Earth is the third planet from the Sun, orbiting our host star at an average distance of one astronomical unit (AU), or about 93 million miles (150 million kilometers) from the center of the star. Unlike the other rocky planets of the inner solar system (Mercury, Venus, and Mars), Earth is mostly covered in liquid water. From a distance, Earth is said to resemble a blue marble due to the extensive liquid water oceans combined with the scattering of blue light in its atmosphere.
At this moment, Earth is the only known planet to host life. Life is so prevalent on Earth that it is visible from space, appearing as swatches of green across the various landmasses.
Earth spins on its axis, completing one rotation roughly every 24 hours. This creates a 24 hour day/night cycle, which dictates the behavior and sleep patterns of many lifeforms on the planet.
Earth “wobbles,” since it is tilted relative to its orbit by about 23.5 degrees. This axial tilt results in one hemisphere of the planet getting more or less solar radiation than the other hemisphere, depending upon which hemisphere is “tilted” toward the Sun at a specific point in the planet’s orbit. These changes affect weather patterns and local climate on the surface. We call these changes seasons, and we recognize four distinct seasons in one orbit (winter, spring, summer, and fall or autumn).
Every orbit takes a little more than 365 days to complete, or roughly a year. Because orbits are seldom perfect circles, the distance between Earth and the Sun changes over the year. Earth is closest to the sun in January, and farthest from the sun in July. (Unlike axial tilt, distance from the sun does not account for the seasons.)
Earth’s atmosphere extends roughly 300 miles from the planet, tapering off into space. A majority of the atmosphere rests roughly 10 miles from the surface, where the air pressure is about 14.7 pounds per square inch (psi). Most of this atmosphere is nitrogen (78%) and oxygen (21%). It also contains less than 1% of argon, and less than 0.05% carbon dioxide. The rest of the atmosphere (air) is made up of neon, helium, methane, krypton, and hydrogen.
Over the past few decades, scientists have confirmed that Earth’s air is more than a collection of these elements. Our planet’s lower atmosphere is teeming with microbial life. The clouds above are home to microscopic organisms that absorb nutrients and reproduce while floating along in air currents for thousands of miles.
Some planets in our solar system have multiple moons — Earth has just one. Our Moon is sometimes called “Luna”, from the Latin, or “Selene”, after the Greek Goddess. The Moon orbits Earth at a distance of between 252,088 miles and 225,623 miles. It completes a single orbit of Earth in a little over 27 days. Like Earth, the Moon spins around an axis. However, the Moon is tidally locked with Earth, meaning it spins at such a speed that, in its orbit around Earth, the same hemisphere (or face) of the Moon faces Earth at all times. Additionally, the Moon’s oscillating gravitational influence results in tides on Earth’s oceans.
Earth is about 4.5 billion years old. Homo sapiens (human beings, you and me) have been around for roughly 200,000 years. Compared to other lifeforms on Earth, we have not been around for very long yet at all. In that time, however, we have learned quite a bit about the planet we live on. And there is still much more to learn!