Introduction

Earth is the third planet from the Sun and the fifth largest. As seen from satellite images it is a beautiful, bright blue planet filled with patches of white water vapour (clouds) that cover about 40% of the earth.
Unlike all other planets, earth actually supports life. Well, it is uncertain whether any of the planets in this solar system has living organisms or used to have.

Scientists believe that earth was formed 4.5 billion years ago when many other planets formed. The earth takes 365 days (1 year) to orbit the Sun and completes one rotation in 24 hours (a day). It is known that the tilt of the earth's axis is 23.5 degrees, which accounts for the seasons on earth. The tilt causes certain parts of the planet to be heated differently thus causing seasonal changes.

Of all the inner planets (Mercury, Venus, Earth, and Mars), Earth is the only one that contains water in its liquid form. This is essential to the development of life on Earth, as scientists believe. Since water was so important for life on earth, scientists today use liquid water as one of the key factors when searching for life on other planets inside and out of our solar system.

Earth has an atmosphere made up of nitrogen, carbon dioxide, hydrogen, oxygen and water vapour. One of the more important layers in the atmosphere is the ozone layer. This layer helps to protect earth from dangerous radiation and reflects most of the radiation from entering earth. The ozone layer is approximately 10 miles up from the ground.

The surface of the earth is covered by land and water. Approximately 72% of the earth's surface is covered by water, the greatest concentration of this being found in the oceans. The remaining 28% forms the landmasses. The land is shaped with many mountains, hills and valleys. The highest sets of peaks are the Himalayan Mountains, with the tallest Mount Everest reaching some 29,035 feet into the air. Although Everest is the tallest peak starting from the land surface, the highest mountain can actually be found on the island of Hawaii. Mauna Kea stretches 13,796 feet above water and 19,680 feet underwater, for a total of 33,476 feet!

Earth's Moon

Moon Facts

Distance from Earth 384,400 km
Equatorial Radius: 1737.4 km
Volume 21,970,000 km3
Mass: 73,483,000,000,000,000,000,000 kg

The regular daily and monthly rhythms of Earth's only natural satellite, the Moon, have guided timekeepers for thousands of years. Its influence on Earth's cycles, notably tides, has also been charted by many cultures in many ages. More than 70 spacecraft have been sent to the Moon; 12 astronauts have walked upon its surface and brought back 382 kg (842 pounds) of lunar rock and soil to Earth.

The presence of the Moon stabilizes Earth's wobble. This has led to a much more stable climate over billions of years, which may have affected the course of the development and growth of life on Earth.

How did the Moon come to be? The leading theory is that a Mars-sized body once hit Earth and the resulting debris (from both Earth and the impacting body) accumulated to form the Moon. Scientists believe that the Moon was formed approximately 4.5 billion years ago (the age of the oldest collected lunar rocks). When the Moon formed, its outer layers melted under very high temperatures, forming the lunar crust, probably from a global "magma ocean.