![]() The moon rocksĭuring the Apollo missions, astronauts brought back 842 pounds of lunar rock, sand, and dust to Earth, allowing scientists to scrutinize the moon’s surface. Even the far side of the moon receives sunlight-we just can’t see it-and the section of the moon illuminated on any given day shifts depending on the moon’s position. (Read about the phases of the moon and which month hosts a Sturgeon moon.)Įven though the same side of the moon always faces Earth, there’s no true “dark side,” as many mistakenly call our lunar orb’s far side. This sequence is a combined result of the moon’s changing position relative to both the Earth and sun, requiring 29.5 days to complete one full lunar cycle. Viewed from Earth, the amount of the moon illuminated by the sun appears to wax and wane, creating the familiar cycle from new moon to crescent to full. Known as synchronous rotation, this celestial dance means that the same lunar face always peers down at us. Photograph courtesy NASAĮvery 27.3 days, the moon makes its way around our planet while also completing one turn on its axis. ( Learn more about the solar system’s many moons.)Ī full moon, captured in 2005 by a crew member of the International Space Station, appears to be rolling along atop Earth's deep-blue stratosphere. It spans about 2,160 miles across, nearly a third the width of Earth, orbiting at an average of 30 Earth-widths away. There are more than 190 moons orbiting the planets and asteroids in our solar system, and Earth’s moon is the fifth largest of the lot. ( Explore 50 years of visits to the moon.) Luna’s shifting face It’s the only other world humans have yet set foot upon-and a prime candidate for future visits. Generations of astronomers have studied this small airless world, from its pockmarked surface to its dense iron core. The moon also preserves many of its ancient features: Unlike Earth, it doesn’t have plate tectonics to continually reface the landscape, nor does it have wind and rain wearing down ancient rocks. This early formation and close ties with young Earth makes the moon one of the most promising places to explore the birth and development of our solar system and home planet. One such collision, between Earth and a Mars-sized orb, likely flung molten rock out into space, some of which coalesced and cooled to form what we now know as the moon. It formed some 4.5 billion years ago, when the solar system was in its infancy and comets, meteors, and asteroids ricocheted throughout. It has circled our planet for billions of years, since before the first sparks of life flickered in the oceans-before Earth was even cool enough to have oceans.īut its seemingly tranquil position in modern night skies stems from a remarkably tumultuous past. The moon, Earth’s sole natural satellite, is our constant companion.
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