EARTH

INTRODUCTION TO THE EARTH
The Earth is the third planet from the Sun in our Solar System. It is the planet we evolved on and the only planet in our Solar System that is known to support life. 

SIZE
The Earth is about 7,926 miles (12,756 km) in diameter. The Earth is the fifth-largest planet in our Solar System (after Jupiter, Saturn, Uranus, and Neptune).

Eratosthenes (276-194 BC) was a Greek scholar who was the first person to determine the circumference of the Earth. He compared the midsummer's noon shadow in deep wells in Syene (now Aswan on the Nile in Egypt) and Alexandria. He properly assumed that the Sun's rays are virtually parallel (since the Sun is so far away). Knowing the distance between the two locations, he calculated the circumference of the Earth to be 250,000 stadia. Exactly how long a stadia is is unknown, so his accuracy is uncertain, but he was very close. He also accurately measured the tilt of the Earth's axis and the distance to the sun and moon. 

The Earth and the moon. Photo taken by NASA's Galileo mission in 1990.

THE MOON
The Earth has one moon. The diameter of the moon is about one quarter of the diameter of the Earth.

The moon may have once been a part of the Earth; it may have been broken off the Earth during a catastrophic collision of a huge body with the Earth billions of years ago. 

MASS, DENSITY, AND ESCAPE VELOCITY
The Earth's mass is about 5.98 x 10²⁴ kg.

The Earth has an average density of 5520 kg/m³ (water has a density of 1027 kg/m³). Earth is the densest planet in our Solar System.

To escape the Earth's gravitational pull, an object must reach a velocity of 24,840 miles per hour (11,180 m/sec). 

LENGTH OF A DAY AND YEAR ON EARTH

Earth rising over the moon. Photo taken by NASA's Apollo 8 mission.

Each day on Earth takes 23.93 hours (that is, it takes the Earth 23.93 hours to rotate around its axis once - this is a sidereal day). Each year on Earth takes 365.26 Earth days (that is, it takes the Earth 365.26 days to orbit the Sun once).

The Earth's rotation is slowing down very slightly over time, about one second every 10 years. 

THE EARTH'S ORBIT

Planet-Sun Orbital DiagramLabel the aphelion (farthest point in orbit) and perihelion (closest point in orbit) of a planet in orbit. Answers

On average, the Earth orbits 93 million miles (149,600,000 km) from the Sun. This distance is defined as one Astronomical Unit (AU). The Earth is closest to the Sun (this is called perihelion) around January 2 each year (91.4 million miles = 147.1 million km); it is farthest away from the Sun (this is called aphelion) around July 2 each year (94.8 million miles = 152.6 million km).

 

Orbital Eccentricity
The Earth has an orbit that is close to being circular; its orbital eccentricity is 0.017. (Eccentricity is a measure of how an orbit deviates from circular. A perfectly circular orbit has an eccentricity of zero; an eccentricity between 0 and 1 represents an elliptical orbit.) 

THE EARTH'S AXIS TILT AND THE SEASONS
The Earth's axis is tilted from perpendicular to the plane of the ecliptic by 23.45°. This tilting is what gives us the four seasons of the year: Summer, Spring, Winter and Autumn. Since the axis is tilted, different parts of the globe are oriented towards the Sun at different times of the year. This affects the amount of sunlight each receives. For more information on the seasons, click here. 

SPEED
At the equator, the Earth's surface moves 40,000 kilometers in 24 hours. That is a speed of about 1040 miles/hr (1670 km/hr). This is calculated by dividing the circumference of the Earth at the equator (about 24,900 miles or 40,070 km) by the number of hours in a day (24). As you move toward either pole, this speed decreases to almost zero (since the circumference at the extreme latitudes approaches zero).

The Earth revolves around the Sun at a speed of about 30 km/sec. This compares with the Earth's rotational speed of approximately 0.5 km/sec (at middle latitudes - near the equator).

For more information on the speed of the Earth, click here. 

The size of the atmosphere in this illustration is greatly exaggerated in order to show the greenhouse effect. The Earth's atmosphere is about 300 miles (480 km) thick, but most of the Earth's atmosphere is within 10 miles (16 km) of the Earth's surface.

TEMPERATURE ON EARTH
The temperature on Earth ranges from between -127°F to 136°F (-88°C to 58°C; 185 K to 311 K). The coldest recorded temperature was on the continent of Antarctica (Vostok in July, 1983). The hottest recorded temperature was on the continent of Africa (Libya in September, 1922).

The greenhouse effect traps heat in our atmosphere. The atmosphere lets some infrared radiation escape into space; some is reflected back to the planet.

For more information on the greenhouse effect, click here. 

ATMOSPHERE
The Earth's atmosphere is a thin layer of gases that surrounds the Earth. It is composed of 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide, and trace amounts of other gases.

The atmosphere was formed by planetary degassing, a process in which gases like carbon dioxide, water vapor, sulphur dioxide and nitrogen were released from the interior of the Earth from volcanoes and other processes. Life forms on Earth have modified the composition of the atmosphere since their evolution.