The Earth is the third planet from the sun. About 71% of its surface is covered by water; the rest by land. The planet has a significant magnetic field. It is orbited by one satellite, the Moon.

The Earth’s atmosphere is primarily Nitrogen (N₂), 78.08 %; Oxygen (O₂), 20.95 %; and Argon, 0.93%; with trace amounts of carbon dioxide and water vapor. The average atmospheric pressure on the surface is 29.91 in.Hg. The temperature on the surface ranges from –127° F to 136° F; the average temperature is 59° F.

Our planet completes its elliptical orbit around the sun in an average solar year, 365.24219 days. Its average distance from the sun is 80,777,537.8 n.mi. For astronomical purposes, this distance is called an astronomical unit (1.0 AU). The Earth’s axis is tilted 23.45 deg away from the perpendicular to its orbital plane. This tilt produces the seasonal climate variations.

The tilt of the Earth’s axis causes different parts of the planet to vary in the way they are oriented towards the sun at different times of the year. During summer, the Sun’s rays hit the Earth at a more direct angle than during winter, and the time between sunrise and sunset is longer. During winter, the sun rays hit the Earth at a shallower angle, and the hours of daylight are shorter.

In the northern hemisphere, summer begins on June 21 or 22, the summer solstice, and winter begins on December 21 or 22, the winter solstice. Winter lasts until the beginning of spring, which in the northern hemisphere occurs on the vernal equinox, March 20 or 21. On this day, and on the autumnal equinox, September 22 or 23, day and night are of equal duration. The autumnal equinox marks the beginning of autumn.

In the southern hemisphere the seasons are reversed. South of the equator, winter starts on June 21 or 22, and summer starts on December 21 or 22.

Since it is useful for calendars to have a whole number of days for the year, the length of a common year in the Gregorian calendar is 365 days, and leap years have 366 days. In leap years, February has 29 days instead of the usual 28. Leap years occur at specified intervals (every four years, except years that are divisible by 100 are not leap years, unless they are also divisible by 400, in which case they are leap years). The Gregorian calendar year has an average length of 365.2425 days, which closely approximates the duration of the solar year.

There are two zones of high-energy charged particles at high altitudes, the Van Allen radiation belts. The particles are held in Earth’s magnetic field, from 400 to 40,000 miles above the surface of the Earth. The inner belt, consisting mostly of protons, is centered at an altitude of about 3,700 miles. The outer zone has helium ions and is centered about 12,500 miles above the surface of the Earth.

The Moon orbits the Earth about once a month (every 29 days, 12 hours, 44 minutes, 2.9 seconds), the same as its period of rotation. For this reason, it always presents the same side to the Earth. The Moon is airless and has a diameter of 2162 miles, about one quarter of the Earth diameter. Its surface gravity is one sixth that of Earth.

The average distance from the Earth to the Moon is 238,857 mi., about 30 times Earth’s diameter. Two NASA astronauts, Neil Armstrong and Edwin Aldrin, first landed on the Moon July 20, 1969, after a four day voyage in an Apollo spacecraft.

The size of the Earth was estimated by the Greek Erastothenes circa 225 BC. Using sun declination observations at Alexandria and Syene (Aswan), located a known distance to the south, the assumption of a spherical Earth, and principles of angular geometry, he estimated the Earth’s radius at 40,000 stadia, or 24,000,000 ft.

By the nineteenth century, it was known that the Earth is not a true sphere. In 1830, its shape was approximated by the English astronomer Sir George Biddell Airy as an ellipsoid with an equatorial radius of 20,923,764 ft. A current estimate, defined by the WGS 84 ellipsoid, gives an Earth equatorial radius of 20,925,646.3 ft and a polar radius of 20,855,486.6 ft.

Earth’s total surface area is 196,950,000 sq. mi. The area covered by the oceans is 139,480,000 sq. mi. Total land area is 57,470,000 sq. mi.

At the Earth’s center is the core, consisting of a solid inner core and a fluid outer core. The flow of molten iron in the outer core generates Earth’s magnetic field. Above the core lies the mantle, which is relatively flexible. The outer layer of the Earth is the crust, relatively light and brittle.

The Earth’s crust is about 6.5 miles thick beneath the oceans, and about 25 miles thick under the continents. The surface is divided into large plates that, over time, change in size and position. Sixty-five million years ago the Earth looked quite different than it does today. There are sites of intense geologic activity at the edges of these plates, where they move against each other. This geologic activity takes the form of mountain building, earthquakes, and volcanoes.

The land masses of the Earth are composed of seven continents: North America, South America, Africa, Europe, Asia, Australia, and Antarctica. The highest point on Earth, 29,082 ft. above sea level, is at Mount Everest in the Himalaya Mountains, Nepal. The lowest land on the surface of the Earth, 1,371 ft. below sea level, is at the shores of the Dead Sea, in Israel and Jordan.

The Earth’s oceans are the Atlantic, Pacific, Indian, Arctic, and Southern oceans. There are several seas, bodies of water partly enclosed by land; among these are the Caribbean, Mediterranean, Bering, Coral, and South China seas.

Terrain types vary widely and include forest, jungle, desert, savannah, tundra, swamp, marsh, and mountain. The circulation of air in the atmosphere, and the circulation of ocean currents, affect the weather in various parts of the planet.

Water moves from the oceans to the atmosphere through evaporation, hence to the land through precipitation, and back to the oceans through the flow of rivers and streams. This movement of water is called the hydrologic cycle. It serves to keep a regular amount of water vapor in the atmosphere.

There are about 300,000 plant species and about 1,400,000 animal species on Earth. The plants fall into three broad groups: mosses, ferns, and flowering plants. Animals are grouped into vertebrates (mammals, birds, reptiles, amphibians, and fish); invertebrates (arthropods and mollusks); and protozoa (flagellates, amoebae, sporozoans, and ciliates). Animals and plants are eukaryotes, organisms whose cells have a nucleus. Algae, fungi and slime molds are also eukaryotes.

Many organisms on Earth are microscopic. These include protozoa, as well as viruses, which have no cellular structure, and archaea and bacteria, which are prokaryotes (cells without nucleus). There are more than ten million species of bacteria, and over five thousand virus species. Only a few hundred species of archaea have been identified, although it is likely that many more exist.

The Earth’s population of about 6,700,000,000 people live in 194 independent countries. All but two countries, the Vatican and Taiwan, are members of the United Nations, a global association of governments facilitating cooperation in international law, security, economic development, and social equity.

Planetary images source: NASA, except as noted.