D explanation cepheid the cepheid variables are the

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d Explanation: Cepheid. The Cepheid variables are the most important link to galaxian distances. The nearest of these is the Clouds of Magellan and are only about 150,000 to 200,000 light years away from the sun. By determining the absolute magnitudes of Cepheids and other observable characteristics, distances to several neighboring galaxies in which Cepheids can be observed are estimated. The distances that separate galaxies are measured in the hundreds of thousands to millions of ________ _______. Explanation: Light Years. There are less than 20 known galaxies within 2.5 million light years (LY), but there are many thousands within 50 million LY. Galaxies extend in all directions as far as we can see. Johannes Kepler was an astronomer and a _________________. Explanation: Mathematician. He became involved with the theory of planetary motion and spent over twenty-five years in this area of study. Kepler developed three laws of _____________ ________. Explanation: Planetary Motion. Kepler's laws were the most important contributions in Commentaries on the Motions of Mars, writings that took him over a decade to research.
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Kepler's First Law states that each planet moves about the sun in an orbit that is an ___________, with the sun at one focus of the ellipse. : Ellipse. You don't need to really understand much of the geometry of ellipses--you just need to know that an ellipse contains two points called foci (singular: focus). According to Kepler's first law, the path that the planet follows will be an ellipse (not a circle as assumed by Copernicus!), and the sun will be located at one of the foci of that elliptical orbit: Kepler’s Second Law states that the straight line joining a planet and the sun sweeps out equal areas in space in equal intervals of _______. Explanation: Time. This one can be difficult to understand without an illustration; take a look at the graphic below. The point of this law is that when a planet is closer to the sun, it moves more quickly. Because planetary orbits are elliptical, the distance between a given planet and the sun varies. When a planet is nearest to the sun (this point is called perihelion) it is at its fastest speed; when it's at the furthest point of its orbit (aphelion), it is slowest. Kepler's Third Law states that the squares of the sidereal periods of the planets are in direct ______________ to the cubes of the semi-major axes of their orbits. Explanation: Proportion. This law sounds a lot more complicated than it is. A "sidereal period" is the amount of time it takes for a planet to do one complete orbit around the sun. The "semi-major" axis of an ellipse is almost like the "radius" of an ellipse. The point of this law is that the period of time it takes a planet to orbit the sun increases rapidly with the radius of its orbit; i.e. Mercury takes 88 days while Pluto takes 248 years . To arrive at his third law, it was not necessary for Kepler to know the actual distances of the planets from the sun, but only the distances in units of the Earth's distance, the astronomical unit.
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An _______________ unit (AU) is approximately equal to the distance from the Earth to the sun. The currently accepted value of the AU is nearly 93 million miles.
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