PROPER MOTION

# PROPER MOTION - degrees in a year What is happening in...

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PROPER MOTION We've seen that diurnal motion of stars is due to the rotation of the earth on its axis. Annual motion of stars is due to the revolution of the earth around the sun. And the motion of planets is due to the actual orbits of the planets around the sun. It was finally discovered, in the eigtheenth century, that the stars do have real motion, called proper motion . We see this as a very small angular displacement, over time, relative to other stars. In telescope it would look as Figure 1 below, with one star displaced after, say, a year relative to a pattern among other stars. This figure looks similar to the one we saw earlier showing the wandering of a planet. The difference is that the angular shift of a star is very much less, typically about 1/1000 of a degree per year (whereas for a planet it might be 90 degrees or 180

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Unformatted text preview: degrees in a year). What is happening in space is shown in Figure 2. Seen against a background of 7 distant stars, the near star is first at point P, aligned with star 2, then at later time (years later) at point Q, aligned with star 6. The stars that exhibit proper motion are always those that are relatively near to us, and their motion seen relative to distant stars. "Near" in this context means from a few light years out to a few tens of light years. The distant stars may be millions of light years away. Great patience and diligence is required to study this motion. One must be assured that the telescope is in exactly the same position and orientation consistently over a period of years, even decades....
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PROPER MOTION - degrees in a year What is happening in...

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