Astronomy is the study of celestial objects, which are objects in the sky, as opposed to terrestrial objects, which are found on Earth. Modern astronomy begins where Earth's atmosphere ends.
The Earth-Moon system
Earth has a single moon, the Moon. The Earth and Moon orbit each other around their common center of gravity. Because the center of gravity is inside the Earth, the Moon is said to orbit the Earth.
However, the Moon is about 27% the size of the Earth, which makes it the largest natural satellite relative to a planet in the solar system. For this reason, some astronomers have suggested that the Earth and Moon should be reclassified as a double planet.
In 1969, the Moon became the first astronomical body to be reached by a person. Since then, robotic probes have reached many other parts of the solar system.
Earth orbits around the Sun. It shares that orbit with several other celestial objects. Asteroid 2010 TK7 is Earth's only known Trojan object, which shares Earth's orbit at Lagrange point 4. Cruithne is an asteroid in 1:1 orbital resonance with the Earth.
The solar system
All objects in the solar system orbit around the Sun. At the time of writing, the solar system consisted of eight planets, five dwarf planets, an asteroid belt, and multiple clusters of asteroids and comets. Earth is the third planet from the Sun.
= Mercury =
The closest planet to the Sun is Mercury. Mercury can sometimes be seen at dawn or dusk, but it is usually lost in the Sun's glare.
Because it is tidally locked to the Sun, the same side of Mercury always faces the Sun, while the other side perpetually faces away from the Sun. Thus, on one side, Mercury's temperature is hot enough to melt lead. On the other side, temperatures are close to the temperature of interstellar space. However, astronomers have recently discovered evidence of liquid water in some of Mercury's craters.
= Venus =
Venus' caustic atmosphere is extremely high in greenhouse gases, which keep its surface much hotter than it would otherwise be. The same cloudy atmosphere gives Venus its brightness. Venus is unmistakable in the night sky. It can sometimes be seen in the early evening on the same side as the sunset, or just before dawn on the same side as the sunrise.
= Mars =
Mars is the fourth planet from the Sun, roughly twice as far from the Sun as the Earth. Mars can sometimes be seen in the night sky as a distinctive red object. Because Earth's orbital period is shorter than that of Mars, Mars appears to be going backwards in the sky whenever Earth passes it in its orbit.
= Asteroid belt =
The asteroid belt consists of hundreds of thousands of objects which range in size from dust all the way up to the largest known, Ceres, which is nearly 600 miles across and is classified as a dwarf planet. Half of the mass of all the objects in the asteroid belt is contained in just four asteroids: Ceres, Vesta, Pallas, and Hygiea.
= Jupiter =
Jupiter is the largest planet in the solar system. All of Earth would fit within the Red Spot, a storm in Jupiter's atmosphere which has lasted for as long as Jupiter has been observed from the Earth with telescopes. Jupiter can sometimes be seen as a bright yellow-white object in the night sky.
Unlike the inner planets, Jupiter is a gas giant which consists mostly of hydrogen, helium, and methane. There is no clear separation between its atmosphere and its surface.
= Saturn =
Saturn's spectacular rings make it a clearly visible white object in the night sky when it is above the horizon at night. Saturn is the most distant planet to be visible with the naked eye.
= Uranus =
Uranus' heavy methane ice composition gives it its distinctive cyan colour, although hydrogen and helium are more common in its atmosphere. It cannot be seen without a telescope.
Its unusual axial means that Uranus spins on its side when compared to the other planets of the solar system. It also spins backwards from most of the planets. Several theories about this exist, but the true explanation is not yet known.
= Neptune =
Neptune is the last of the gas giants. Its methane ice gives it a deep aquamarine blue colour. Like Uranus, it cannot be seen from Earth without a telescope.
= Pluto =
Pluto is a dwarf planet with an unusually elliptical orbit. Most times it is farther away from the Sun than Neptune, but sometimes it is closer. It was reclassified as a dwarf planet for several reasons, including that it has failed to clear its orbit of other objects.
Pluto and its satellite Charon could also be considered to be a double planet. Charon is even larger relative to Pluto than the Moon is relative to the Earth. Their common center of gravity is outside Pluto.
The scattered disk and the Oort Cloud
Many other objects exist in the solar system, including comets and asteroids outside the asteroid belt. Most are believed to have gathered together into bands similar to the asteroid belt.
The Oort Cloud extends all the way from the scattered disk into interstellar space. This limit is the place where the Sun's gravity is balanced by interstellar gravity. Voyager I is the first manmade object to leave the Oort Cloud and head out into interstellar space.
The Milky Way
The sun is just a single yellow dwarf star in the Milky Way, a galaxy which contains over 100 billion stars. Dwarf stars are so small in the stellar scale that even the closest yellow and red dwarf stars cannot be seen from Earth without a telescope.
The Milky Way is just a single galaxy in a universe filled with known galaxies. While there may be billions of galaxies in the observable universe, only a few thousand galaxies have been identified so far.