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The mass is located within the circle of the Sun's orbit through the galaxy is about 100 billion times the mass of the Sun.Based on a distance of 30,000 light years and a speed of 220 km/s, the Sun's orbit around the center of the Milky Way once every 225 million years. The Sun has orbited the galaxy, more than 20 times during its 5 billion year lifetime. Some of the galaxy's mass is inside the sun's orbit and some of it is outside.Astronomers focusing on a star at the center of the Milky Way say they have measured precisely for the first time how long it takes the sun to circle its home galaxy: 226 million years.The last time the sun was at this exact spot of its galactic orbit, dinosaurs ruled the world.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.This is not a question that we are going to answer any time soon as some of the greatest minds in philosophy, physics, and psychology have grappled with it.
The centrifugal force caused by the rotation balances out the gravitational force, which draw all the matter toward the center.
Now we are getting more specific, more mathematical, but it still doesn't tell us what time is.
I will attempt to give an operating definition here, one that is useful in understanding time travel. To measure time we need periodic and repetitious motion.
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.
That means it takes the solar system about 226 million years to orbit the Milky Way and puts the most precise value ever determined on one of the fundamental motions of the Earth and its sun, said James Moran of Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
The Sun's orbital period is determined by the galaxy's mass within the orbit of the Sun.