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Astronomy: Understanding the Universe by Sherman Hollar

By Sherman Hollar

Fathoming the boundlessness of area and the universe, we're instantly full of interest approximately our personal origins and sweetness concerning the items, lifestyles types, and subject that populate the cosmos. in the course of the observations and paintings of astronomers through the years, now we have slowly been capable of decrease the variety of unknowns and improve reasons or theories for a few of the celestial items and phenomena we see. This space-traveling survey recounts many of the significant discovery milestones within the box of astronomy and examines the instruments and strategies at present utilized by astronomers to review the universe.

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These are much larger and more massive than the inner planets. Jupiter has 318 times Earth’s mass and in fact is more massive than all the other planets combined. Being made mostly of hydrogen and helium (mainly in liquid forms), the Jovian planets are also much less dense than the inner planets. Natural Satellites Six of the planets—Earth, Mars, Jupiter, Saturn, Uranus, and Neptune—are known to have satellites. Dwarf planets and asteroids can also have moons. Because the Moon is large in comparison with Earth, the EarthMoon system is sometimes called a double planet.

The German astronomer Johannes Kepler was finally able to describe planetary motions using three mathematical expressions, which came to be known as Kepler’s laws of planetary motion. In carefully studying Mars, Kepler found that its orbit is not circular, as had been assumed. Rather, the orbits of the planets are elliptical, with the Sun at one of two 34 The Solar System fixed points in the ellipse called foci. Also, as a planet travels around the Sun, its speed is greater when it is closer to the Sun.

Some force must be attracting them to the Sun. The English scientist Isaac Newton calculated that in order for Kepler’s laws to have the form they do, this force must grow weaker with increasing distance from the Sun, in a particular way called an inverse square law. He also realized that the Moon’s curved path around Earth was a type of weak acceleration toward Earth. He calculated this acceleration to be much less than that of an apple falling from a tree. In comparing these accelerations, he found their difference to be described by the same inverse square law that described the force the Sun exerted on the planets.

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