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# How does gravity work?

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### Quick Answer

To the extent that scientists understand gravity, it is a product of geometry; gravity is a curvature in the space-time continuum, and the extent of the curvature is determined by the extent of an object's mass. The curvature around Earth makes the moon stay nearby, and the curvature around the sun causes the planets of our solar system to orbit it.

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### Full Answer

An analogy often used to help people understand gravity is this: place a large marble on a rubber sheet and observe how the sheet indents; the marble represents the sun and the indentation represents its gravitational pull. Place a smaller marble representing Earth on the sheet and observe how the indentation of the rubber sheet causes the smaller marble to roll toward the larger.

As of 2015, though, science's understanding of gravity has not yet been reconciled with its understanding of particle physics. Dividing the two topics is a threshold called the Planck length, which is extraordinarily small. Anything smaller than the Planck length is best understood through a theory known as quantum mechanics, while anything larger is best understood using Albert Einstein's theory of relativity. Until science has a unified field theory reconciling relativity and quantum mechanics, science cannot honestly claim a full understanding of gravity.

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Gravity is one of the fundamental forces of the universe, and it keeps us on Earth because it is relentlessly attractive. Every massive particle exhibits gravity by pulling closer to every other particle. The strength of this interaction is dependent on two quantities: the amount of mass and the distance between objects. It is possible to escape Earth's gravity if you have enough energy to speed up sufficiently.

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A low-mass star, like the sun, is a star that was created by the gravitational collapse of large molecular clouds. The collapse of these clouds causes the matter to be pulled through a disk-like center, which eventually forms into what is called a protostar.

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A high-mass protostar is a precursor to a star; it is a star in the nursery stage that is still fundamentally a high-mass cloud of gas. All stars must go through the nursery stage before becoming full-fledged stars. The nursery stage consists of passing through the nebula.

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Astrolabes work by presenting users with a graphic interface that makes it relatively easy to compute the positions of the sun, the moon and the stars. Astrolabes can be used for navigation as well as predicting the exact times for sunrise and sunset. Turned on its side, an astrolabe can be used as a mechanical calculator akin to a slide rule.

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