Characteristics of the Earth
by Ron Kurtus (revised 19 June 2010)
Our planet Earth is a rotating sphere that orbits the Sun. The Earth's axis of rotation is at a constant tilt with respect to its orbit around the Sun, resulting in the change of seasons.
The physical characteristics of the Earth include its size and composition. The Earth also has gravity and magnetic force fields.
Questions you may have include:
- What is the Earth's relationship with the Sun?
- What are the Earth's physical characteristics?
- What do the Earth's force fields do?
This lesson will answer those questions.
Earth's relationship with Sun
The Earth is the third planet from the Sun, with Mercury and Venus being closer. Its unique position gives the Earth a temperate climate, allowing the chemical reactions necessary to sustain life. But over the years, the climate has varied enough to melt the ice caps near the North and South poles, or to cause glaciers to move south in an Ice Age.
Revolves around the Sun in an orbit
The Earth goes around the Sun in a counterclockwise orbit, taking a year or about 365 days to make one revolution. The orbit is an ellipse, so the Sun is slightly off-center.
The Earth orbits the Sun
The average distance from the Sun is 149,597,890 kilometers (92,955,820 miles). This distance is so great that it takes light about 8 minutes to reach the Earth from the Sun.
Spins on its axis
The Earth spins or rotates on its axis in a counterclockwise motion, as viewed from above the North Pole. It makes one revolution in 24 hours.
The spinning of the Earth on its axis is what causes the Sun to appear to move across the sky. The effect the apparent movement of the Sun is similar to looking out the window of a moving car, where things outside appear to be moving past you.
Tilt affects seasons
The Earth's axis also tilts with respect to the Sun, causing the changes of the seasons. In summer, the Earth is tilted such that the Sun falls more directly, while in winter the Sun looks lower in the horizon and the light comes at a glancing angle. The tilt of the Earth also causes the summer days to be longer than the nights. In winter the days are shorter and there is less light to heat the ground.
The light from the Sun is more direct in summer than
in winter at the same latitude
The further north you go, the more the effect of the tilt of the Earth is apparent. Above the Arctic Circle, daylight can be seen for a full 24 hours in the summer or night can be 24 hours in the winter. That is why they call the area "the land of the midnight sun."
(See Motion of the Earth for more information.)
The Earth has only one moon, while Mars has two moons and Jupiter has 9 moons.
(See The Moon for more information.)
Physical characteristics of Earth
Physical characteristics include shape, size and composition.
Spherical in shape
Just as the Sun and Moon appear as spheres, so too is the Earth spherical in shape. To people on Earth, the planet appears to be generally flat (not counting for hills and valleys), but in reality the surface of the Earth has a slight curve. This can be noticed when looking out on a large lake or the ocean and seeing a ship come up along the horizon.
Ship coming up over the horizon
The shape of the Earth has been proven by ships circling the Earth, as well as from pictures of the Earth taken from the space vehicles.
View of Earth from space
Its shape is actually slightly flattened at the poles.
The diameter of the Earth at the equator is 12,756 km (7,926 miles), and its circumference or distance around the Earth at the equator is 40,075 km (24,901 miles).
The composition of the Earth consists of the solid and liquid portion and the atmosphere or gaseous portion.
Solid and liquid
The percentage composition of the Earth's solid and liquid materials (by mass) is:
Oxygen is chemically combined with many substances to produce liquid and solid compounds. Although water (H2O) is a dominant compound on Earth, Hydrogen is not listed above because of its small mass.
Silicon Dioxide (SiO2) is sand, and that compound makes up a large portion of the Earth's mass. Much of the Iron is in the Earth's core and is responsible for the Earth's magnetic field.
Although most people think air is mainly Oxygen, the atmosphere of the Earth actually consists of 79% Nitrogen (N2), 20% Oxygen (O2) and 1% of other gases such as Carbon Dioxide (CO2).
The Earth has two major force fields: gravity and magnetism.
Gravity is the force at a distance that attracts objects of mass toward each other. The force of gravity from the Earth holds down our atmosphere, oceans and everything else.
Some planets and moons that have less gravity than Earth have lost their atmosphere because it wasn't sufficient to hold the gas close to the surface.
When you throw a ball or shoot a bullet upward, it will slow down due to the Earth's gravity, until it finally falls back to the ground. You would have to shoot the object at 40,248 km/hr (25,009 mph) for it to escape the Earth's gravity and fly out into space. This is called the Earth's Escape Velocity.
(See the lesson on Gravity for more information.)
The Earth is like a giant magnet with a magnetic pole near the North Pole and the opposite near the South Pole. The north pole of a magnet seeks the North Magnetic Pole. Through the ages, indications are that the poles switched directions. No one is sure why this happened.
The rotation of the Earth and the fact that the core of the Earth is made of iron are major factors in creating the magnetic field.
One thing the magnetic field does is to attract charged particles that have been emitted from the Sun. The focusing of these particles at the poles may help to prevent us from being harmed by the high energy particles.
These particles cause the air in the upper atmosphere to glow. This is called the northern lights (aurora borealis) or southern lights (aurora australis).
(See the lesson on Magnetism for more information.)
The Earth is a rotating sphere that orbits the Sun. The axis of rotation of the Earth is at a constant tilt with respect to its orbit around the Sun, thus resulting in the change of seasons. Iron and oxygen make up 65% of the Earth's mass. The Earth also has gravity and magnetic force fields.
This is the only Earth we have. Keep it a good place to live.
Resources and references
Earth Facts and Figures - From NASA
The Earth - Details from Nine-Planets website
Earth's Atmosphere - From the University of Tennessee Department of Physics
Space Weather - News about Earth-Sun environment
Earth's Magnetic Field Is Fading by John Roach for National Geographic News September 9, 2004
Books on the Earth
Questions and comments
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Characteristics of the Earth