Located nearly 93 million miles, or 1 Astronomical Unit (AU) away from the Sun, the planet Earth is the largest of the terrestrial planets—or the four rocky planets closest to the Sun in our solar system. (The rest are gas giants).
Earth is the only planet in our solar system that is not named after Greek and Roman gods and goddesses. The name Earth is at least 1,000 years old and is an English/German word that simply means ground.
It takes roughly 24 hours for the Earth to complete a full rotation, but that is gradually slowing. This deceleration is almost imperceptible, but has the effect of lengthening our days. It is happening so slowly, though, that it could be 140 million years before the length of a day increases to 25 hours.
The first photo of Earth from space was taken on October 24, 1946, by a V-2 test rocket launched from New Mexico. From there, we turned our attention to the closest body in our solar system: the Moon.
While the Moon, Earth’s only natural satellite, is only the fifth largest in our solar system, in terms of percentage of the size of the body it orbits, the Moon is the largest satellite of any planet in our solar system.
Astronomers hypothesize that the Moon was formed 4.5 billion years ago, not long after Earth, from the debris left over after a giant impact between Earth and a Mars-sized body.
The first unmanned spacecraft to reach the Moon was one from the Soviet Union’s Luna program in 1959, and the first manned lunar landing being Apollo 11 in 1969. The last manned spacecraft was Apollo 17 in 1972, and since then, the Moon has only been visited by unmanned spacecraft.
NASA started to plan to resume manned missions for the construction of a lunar base by 2024, but the program was cancelled in favor of a manned asteroid landing by 2025 and a manned Mars orbit by 2035.