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Re: [Phys-L] parallax in astronomy


Hi Ludwik,

Astronomers commonly use the equation (when determining stellar distances) in a slightly different form. If you use arcsecond as a unit of angle, the equation can be written by distance = 1/parallax_angle. Distance here is given in parsecs (the distance from which the average Earth-Sun distance is seen in an angle of one second of arc). In addition, you have to use half of the observed parallax angle (ie. the maximum shift of the star according to background stars) because you use a base of 1 astronomical unit to define the parsec. For a diagram, take a look:

http://hyperphysics.phy-astr.gsu.edu/hbase/astro/para.html

The link you gave in your essay seems to explain the base incorrectly (in the last lines of the page) - if you take parallax angle as the angle through which the star appears to move (in a period of six months), the baseline would be two AUs.

Astronomers use different baselines to measure distances, (for example diurnal rotation of the Earth for asteroid distances) and more sophisticated techniques for distances exceeding 100 parsecs.

It helps to understand the method if you apply it in a classroom and use common units of distance!

Kauko
Kuopion Lyseo Upper Secondary school
Kuopio Finland

Dear friends,

I have not been teaching since I retired in 2004. But I often dream about teaching situations. This happened two days ago; In that dream I was not able to explain the parallax phenomenon. What follows is the link to a short essay I started composing after waking up. Today I finished writing it and posted the essay at:
http://pages.csam.montclair.edu/~kowalski/about_parallax.html

Is my explanation correct and accurate; I am not an astronomer. Comments will be appreciated.
Ludwik Kowalski


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