Chronicles the formation of Earth from solar system dust particles that coalesced and became one of the four rocky planets closest to the sun. Shows how scientists examine meteorites to determine the chemical composition of the dust grains that helped build Earth. Explains that scientists estimate Earth to be about 4.6 billion years old, the average age of most meteorites discovered. Describes the theory of the Iron Catastrophe, thought to have occurred almost 50 million years after Earth's formation, when internal heat from trapped radioactive elements and external heat from surface collisions caused the planet's iron to melt, sink, and form Earth's core. Tells how convection currents in Earth's core generate the planet's magnetic field and relates the migration of Earth's magnetic north pole. Looks at one theory of how the moon formed—a massive collision of Earth with a Mars-sized planetesimal produced debris that combined to form the moon some 50,000 years after Earth formed. Reveals the finding that water may have been present about 200,000 years after Earth formed and details one theory that Earth's water came from comets. Shows how scientists have tried to verify this theory through spectroscopy by examining and comparing the water in passing comets with that of Earth's water.
For many centuries, our ancestors thought the Earth was the center of the universe. Now, science lets us know that our ancestors were incredibly bad guessers. The Earth is located in the Orion Spur off the Perseus Arm in the Milky Way galaxy. Just where the Milky Way galaxy falls in the scope of the entire universe is still to be determined.
Earth is the largest terrestrial planet in the solar system, and is the third planet from the sun. Known as "Terra" by some, the planet Earth is home to millions of species and is the only known planet in the universe where life can exist.
The planet is an oblate sphere composed of rocky material, like the other three terrestrial planets in our solar system. However, it is the only planet that has active tectonic plates. Earth is made up of 70 percent water and 30 percent land.
Due to the position of the Earth on its axis, the amount of sunlight that reaches an area varies during different times of the year. This axial position, along with the planet's orbit around the sun, creates seasonal change in climate from region to region. This is why there are some regions on Earth that experience extreme heat waves, while others experience long periods of time with no sunlight.
The Milky Way galaxy alone is estimated to measure 1,000 light years from side to side. If the Milky Way were an ocean, the Earth would not even be as large as a drop of water. But how large is the universe? This we do not know, because we only can estimate the size of the observable universe, which has a diameter 280 billion light years across. Beyond that is a mystery, especially as light can't travel fast enough to go past the observable universe and give us a picture.
Galaxies rotate, although they do it very slowly. Since galaxies rotate, the theory is the entire universe also rotates. The Milky Way galaxy looks like a spiral with two major arms. The Earth is located in a minor arm (Orion) that connects to a major arm (Perseus). The universe never really stays still. Also, it's now thought that other dimensions intersect with our 3-dimensional universe. So determining the size of the universe, to figure out where in it the Earth falls, is next to impossible to do.
Another theory is that the universe is growing, and will at some point--billions and billions of years into the future--collapse into one giant black hole. The gist is that Earth and the Milky Way keep shifting positions in the universe as the universe itself changes.
Throughout history, many religious doctrines taught that the Earth was the center of the universe--which would explain why God bothers so much with it. As it becomes ever clearer that we cannot find the center of the universe, let alone our place in it, our importance as the pinnacle of creation dwindles.
There has been an attempt by leading scientists at Princeton to make a 2-dimensional map of the universe, in which one end of the universe is where the Big Bang began and the other is the surface of the Earth. According to this map, the Milky Way galaxy is to the far right of the universe (see Resources). Also according to the map, the center of the universe is a galaxy known only as PSR-P1620-24.
温馨提示：答案为网友推荐，仅供参考