Trigonometric parallax is used to measure the distances of the nearby stars

# Trigonometric parallax is used to measure the distances of the nearby stars

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Trigonometric parallax is used to measure the distances of the nearby stars. The stars are so far away that observing a star from opposite sides of the Earth would produce a parallax angle much, much too small to detect. As large a baseline as possible must be used. The largest one that can be easily used is the orbit of the Earth. In this case the baseline is the distance between the Earth and the Sun---an astronomical unit (AU) or 149.6 million kilometers! A picture of a nearby star is taken against the background of stars from opposite sides of the Earth's orbit (six months apart). The parallax angle p is one-half of the total angular shift. However, even with this large baseline, the distances to the stars in units of astronomical units are huge, so a more convenient unit of distance called a parsec is used (abbreviated with ``pc''). A parsec is the distance of a star that has a par allax of one arc sec ond using a baseline of 1 astronomical unit. Therefore, one parsec = 206,265 astronomical units. The nearest star is about

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Unformatted text preview: 1.3 parsecs from the solar system. In order to convert parsecs into standard units like kilometers or meters, you must know the numerical value for the astronomical unit---it sets the scale for the rest of the universe. Its value was not know accurately until the early 20th century (see the planetary science chapter ). In terms of light years, one parsec = 3.26 light years. Which unit should you use to specify distances: a light year or a parsec? Both are fine and are used by astronomers (including myself) all of the time, just as you may use ``feet'' and ``yards'' in everyday talk. However, when the parsec unit is used on this web site, the equivalent in light years will usually be given as well, but when the light year unit is used, the parsec equivalent will usually not be given. Using a parsec for the distance unit and an arc second for the angle, our simple angle formula above becomes extremely simple for measurements from Earth: p = 1/ d...
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## Trigonometric parallax is used to measure the distances of the nearby stars

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