The solar system consists of eight planets and five dwarf planets rotating around a nearby star, the sun. The sun's massive amount of gravity keeps the solar system together. Tracking the movements of the Earth and moon can be part of a stargazing hobby, or part of scientific research into the way the solar system works.
The sun is a giant star, and is made of very hot gas fueled by nuclear reactions. The sun is the closest star to the Earth, and is one of many stars in the Milky Way galaxy. A galaxy is a massive collection of stars, planets and other materials that is bound together by gravity.
The sun moves in a circular orbit around the center of the Milky Way galaxy, traveling at about 137 miles per second. Even at that high speed, it takes the sun 230 million years to complete one revolution of the galactic center.
The Earth is the third planet from the sun, orbiting it at a distance of about 149 million km. The Earth is constantly spinning like a top around its axis, and it takes one day for it to complete a spin. This spinning is what causes the Earth's day and night cycle.
The Earth is also constantly traveling around the sun in an oval-shaped orbit. It takes one year to complete a revolution. Since the orbit is not a perfect circle, the Earth is at times closer or farther from the sun, which makes it cooler or warmer on the planet. This, combined with the tilt of the Earth's axis, is the cause of Earth's seasons.
The moon is Earth's natural satellite, meaning it revolves around the Earth in the same way the Earth travels around the sun. The moon is about 384,000 km from the Earth and it takes about 27 days to make one trip around the Earth. The moon spins around on its own axis in the same amount of time it takes to make one revolution of the Earth. This is why the same side of the moon always faces the Earth. The moon's travel around the Earth impacts Earth's tides and causes eclipses when certain alignments of the Earth, sun and moon occur.
The other major planets in the solar system are Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune. Mercury takes 88 Earth days to travel around the sun and Venus takes 226 days. Those are the only planets with shorter years than Earth. Mars takes 694 Earth days to travel around the sun, Jupiter takes nearly 12 years, Saturn takes nearly 30 years, Uranus takes about 84 years and Neptune takes more than 164 years.
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