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Every January 8th
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#EarthsRotationDay

Most everyone knows from science class in school that the earth rotates on its axis and one full rotation takes 24 hours to complete. It would be very surprising if there was someone who didn’t already know this! But what some people may not know is that the speed of the earth’s rotation changes slightly from day to day and year to year.

So a true solar day is not exactly 24 hours, but the variations are a matter of seconds. A mean solar day is based on the yearlong average, but the basic concept of a solar day itself is the length of time for the earth to complete one full rotation on its axis.

And that’s one of the reasons that Earth’s Rotation Day is celebrated!

History of Earth’s Rotation Day

At the World’s Fair in 1851, the French physicist Léon Foucault (pronounced ‘foo koh’) demonstrated how the earth rotates by suspending a lead-filled brass ball from the top of the Panthéon in Paris. This device, now known as the Foucault Pendulum, showed that the plane of the swing of the pendulum would rotate relative to the Earth’s own rotation.

When a Foucault Pendulum starts swinging in one direction, after a few hours the direction will change. Although it seems like the floor is the stable part of the contraption and the change is the way the pendulum is swinging, in reality, the change is the fact that the earth beneath the feet is slowly rotating while the pendulum stays the same.

Many installations of the Foucault Pendulum also include a series of pegs that are arranged around the center, indicating the passage of time. If one was located on the north pole or south pole, then it would take almost exactly 24 hours for the series to be finished, but the amount of time changes slightly as the installations are moved around the globe.

Foucault Pendulums can now be found in science museums across the world and they are an important tool for understanding and learning about solar science. While Isaac Newton discovered gravity, he did not actually explain the cause behind it, merely that it exists as a force and so this scientific knowledge was expanded through Foucault.

Earth’s Rotation Day honors Foucault’s first public demonstration in Paris and this day seems to have been historically celebrated on the anniversary of that occasion. That being said, it is a little less clear on who first marked the occasion of Earth’s Rotation Day or when they decided to do it. It probably didn’t happen in Foucault’s lifetime, but no one can be completely certain either way.

How to Celebrate Earth’s Rotation Day

Sure, it’s true that the earth rotates every day. But not every day is Earth’s Rotation Day, so it is important to celebrate this annual event! Try out these ideas:

Visit a Space and Science Museum

For those who would like to see a version of Foucault’s Pendulum in real life, try visiting the nearest space and science museum, as many of them have one. They’re actually quite interesting to look at in action and much can be learned from the experience. Another great reason to visit a local space and science museum is to try to learn more about the earth’s rotation!

Certainly, the ideal place to go might be to view the life-sized replica of the original Foucault’s Pendulum on display at the Pantheon in Paris. But if that’s a bit too far to travel, here are just a few space and science museums that offer an opportunity to get started with viewing a Foucault Pendulum:

  • Houston Museum of Natural Science
    Part of the permanent exhibit at this museum located in Texas is the Herzstein Foucault Pendulum. Visitors can get up close and personal with this installation that demonstrates the activity and movement of the earth as Foucault originally discovered and displayed. 
  • Science Center of Iowa in Des Moines
    One of the most beloved features at this science center has been hanging in this city for more than 45 years.
  • California Academy of Sciences in San Francisco
    Hanging in the East Pavilion of the Academy is a replica of Foucault’s original installation from Paris in 1851. Although the original one hung from a 219 foot wire, this one is a smaller version, hanging from a 30 foot cable. 
  • United Nations Headquarters in New York City
    This installation in the main lobby of the UN Assembly contains a 200 pound gold plated sphere that is suspended 75 feet above the floor. It was presented as a gift to the United Nations from the Netherlands government in 1955, just over 100 years following Foucault’s original display in Paris. 

Throw an Earth’s Rotation Day Party

Join in on the celebration with a variety of sciency friends, family members and coworkers by hosting a party in honor of Earth’s Rotation Day! Have guests dress up as their favorite scientist, such as Leo Foucault or Isaac Newton.

Decorate the party room with planet earth themed balloons in blue and green. Entertain guests by playing party games, like dividing up into teams and holding a competitive quiz to see who can best answer questions related to science, space and the earth’s rotation. Other entertainment might be to watch a video or documentary to learn all about Foucault’s Pendulum and his discovery of the way the earth rotates on an axis.

Of course, no party would be complete without access to snacks, so be sure to create some treats that revolve around the theme of the earth in space. Bake round cookies and ice them with blue and green frosting to resemble the earth. Serve popcorn that is dyed blue and green to represent the blue and green colors of the plant. Or make cake pops that are decorated to look like the earth.

Learn More About Leon Foucault

Of course, the celebration of this day is about the importance of the scientific discovery made by Frenchman Leon Foucault, so this would be a great day to learn a little bit more about him. Here are some interesting facts to learn and share on Earth’s Rotation Day:

  • Leon Foucault was born in Paris, studied at home and attended College Stanislas where he studied medicine and then eventually changed his major to physics.
  • One passion of Foucault’s was photography and it was during this time he learned that he discovered a unique way to take photographs through a microscope with the use of a very strong light.
  • Foucault worked with Armand Hippolyte Louis Fizeau as a photographer and eventually took over his job as the editor of Journal des Debats.
  • For his work on science and earth’s rotation, Foucault received the Copley Medal of the Royal Society.

Read Kids’ Books About Gravity

Trying to explain these difficult concepts to little readers? Check out these amazing children’s books to help make things simple:

  • On Earth by G. Brian Karas (2008)
    With beautiful artistic pictures, this book illustrates the concepts of the earth’s rotations, cycles and revolutions on a level that even small kids can understand. 
  • Earth: Our Planet in Space by Seymour Simon (2003)
    Kids will enjoy learning about the relationship between the earth, sun and moon and how that impacts the world around them. Recommended for ages 7 and up. 
  • Earth, Sun and Moon: Rotation and Revolution by Bob De Weese (1994)
    Older kids can learn about the ways that the activities of the earth, moon and sun are all integrated and affect each other. 
  • Gravity by Jason Chin (2014)
    Not specifically about the earth’s rotation, but certainly related, this innovative children’s book makes the concept simple and provides beautiful photographs for readers to enjoy. 

Do Some Research About the Earth

Another excellent way to spend time on Earth’s Rotation Day would be to do some research on the history of how the view of the earth’s relation to the universe has changed. At one time, it was a widely held belief that the earth was the center of the universe and everything revolved around it – the planets, the sun, the moon, and the stars. Of course, at one time people also believed that the earth was flat and explorers would eventually fall off the edges! Later, the theory arose that the sun was actually the center of the universe, but eventually that was also exposed as a myth when scientists realized that neither was actually the case. In other words, no, the earth is not actually a fixed sphere at the center of the universe, simply pulling everything towards it. It rotates on its axis and Earth’s Rotation Day is celebrated in honor of that fact.

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