Our home planet, Earth, holds a remarkable distinction as the third planet from the Sun and the only known haven for life. Nestled within the vast expanse of the universe, Earth is a captivating oasis teeming with diverse ecosystems, breathtaking landscapes, and a rich tapestry of life forms.

With a radius of 3,959 miles, While Earth is only the fifth largest planet in the solar system, it is the only world in our solar system with liquid water on the surface. Just slightly larger than nearby Venus, Earth is the biggest of the four planets closest to the Sun, all of which are made of rock and metal.

Earth is the only planet in the Solar System whose English name does not come from Greek or Roman mythology. The name was taken from Old English and Germanic. It simply means “the ground.” There are, of course, many names for our planet in the thousands of languages spoken by the people of the third planet from the Sun.

A 3D model of Earth, our home planet. Credit: NASA Visualization Technology Applications and Development (VTAD)

Our home planet is the third planet from the Sun, and the only place we know of so far that’s inhabited by living things. While Earth is only the fifth largest planet in the solar system, it is the only world in our solar system with liquid water on the surface. Just slightly larger than nearby Venus, Earth is the biggest of the four planets closest to the Sun, all of which are made of rock and metal.


The name Earth is at least 1,000 years old. All of the planets, except for Earth, were named after Greek and Roman gods and goddesses. However, the name Earth is a Germanic word, which simply means “the ground.”

Potential for Life

Earth has a very hospitable temperature and mix of chemicals that have made life abundant here. Most notably, Earth is unique in that most of our planet is covered in liquid water, since the temperature allows liquid water to exist for extended periods of time. Earth’s vast oceans provided a convenient place for life to begin about 3.8 billion years ago.

Some of the features of our planet that make it great for sustaining life are changing due to the ongoing effects of climate change. To find out more visit our sister website,

Size and Distance

With a radius of 3,959 miles (6,371 kilometers), Earth is the biggest of the terrestrial planets and the fifth largest planet overall.

From an average distance of 93 million miles (150 million kilometers), Earth is exactly one astronomical unit away from the Sun because one astronomical unit (abbreviated as AU), is the distance from the Sun to Earth. This unit provides an easy way to quickly compare planets’ distances from the Sun.

It takes about eight minutes for light from the Sun to reach our planet.

Orbit and Rotation

With remarkable consistency, Earth completes a full rotation on its axis every 23.9 hours, defining the rhythm of our daily lives. From the emergence of a golden sunrise to the enchanting hues of a sunset, each rotation marks a complete cycle of day and night.

Yet, Earth’s celestial choreography extends far beyond the daily cycle. Over the course of 365.25 days, our planet embarks on a grand voyage around the Sun. However, the addition of a quarter of a day presents a challenge for our calendar system, which traditionally counts a year as 365 days. To harmonize our calendars with Earth’s orbit, we embrace the concept of the leap year. Every four years, we add an extra day, aptly named the leap day, to synchronize our timekeeping with the cosmic rhythm.

The axis of Earth, tilted at an angle of 23.4 degrees relative to the plane of its orbit, holds the key to another captivating phenomenon—the cycle of seasons. As our planet revolves around the Sun, this tilt plays a crucial role in shaping our yearly experiences. During one part of the year, the northern hemisphere leans closer to the Sun, basking in its radiant warmth and illumination. Meanwhile, the southern hemisphere tilts away, experiencing cooler temperatures and shorter days. This asymmetry gives rise to summer in the north and winter in the south, with the Sun soaring higher in the sky in the summer months.


Earth is the only planet that has a single moon. Our Moon is the brightest and most familiar object in the night sky. In many ways, the Moon is responsible for making Earth such a great home. It stabilizes our planet’s wobble, which has made the climate less variable over thousands of years.

Earth sometimes temporarily hosts orbiting asteroids or large rocks. They are typically trapped by Earth’s gravity for a few months or years before returning to an orbit around the Sun. Some asteroids will be in a long “dance” with Earth as both orbit the Sun.

Some moons are bits of rock that were captured by a planet’s gravity, but our Moon is likely the result of a collision billions of years ago. When Earth was a young planet, a large chunk of rock smashed into it, displacing a portion of Earth’s interior. The resulting chunks clumped together and formed our Moon. With a radius of 1,080 miles (1,738 kilometers), the Moon is the fifth largest moon in our solar system (after Ganymede, Titan, Callisto, and Io).

The Moon is an average of 238,855 miles (384,400 kilometers) away from Earth. That means 30 Earth-sized planets could fit in between Earth and its Moon.

A 3D model of Earth’s Moon. Credit: NASA Visualization Technology Applications and Development (VTAD)


When the solar system settled into its current layout about 4.5 billion years ago, Earth formed when gravity pulled swirling gas and dust in to become the third planet from the Sun. Like its fellow terrestrial planets, Earth has a central core, a rocky mantle, and a solid crust.


Earth is composed of four main layers, starting with an inner core at the planet’s center, enveloped by the outer core, mantle, and crust.

The inner core is a solid sphere made of iron and nickel metals about 759 miles (1,221 kilometers) in radius. There the temperature is as high as 9,800 degrees Fahrenheit (5,400 degrees Celsius). Surrounding the inner core is the outer core. This layer is about 1,400 miles (2,300 kilometers) thick, made of iron and nickel fluids.

In between the outer core and crust is the mantle, the thickest layer. This hot, viscous mixture of molten rock is about 1,800 miles (2,900 kilometers) thick and has the consistency of caramel. The outermost layer, Earth’s crust, goes about 19 miles (30 kilometers) deep on average on land. At the bottom of the ocean, the crust is thinner and extends about 3 miles (5 kilometers) from the seafloor to the top of the mantle.


Like Mars and Venus, Earth has volcanoes, mountains, and valleys. Earth’s lithosphere, which includes the crust (both continental and oceanic) and the upper mantle, is divided into huge plates that are constantly moving. For example, the North American plate moves west over the Pacific Ocean basin, roughly at a rate equal to the growth of our fingernails. Earthquakes result when plates grind past one another, ride up over one another, collide to make mountains, or split and separate.

Earth’s global ocean, which covers nearly 70% of the planet’s surface, has an average depth of about 2.5 miles (4 kilometers) and contains 97% of Earth’s water. Almost all of Earth’s volcanoes are hidden under these oceans. Hawaii’s Mauna Kea volcano is taller from base to summit than Mount Everest, but most of it is underwater. Earth’s longest mountain range is also underwater, at the bottom of the Arctic and Atlantic oceans. It is four times longer than the Andes, Rockies and Himalayas combined.


The Earth’s atmosphere is a remarkable and dynamic envelope of gases that surrounds our planet. Spanning from the Earth’s surface to the outer reaches of space, the atmosphere plays a vital role in sustaining life and providing protection against the harshness of the cosmos.

Composition: Composed primarily of nitrogen (78%), oxygen (21%), and traces of other gases such as argon, carbon dioxide, and neon, the atmosphere creates a balanced and breathable mixture of gases. These essential components support the diverse array of life forms that thrive on Earth.

Layers: The atmosphere is divided into distinct layers, each with its own unique characteristics and functions. Starting from the Earth’s surface and moving upward, we encounter the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. These layers vary in temperature, density, and composition, creating a complex atmospheric system.

Troposphere: The troposphere is the lowest layer of the atmosphere, extending from the Earth’s surface up to an average height of about 8 to 15 kilometers. This layer is where weather phenomena occur, and it is characterized by a decrease in temperature with increasing altitude.

Stratosphere: Above the troposphere lies the stratosphere, extending from the tropopause to an average height of about 50 kilometers. The stratosphere hosts the ozone layer, a region where ozone molecules absorb harmful ultraviolet (UV) radiation from the Sun, shielding life on Earth from its detrimental effects.

Mesosphere: The mesosphere extends from the stratopause to an average height of about 85 kilometers. In this layer, temperatures decrease with altitude, reaching extremely cold temperatures. The mesosphere is where meteors burn up upon entering Earth’s atmosphere, creating mesmerizing streaks of light known as shooting stars.

Thermosphere: The thermosphere is the layer above the mesosphere, extending upward to about 600 kilometers or more. Despite its high altitude, this layer experiences temperatures that can reach thousands of degrees Celsius due to the absorption of intense solar radiation. The thermosphere is also home to the International Space Station (ISS) and other satellites.

Exosphere: The outermost layer of the Earth’s atmosphere is the exosphere, where the atmosphere gradually transitions into the vacuum of space. The gases in this region are sparse and continuously escape into space due to the weak gravitational pull of Earth.


Our planet’s rapid rotation and molten nickel-iron core give rise to a magnetic field, which the solar wind distorts into a teardrop shape in space. (The solar wind is a stream of charged particles continuously ejected from the Sun.) When charged particles from the solar wind become trapped in Earth’s magnetic field, they collide with air molecules above our planet’s magnetic poles. These air molecules then begin to glow and cause aurorae, or the northern and southern lights.

The magnetic field is what causes compass needles to point to the North Pole regardless of which way you turn. But the magnetic polarity of Earth can change, flipping the direction of the magnetic field. The geologic record tells scientists that a magnetic reversal takes place about every 400,000 years on average, but the timing is very irregular. As far as we know, such a magnetic reversal doesn’t cause any harm to life on Earth, and a reversal is very unlikely to happen for at least another thousand years. But when it does happen, compass needles are likely to point in many different directions for a few centuries while the switch is being made. And after the switch is completed, they will all point south instead of north.​


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AP Calculus AB 2021 FRQ – Question No.1

The density of a bacteria population in a circular petri dish at a distance r centimeters from the center of the dish is given by an increasing, differentiable function f , where f (r) is measured in milligrams per square Values of f (r) for selected values of r are given in the table above.

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