ASTRONOMERS MUST BE CREATIVE when figuring out how much stuff there is in the universe. You can't exactly roll a galaxy onto an industrial scale (even if you could, as Isaac Newton figured out, you'd still have to factor in the influence of Earth's gravity to get its mass). The classical way of determining the amount of matter in an object like a moon, planet, or star is to measure its gravitational interaction with other objects. These early calculations combined Newton's law of gravity with Johannes Kepler's laws of planetary motion-the relationship between the planet's orbital speed and distance from that other object.
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