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June 01, 1999, 09:40 p.m.
Radio astronomers measure sun's orbit around Milky WayBy PAUL RECER
CHICAGO -- 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.
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.
A report on the finding was presented Tuesday at a national meeting of the American Astronomical Society.
"Our new figure of 226 million miles is accurate to within 6 percent," Mark Reid, a Harvard-Smithsonian astronomer and leader of the team that made the measurements, said in a statement.
The sun is one of about 100 billion stars in the Milky Way, one of billions of ordinary galaxies in the universe.
The Milky Way is a spiral galaxy, with curving arms of stars pinwheeling out from a center.
The solar system is about halfway out on one of these arms and is about 26,000 light years from the center. A light year is about 6 trillion miles.
Reid and his team made the measurement using the Very Long Baseline Array, a system of 10 large radio-telescope antennae placed 5,000 miles across the United States, from the U.S. Virgin Islands to Hawaii.
Working together as a single unit, the antennae can measure motions in the distant universe with unprecedented accuracy.
The accuracy is such that the VLBA can look at a bit of sky that has an apparent size one ten- thousandth the diameter of a human hair held at arms length.
For their solar system measurement, the astronomers focused on Sagittarius A, a star discovered two decades ago to mark the Milky Way's center. Over a 10-day period, they measured the apparent shift in position of the star against the background of stars far beyond.
The apparent motion of Sagittarius A is very, very small, just one-600,000th of what could be detected with the human eye, the astronomers said.
Reid said the measurement adds supports to the idea that the Milky Way's center contains a supermassive black hole.
"This ... strengthens the idea that this object, much smaller than our own solar system, contains a black hole about 2.6 million times more massive than the sun," Reid said in a statement.
Moran said the new measurement of the solar system orbit adds new accuracy to a fundamental fact of the universe: Everything is moving constantly.
The Earth rotates on its axis at about 1,100 miles an hour, a motion that creates day and night.
The Earth orbits the sun at about 67,000 miles an hour, a motion that takes one year.
The sun circles the Milky Way at a speed of about 486,000 miles per hour. And every object in the universe is moving apart from the other objects as the universe expands at a constantly accelerating rate.