Friday is Earth Day, an annual celebration of the planet we live on and the environmental movement to protect it.

The first Earth Day was held in 1970. Sen. Gaylord Nelson, D-Wis., came up with a plan to host a “national teach-in on the environment,” and the rest is history, explains. Today it’s recognized in nearly 200 countries.

If you can, plant a tree Friday in honor of the Blue Marble. If you can’t, celebrate by reading these facts about the Earth and the life on it, collected primarily from, livescience and Better Homes and Gardens:

The Earth is the densest planet in the solar system, with about 5.5 grams per cubic centimeter.

About 70 percent of the Earth is covered with water.

The planet’s circumference is 24,901 miles.

The Earth is estimated to be more than 4.5 billion years old.

Almost half of all bottled water sold by vendors in the world is just tap water.

Cranberries float because they have little pockets of air inside them.

Potatoes were cultivated about 7,000 years ago.

It's said that more than 99 percent of species that have ever lived on the planet have gone extinct. According to the International Union for Conservation of Nature and Natural Resources, there are more than 830 species that have done so, among them the dodo bird, great auk and Pyrenean ibex.

Texas is the state with the most tornadoes every year. 

Hurricanes, to be classified as such, must have winds of at least 74 miles per hour. 

The coldest temperature on record was −128.6 degrees Fahrenheit in Antarctica in 1983.

The hottest was 134 degrees Fahrenheit in 1913 in California.

Dogs can predict big storms by feeling changes in the air pressure.

Gravity is weaker in Hudson Bay, Canada, due to now-melted glaciers.

The moon is 230,100 miles from Earth.

Rats are ticklish and can laugh.

The deepest spot on Earth is the Mariana Trench in the Pacific Ocean. The highest spot — when considering the point farthest from the Earth's core — is Mount Chimborazo in Ecuador.

A group of bears is called a sloth, a group of hamsters is called a horde and a group of wombats is called a wisdom.

There are about 195 countries in the world, depending on the source you’re consulting. 

Last year was the warmest on record since the 1800s.