As far as we know, Earth is the only planet that’s capable of sustaining life. Earth, our home planet, is the most beautiful one in the entire solar system. It looks like a bright blue gem with white clouds dazzling over its blue, green, and brown surface. Earth is the third planet from the sun. Earth is the only planet that has a single moon. Our Moon is the brightest and most familiar object in the night sky. It is our only natural satellite. Unlike other planets like Saturn and Jupiter, Earth has no rings.
Earth is distinguished from all other planets in the Solar system by two very important factors:
Planet Earth is approximately 5 billion years old. Life began on Earth 200 million years ago. Therefore, life has been on Earth for a long time now. The name Earth is at least 1000 years old. Every other solar system planet was named for a Greek or Roman deity, but for at least a thousand years, some cultures have described our world using the Germanic word “earth,” which means simply “the ground.” Do you know we had a twin once? Scientists believe that there were two planets sharing orbit for millions of years until they at one time collided. Earth absorbed Theia on collision and acquired the gravity we now utilized on a day-to-day basis.
Size and Distance
Earth has a radius of 3,959 miles. It is the fifth largest planet in our solar system. It is only slightly larger than Venus and is the largest and densest of the four terrestrial or rocky inner planets in the Solar System.
With an average distance of 93 million miles (150 million kilometers), Earth is exactly one astronomical unit away from the Sun because one astronomical unit is the distance from the Sun to Earth. The astronomical unit is used to measure distances throughout the solar system. It is an easy way to quickly compare the planets’ distance from the sun. For example, Jupiter is 5.2 astronomical units from the sun and Neptune is 30.07 astronomical units from the sun.
To measure long distances, astronomers use ‘light-years’ or the distance that light travels in a single Earth year which is equal to 63, 239 astronomical units. For example, Proxima Centauri, the closest star to the Sun, is 4.25 light years away from Earth. It takes about eight minutes for light from the Sun to reach our planet.
Earth’s orbit
Like all other celestial bodies in the solar system, Earth also orbits around the sun. Earth’s orbit is the trajectory along which Earth travels around the sun. Earth’s orbit is not a perfect circle; it is shaped more like an oval or an ellipse. Over the course of a year, Earth moves sometimes closer to the sun and sometimes farther away from the sun. Earth’s closest approach to the sun, called perihelion, comes in early January and is about 91 million miles (146 million km), just less than 1 astronomical unit. This happens 2 weeks after the December Solstice when it is winter in the Northern Hemisphere. The farthest from the sun, Earth gets is called aphelion. It comes in early July and is about 94.5 million miles (152 million km), just over 1 astronomical unit. This comes 2 weeks after the June Solstice when the Northern Hemisphere is enjoying warm summer months.
Tilting of Earth’s Axis
Did you know that Earth is titled? Earth leans a little to one side. Earth’s axis is an imaginary line running from the North Pole to the South Pole. Earth spins around its tilted axis. Earth’s axis of rotation is tilted 23.4 degrees with respect to the plane of Earth’s orbit around the Sun, and because of this tilt, we experience day/night and yearly four seasons.
Rotation
The spinning motion of Earth is called rotation. Thanks to the Earth’s rotation, at any given moment, we are all moving at a speed of about 1,674 kilometers an hour. It causes the cycle of day and night. Earth completes its rotation about its axis in approximately 24 hours. We call this period of time one Earth Day. During one day, half of Earth always faces towards the sun, and the other half faces away from the sun. It’s daytime on the part of the Earth that faces the Sun and it is nighttime on the part of the Earth that faces away from the Sun. The imaginary line that divides Earth’s day side from the night side is called the terminator.
Revolution
The movement of the Earth around the sun in a fixed path is called the revolution. The Earth revolves from west to east i.e. in an anticlockwise direction. Earth makes a complete revolution around the sun every 365.25 days — one year. That extra quarter of a day presents a challenge to our calendar system, which counts one year as 365 days. To keep our yearly calendars consistent with our orbit around the Sun, every four years we add one day. That day is called a leap day, and the year it's added to is called a leap year.
As the Earth orbits the sun, its tilt causes the seasons. It is summer on the part of the Earth tilted towards the Sun. It is winter on the part of the Earth tilted away from the Sun. During this part of the year, the northern hemisphere is tilted towards the sun, and the southern hemisphere is tilted away. With the sun higher in the sky, solar heating is greater in the northern hemisphere producing summer there. Less direct solar heating produces winter in the southern hemisphere. Six months later, the situation is reversed. The hemisphere tilted toward the sun has more daylight hours than the hemisphere tilted away from the sun. The combination of direct rays and more hours of sunlight heats the surface more than at any other time of the year.
On two days each year, the sun reaches its greatest distance north or south of the equator. Each of these days is known as a solstice. This usually happens around June 21st (Summer Solstice) and December 21st (Winter Solstice). These days are known as solstices. On these solstices, the rays of the Sun shine directly on one of the two Tropics. During the June (summer) Solstice the rays of the Sun shine directly on the Tropic of Cancer. During the December (winter) Solstice, the Sun’s rays shine on the Tropic of Capricorn.
As the Earth moves around its orbit, it reaches two points during the year where the tilt of its axis causes it to be straight relative to the Sun, neither hemisphere is tilted towards the Sun. This happens during autumn and spring. On those two days, the noon sun is directly overhead at the equator. Each of these days is known as an equinox, meaning “equal night.” During an equinox, the length of nighttime and daytime is about the same. This happens on approximately March 20th and September 22nd.
Solar versus Sidereal Day
A sidereal day is the time it takes for the Earth to rotate about its axis so that the distant stars appear in the same position in the sky. This is for about 23.9344696 hours. A solar day is the time it takes for the Earth to rotate about its axis so that the Sun appears in the same position in the sky. The sidereal day is 4 minutes shorter than the solar day. This is 24 hours.
Structure of Earth
Scientists study seismic waves to understand the structure of the Earth’s interior. There are two types of seismic waves – a shear wave and a pressure wave. Waves that won’t travel through the liquid is called shear wave; waves that move through both liquid and solids are called pressure waves. These waves reveal that there are three layers within the Earth – the crust, the mantle, and the core. These are classified by the different types of rocks and minerals that make them up. Also, each of Earth’s layers has unique properties based on both their composition and depth.
The crust is the outermost and thinnest layer of the Earth’s surface. The temperature of the crust is around 22°C and it is a solid. The crust is divided into two types – oceanic crust (sima) and continental crust (sial). The land is made of continental crust, which is 22 miles thick and made mostly from a rock called granite, sedimentary rocks, and metamorphic rocks. The layer beneath the ocean bed is made of oceanic crust, which is about 3 to 6 miles thick and made mainly from a rock called basalt.
The mantle is the layer right below the crust is the mantle. The mantle has both solid and liquid parts. The mantle is the largest layer within the earth, spanning about 1800 miles. The composition of the mantle isn’t that much different from that of the crust. The elements in it are largely the same, just with more magnesium and less aluminum and silicon. The increasing heat melts rocks in the mantle, forming magma.
The core is the innermost layer of the earth. The core of the earth splits into two layers – inner and outer. Both the outer and inner layers of the core are composed of iron and nickel, but the outer layer is a liquid and the inner layer is solid.
Earth's Surface
Like Mars and Venus, Earth has volcanoes, mountains, and valleys. Earth's lithosphere, which includes the crust and the upper mantle, is divided into huge plates that are constantly moving. The plates are like the skin of the planet and are also known as tectonic plates. Directly under the lithosphere is another layer called the asthenosphere. It’s a flowing area of molten rock. The center of the Earth gives off constant heat and radiation which heats the rocks and melts them. The tectonic plates are floating on top of the molten rock and moving around the planet. It’s like ice floating at the top of your soda. When the continents and plate change their position, it’s called continental drift. Tectonic plates are constantly moving around the planet. When we say constantly moving, we're talking centimeters each year. You can’t really feel it except when there is an earthquake.
Atmosphere
Down here on Earth, we are protected by a layer of air that covers the entire Earth. It is like our shield from the harmful rays of the sun. This layer of air consists of different gases. Earth's atmosphere is about 300 miles (480 kilometers) thick, but most of it is within 10 miles (16 km) of the surface. Air pressure decreases with altitude. There is also less oxygen to breathe in higher altitudes.
Near the surface, Earth has an atmosphere that consists of 78 percent nitrogen, 21 percent oxygen, and 1 percent other gases such as argon, carbon dioxide, and neon. High above the planet, the atmosphere becomes thinner until it gradually reaches space.
The atmosphere affects Earth's long-term climate and short-term local weather and shields us from much of the harmful radiation coming from the Sun. It also protects us from meteoroids, most of which burn up in the atmosphere, seen as meteors in the night sky, before they can strike the surface as meteorites. It traps heat, making Earth a comfortable temperature and the oxygen within our atmosphere is essential for life.
The atmosphere is divided into five layers – the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere.
Over the past century, air pollutants like greenhouse gases released into the atmosphere have been causing climatic changes like acid rain, global warming and ozone holes which are threatening the potential of life on our planet.
Gravity
Did you ever think why a ball comes back down when you throw it into the air, instead of just traveling higher and higher? It is because of ‘gravity’. If gravity does not exist, we wouldn’t be able to stay put on Earth’s surface and would fall right off of Earth’s surface and float away. Gravity is the force of attraction that pulls together all. The bigger an object higher will be its gravitational pull. This means big objects like planets and stars have a stronger gravitational pull.
Sir Isaac Newton discovered gravity about 300 years ago. The story is that Newton saw an apple fall out of a tree. When this happened he realized there was a force that made it occur, and he called it gravity. The gravitational pull of an object also depends on how close the object is to the other object. For example, the Sun has much more gravity than Earth, but we stay on Earth’s surface instead of being pulled to the Sun because we are much closer to Earth. Gravity is also the force that keeps the Earth in orbit around the Sun, as well as helping other planets remain in orbit. The high and low tides in the ocean are also caused by the moon’s gravity.
And did you know that our weight is based on gravity? Weight is actually the measurement of the force of gravity pulling on an object. For example, how hard gravity is pulling us towards the earth’s surface determines our weight. If we travel to other planets, our weight will vary. If we go to a smaller planet, we will weigh lighter; and if we go to a larger planet, we will weigh heavier. The moon’s gravity is 1/6 of Earth’s gravity, so objects on the moon will weigh only 1/6 of their weight on Earth. So if a person/object weighs 120 pounds here on Earth, it would weigh about 20 pounds on the moon.
