Eratosthenes used a gnomon (the shadow-casting part of a sundial) to measure the circumference of the Earth with remarkable accuracy.

Photograph by Alexandre Mirgorodski, courtesy Wikimedia

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  • On June 19, 240 BCE, Eratosthenes of Cyrene measured the shadow angle of a sundial in Alexandria, Egypt. From this measurement, he estimated the circumference of the Earth with surprising accuracy.


    Eratosthenes knew that at noon on the longest day of the year—June 19—the sun would appear at its zenith in Syene (today, Aswan) Egypt. It would not cast a shadow. Measuring the angle of a shadow cast by the sun at noon in Alexandria, Eratosthenes found it to be 1/50th of a circle. Eratosthenes then calculated the distance between Syene and Alexandria, and correlated that distance to the circumference of his hypothetical circle.


    Eratosthenes estimated the Earth to be about 46,620 kilometers (28,968 miles) in circumference, although historians and mathematicians disagree about how to interpret his conclusions. Using modern technology, we know the Earth is about 40,075 kilometers (24,902 miles) around the Equator—a difference of about 16 percent. Not bad. 

  • Term Part of Speech Definition Encyclopedic Entry
    accuracy Noun

    condition of being exact or correct.

    angle Noun

    slanting space between two lines that ultimately meet in a point.

    arc Noun

    part of the outline of a circle.

    circumference Noun

    distance around the outside of a circle.

    elevation Noun

    height above or below sea level.

    Encyclopedic Entry: elevation
    Equator Noun

    imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

    Encyclopedic Entry: equator
    measurement Noun

    process of determining length, width, mass (weight), volume, distance or some other quality or size.

    technology Noun

    the science of using tools and complex machines to make human life easier or more profitable.

    zenith Noun

    point on the celestial sphere directly above a given position.