The Physics
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Speed of the Sun

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Chaisson, Eric, & McMillan, Steve. Astronomy Today.New Jersey: Prentice-Hall, 1993: 533. "Measurements of gas velocities in the solar neighborhood show that the sun, and everything in its vicinity, orbits the galactic center at a speed of about 220 km/s…." 220 km/s
"Milky Way Galaxy. " The New Encyclopedia Britannica.15th ed. Chicago: Encyclopaedia Britannica, 1998: 131. "The Sun, which is located relatively far from the nucleus, moves at an estimated speed of about 225 km per second (140 miles per second) in a nearly circular orbit." 225 km/s
Goldsmith, Donald. The Astronomers.New York: St. Martin's Press, 1991: 39. "If the solar system… were not moving in orbit around the center, we would fall straight in toward it, arriving a hundred million years from now. But because we do move (at about 150 miles per second) along a nearly circular path…." 240 km/s
Norton, Arthur P. Norton's Star Atlas.New York: Longman Scientific & Technical, 1978: 92. "the sun's neighborhood, including the Sun itself, are moving around the centre of our Galaxy in approximately circular orbits with velocities of the order of 250 km/s." 250 km/s
Recer, Paul (Associated Press). Radio Astronomers Measure Sun's Orbit Around Milky Way. Houston Chronicle. 1 June 1990. "Using a radio telescope system that measures celestial distances 500 times more accurately than the Hubble Space Telescope, astronomers plotted the motion of the Milky Way and found that the sun and its family of planets were orbiting the galaxy at about 135 miles per second."
"The sun circles the Milky Way at a speed of about 486,000 miles per hour."
217 km/s

The Sun is one of some 300 billion stars that travel around the Milky Way in a near circular orbit. These stars are closer to the galactic center than the Sun. The distance from the center of our Galaxy to the Sun is about 26,000 light years (a light year is about 6 trillion miles), which is approximately halfway out on one of the Milky Way's curving arms. The Sun and its planets take a period of 225 million years to revolve around the galactic center. The time it takes for each orbit is sometimes referred to as a cosmic year or a galactic year. The Sun has completed about 20 orbits since the solar system was formed. For each orbit, the Sun traveled 150,000 light years of distance.

The orbit of the Sun around the Milky Way is influenced by the galaxy's matter, which does not solely occupy the galactic center. Instead, it is distributed all over space. Some of the galaxy's mass is inside the sun's orbit and some of it is outside. The Sun's orbital period is determined by the galaxy's mass within the orbit of the Sun.

Newton's explanation of the speed of stars in the Milky Way is as follows. He showed that stars closer to the galactic center, including the Sun, experience a gravitational pull equal to the pull created by the mass that is equal to that of all the stars closer to the galactic center. Hence, the mass of the galactic center is equal to the total mass of all the stars closer to the center.

He also showed that stars farther from the center have a combined gravitational force of zero. Those stars pull in all different and opposite directions, canceling out one another. Therefore, the stars closer to the center experience a gravitational pull towards the center and they move at greater speeds, since there is more force acting upon them. Conversely, more distant stars have less force acting upon them and in turn, they travel at lower speeds. In addition, stars beyond this distance have speeds that stop decreasing and eventually remain constant.

Angela Chan -- 2001

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