across the Place de la Concorde in Paris, or seen any rendering of ancient Egypt in its glory, you are very familiar with obelisks: vertical stone columns that taper as they rise, topped by a pyramid. Washington’s Monument and the Fascinating History of the Obelisk, by John Steele Gordon, is an absorbing account of the obelisk’s place in human civilization. Here are seven things revealed by Gordon that you might not know about obelisks.

1. THEY WERE BUILT BY THE ANCIENT EGYPTIANS, THOUGH ONLY A FEW REMAIN IN EGYPT.

The ancient Egyptians placed pairs of obelisks at the entrances of their temples. According to Gordon, the columns were associated with the Egyptian sun god, and perhaps represented rays of light. They were often topped with gold, or a natural gold-and-silver alloy called electrum, in order to catch the first rays of the morning light. Twenty-eight Egyptian obelisks remain standing, though only six of them are in Egypt. The rest are scattered across the globe, either gifts from the Egyptian government or plunder by foreign invaders.

2. AN OBELISK WAS USED IN THE FIRST CALCULATION OF THE CIRCUMFERENCE OF THE EARTH.

Around 250 B.C., a Greek philosopher named Eratosthenes used an obelisk to calculate the circumference of the Earth. He knew that at noon on the Summer Solstice, obelisks in the city of Swenet (modern day Aswan) would cast no shadow because the sun would be directly overhead (or zero degrees up). He also knew that at that very same time in Alexandria, obelisks did cast shadows. Measuring that shadow against the tip of the obelisk, he came to the conclusion that the difference in degrees between Alexandria and Swenet: seven degrees, 14 minutes—one-fiftieth the circumference of a circle. He applied the physical distance between the two cities and concluded that the circumference of the Earth was (in modern units) 40,000 kilometers. This isn’t the correct number, though his methods were perfect: at the time it was impossible to know the precise distance between Alexandria and Swenet.

If we apply Eratosthenes's formula today, we get a number astonishingly close to the actual circumference of the Earth. In fact, even his inexact figure was more precise than the one used by Christopher Columbus 1700 years later. Had he used Eratosthenes’s estimation, Columbus would have known immediately that he hadn’t reached India.

3. TRUE OBELISKS ARE MADE OF A SINGLE PIECE OF STONE.

True obelisks as conceived by the ancient Egyptians are “monolithic,” or made from a single piece of stone. (The literal translation of monolith—a Greek word—is “one stone.” On that note, the word “obelisk” is also Greek, derived from obeliskos, or skewer. An ancient Egyptian would have called an obelisk a tekhen.) The obelisk at the center of Place de la Concorde, for example, is monolithic. It is 3300 years old and once marked the entrance to the Temple of Thebes in Egypt. So difficult is the feat of building a monolithic obelisk that Pharaoh Hatshepsut had inscribed at the base of one of her obelisks the proud declaration: “without seam, without joining together.”

4. THEY WERE REALLY, REALLY HARD TO BUILD.

Nobody knows exactly why obelisks were built, or even how. Granite is really hard—a 6.5 on the Mohs scale (diamond being a 10)—and to shape it, you need something even harder. The metals available at the time were either too soft (gold, copper, bronze) or too difficult to use for tools (iron’s melting point is 1,538 °C; the Egyptians wouldn’t have iron smelting until 600 B.C.).

The Egyptians likely used balls of dolerite to shape the obelisks, which, Gordon notes, would have required “an infinity of human effort.” Hundreds of workers would have each had to pound granite into shape using dolerite balls that weighed up to 12 pounds. This doesn’t even address the issue of how one might move a 100-foot, 400-ton column from the quarry to its destination. While there are many hypotheses, nobody knows precisely how they did it.

5. AN OBELISK HELPED ARCHAEOLOGISTS TRANSLATE HIEROGLYPHICS.

Until the 19th century, hieroglyphics were thought to be untranslatable—mystical symbols with no coherent message beneath. Jean-François Champollion, a French Egyptologist and linguist, thought differently, and made it his life’s purpose to figure them out. His first success came from the Rosetta Stone, from which he divined the name “Ptolemy” from the symbols. In 1819, “Ptolemy” was also discovered written on an obelisk which had just been brought back to England—the Philae obelisk. The “p,” “o,” and “l” on the obelisk also featured elsewhere on it, in the perfect spots to spell the name “Cleopatra.” (Not that Cleopatra; the much earlier Queen Cleopatra IX of Ptolemy.) With those clues, and using this obelisk, Champollion managed to crack the mysterious code of hieroglyphics, translating their words and thus unlocking the secrets of ancient Egypt. (Almost 200 years later, the European Space Agency’s mission to land a spacecraft on a comet commemorated these events; the spacecraft is named Rosetta. The lander is named Philae.)

6. THE OLDEST REMAINING OBELISKS ARE AS OLD AS RECORDED HUMAN HISTORY.

The oldest obelisks are almost impossibly old—ancient even by the standards of antiquity. Seaton Schroeder, an engineer who helped bring Cleopatra’s Needle to Central Park, called it a “might monument of hoary antiquity,” and commented eloquently, “From the carvings on its face we read of an age anterior to most events recorded in ancient history; Troy had not fallen, Homer was not born, Solomon’s temple was not built; and Rome arose, conquered the world, and passed into history during the time that this austere chronicle of silent ages has braved the elements.”

7. THE TALLEST OBELISK IN THE WORLD IS THE WASHINGTON MONUMENT.

First conceived in 1832, the Washington Monument took decades to build. It is, by law, the tallest structure in the District of Columbia, and is twice as tall as any other obelisk in the world. Gordon notes that it stands unique among memorials in Washington. Whereas people visit memorials to Lincoln and Jefferson (among others) to see giant statues of the men they commemorate, the highlight of the Washington Monument is the monument itself. The statue of Washington inside receives little notice. As Gordon writes in Washington’s Monument, “The obelisk, silent as only stone can be, nonetheless seems to say as nothing else can, ‘Here is something significant.’”