## Measuring the distance of stars

Is the parallax method still the one most widely used (for the relatively nearer stars at least)? More particularly, is it still the case that we take one measurement then wait six months until the earth is on the other side of the sun and take a second measurement? Or has this been superseded by something else?

Also, with the parallax method we need to know the distance from earth to Sun - so how did we first figure out how far away the Sun is?

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 Quote by neoweb Also, with the parallax method we need to know the distance from earth to Sun - so how did we first figure out how far away the Sun is?
If you measure the position of the stars you will note that they have an apparent displacement which is greatest when the Earth is moving at a right angle to the light arriving from the star. This displacement is caused by the aberration of light and its angle depends on the Earth's orbital velocity. Measuring this displacement allows us to determine the Earth's orbital velocity. given the length of the year and the orbital velocity, we can calculate the circumference of the Earth's orbit, and dividing this by 2 pi gives us the mean radius or distance from Sun to Earth.
 Recognitions: Gold Member Science Advisor The earth to sun distance was first estimated by a dude named Aristarchus about 2300 years ago. Parallax was the first method used to gage the distance to other stars. It is arguably the most reliable since it relies on well known and accepted geometrical principles. It is still useful today. Other more sophisticated methods came into use more recently. The first method to emerge after parallax was the Cepheid yardstick. See here for more information. http://curious.astro.cornell.edu/que...php?number=400 http://curious.astro.cornell.edu/que...php?number=128

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## Measuring the distance of stars

Yes. Hipparcos has given us probably the most accurate distances to nearby stars to date. The principles on which it worked (the satellite has long since burned up) are a little arcane, but at the core is parallax.

The 'first' methods of determining the distance to the Sun were also parallax, but in an interesting way - a Greek astronomer (Aristarchus?) estimated the distance to the Sun by the shadow of the Earth on the Moon, during a lunar eclipse! (It's simple geometry). However, as several of his input parameters were wrong, he got the distance to the Sun wrong too.

More generally, since the Moon and Sun have the same angular diameter (as evidenced by solar eclipses), some simple geometry shows that once you have one distance (e.g. Earth to Moon), you can get the other. It gets a bit more complicated if you want to get accurate (e.g. orbits aren't perfect circles).

With Newton and accurate orbits of the planets and asteroids, the distance to the Sun was fairly accurately determined; however, only when radar astronomy was able to get an echo (from Eros, IIRC, though it might have been Venus) could the solar system distance scale be nailed down to ~km (or maybe more accurately). Today we have transponders on interplanetary probes, and the 'light return time' gives very accurate distances (we also need good ephemerises). Anyway, the distance to the Sun need not be all accurately known to work out distances to nearby stars ... the parallax angle (pre-Hipparcos) could only be measured to ~1 decimal place!

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