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The International Space Station (ISS) above the Earth. Image credit: NASA

The Earth is the third planet from the Sun and the fifth largest of the eight planets in our Solar System. It is located between Mars and Venus, which are also terrestrial planets. Earth has a number of nicknames including the Blue Planet and Terra. The most remarkable thing about our planet is the life that thrives all over our planet. Not only are there plants, but there are also animals, sea creatures, birds, and other diverse species.

Civilizations throughout the centuries have had different names for our planet as well as various concepts of what it looked like. The Mayans believed that the Earth was flat and that there is a jaguar, known as a bacab, at each corner of the Earth, which held up the sky. Gaea represented the earth for the early Greeks; she was believed to have given birth to the universe in Greek mythology.

Earth’s average distance from the Sun is 150 million kilometers. Astronomers call this distance 1 astronomical unit (AU), and then use this as a comparison for the other planets in the Solar System. For example, Mars is located 1.52 AU from the Sun, and Saturn is 9.58. It takes sunlight a full 8 minutes to go from the Sun to reach Earth.

The Earth rotates on its axis in less than 24 hours. It is actually 23 hours, 56 minutes and 4 seconds. This is the sidereal day. The Earth orbits the Sun every 365.2564 days. It’s this extra .2564 days that creates the need for leap years. That’s why we tack on an extra day in February every year divisible by four unless it’s divisible by 100 (1900, 2100, etc)… unless it’s divisible by 400 (1600, 2000, etc).

Comparing a stat's habitable zone based on its size. Credit: Fine Art America/Detlev Van Ravenswaay

Chart comparing a star’s habitable zone (aka. “Goldilocks zone”) based on its size. Credit: Fine Art America/Detlev Van Ravenswaay

Scientists have often tried to discover what makes earth such an ideal place for sustaining life. The Earth has been described as being in the habitable zone, which is a theoretical area in a Solar System where the conditions are optimal for the existence of life. Scientists are using the habitable zone to find extrasolar planets that may be able to support life.

Earth is also the only planet in our Solar System with tectonic plates. The plate tectonics help protect our planet from overheating like Venus because the carbon in plants makes its way back into the Earth and is recycled. Liquid water is also an important element for life. Just over 70% of Earth’s surface area is water.

The air that we breath is mixture of gases. Usually the air composition is 78% nitrogen, 21% oxygen, and 1% various other gases including: xenon, carbon dioxide, hydrogen, argon, neon, helium, and krypton. There are a few other trace gasses mixed in there, too. Let’s not forget the water vapor, pollen, dust, and other solids that get mixed in there.

Heat Shield

The layer’s of Earth’s atmosphere can be seen when viewed from orbit. Image Credit: NASA

The majority of the planet itself is actually made of four basic materials. If you could separate the Earth out into piles of material, you’d get 32.1 % iron, 30.1% oxygen, 15.1% silicon, and 13.9% magnesium. Of course, most of this iron is actually down in the Earth’s core. The core has never been sampled, but scientists think that it would be 88% iron.

Earth is the densest planet in the Solar System, with an average density of approximately 5520 kg/m 3, which is more than five times the density of water. The planet is so dense because it is composed of large amounts of iron. Most of the iron is concentrated in the core which is 88% iron. Because the planet is so dense, its gravity is great compared to its size.

The temperature on our planet is not as extreme as the temperature of the other planets. The lowest recorded temperature on Earth is approximately -89°C in Antarctica while the highest recorded temperature is 58°C.  The diameter of the planet is 12,742 kilometers, and the volume is 1,080,000,000,000 km3.

The planet is not a perfect sphere. It is actually an oblate spheroid. That means it is squished at the poles, so it bulges out at the equator. This is caused by the speed of the Earth’s rotation and is common with all the planets.

NASA created these two images to exhibit high-resolution global composites of Moderate Resolution Imaging Spectroradiometer (MODIS) data. The land surface data were acquired from June through September of 2001. The clouds were acquired on two separate days--July 29, 2001, for the northern hemisphere and November 16, 2001 for the southern hemisphere. The images were rendered in Electric Image and composited in Adobe Photoshop in late January, 2002.

“The Blue Marble”. These images of planet Earth created using data acquired by NASA between June through September of 2001. Credit: NASA

The Earth is 70% water. That kind of limits the places where we can live doesn’t it? The atmosphere is thickest within the first 50 km of the surface, but it actually reaches out to about 10,000 km above the the planet. This outermost layer of the atmosphere is called the exosphere, and starts about 500 km above the surface of the Earth.

At this point, free-moving particles can actually escape the pull of Earth’s gravity, and be blown away by the solar wind. Over 75% of the Earth’s atmosphere is is contained within the first 11km. Above that the air becomes too thin to support most life.

Earth is thought to be the only planet in our solar system that has plate tectonics. There are multiple continental plates that are constantly moving and helping to shape and form our planet. These plates are parts of the Earth’s crust that are floating on the molten core. This process has many important effects, but one of the most important ones is the recycling of carbon.

Over long periods of time, the remnants of this life, rich in carbon, are carried back into the interior of the Earth and recycled. This pulls carbon out of the atmosphere, which makes sure we don’t get a runaway greenhouse effect. If other planets had these plates, there might be more habitable planets in our galaxy.

Our planet has one satellite, which is known simply as the Moon. Once scientists discovered that other planets also had moons our satellite’s name was capitalized. The Moon is tidally locked with our planet, which means that it takes as long for it to orbit the Earth as it does for the satellite to rotate once. Because of this, the same side of the Moon is always facing our planet – although in reality we see more than 50% of the satellite due to several factors – so we never saw the other side of the Moon until we sent up a probe to take pictures.

Astronomy Cast – Episode 51: Earth.
NASA Earth Observatory
NASA: Speed of the Earth’s Rotation
Corkscrew Asteroids
NASA: What Is Earth?
NASA Earth Observatory


Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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