The Super Moon, and Isaac Newton’s “Principia” and Law of Universal Gravitation
Is that full moon on the horizon closer to us, or is it just an optical illusion? Possibly both. Although the cause of the optical illusion is debatable, the fact that the moon is closer is not. Because of its elliptical orbit, when the moon comes closest to earth at perigee (also called a “super moon“), it is approximately 30,000 miles closer to earth than it will be from its farthest point (apogee). So, why does the moon travel in an elliptical path around us?
Last week, 327 years ago on July 5, 1687, Isaac Newton published Philosophiae Naturalis Principia Mathematica, or Mathematical Principles of Natural Philosophy. In it, Newton proposed his laws of motion that laid the foundation for classical mechanics. But he also established a mathematical principle using the inverse square law to explain his Law of Universal Gravitation, and thus the motion of the planets and their moons, including our own. Although Johannes Kepler established the laws of planetary motion, it was Isaac Newton who explained mathematically why the moon, and other celestial bodies in the solar system, including earth, moved elliptically.
The Law of Universal Gravitation states that every object in the universe attracts every other object with a force directed along the line of centers for the two objects that is proportional to the product of their masses, and inversely proportional to the square of the separation between the two objects.(1) In other words, the force of gravity grows with the mass of two objects, and the force gets weaker the more distant the objects are from each other. With this principle, Newton extended Kepler’s three laws of planetary motion to explain what caused the planets to move in an elliptical path.
On Saturday, July 12 a “super moon” will appear in the sky at its closest point to earth, or at perigee. In the United States, look for the moon to rise between approximately 8:00 pm. and 9:15 p.m.
You can find out more about Newton’s Law of Universal Gravitation, the motions of the planets, moons, and the laws of motion, along with other topics and subjects covered by Newton’s Principia by searching Research Topics and other documents in eLibrary.
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