81. Lecture 5: Stellar Distances This means that from the ground, the method of Trigonometric Parallaxes has the following limitations. good out to 100 pc; only get 10% distances out to a http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit1/distances.html

Astronomy 162: Prof. Richard Pogge, MTWThF 9:30 Lecture 5: Distances of the Stars Readings: Ch 19, section 19-1 Key Ideas Method of Trigonometric Parallaxes Direct geometric method of finding distances Units of Cosmic Distance: Light Year Parsec ( Par allax sec ond) Why are Distances Important? Distances are necessary for estimating: Total energy emitted by an object (Luminosity) Masses of objects from their orbital motions True motions through space of stars Physical sizes of objects The problem is that distances are very hard to measure... The problem of measuring distances Question: How do you measure the distance of something that is beyond the reach of your measuring instruments? Examples of such problems: Military range finding to targets Measuring distances to any astronomical object Answer: You resort to using GEOMETRY to find the distance. The Method of Trigonometric Parallaxes Nearby stars appear to move with respect to more distant background stars due to the motion of the Earth around the Sun. This apparent motion (it is not "true" motion) is called Stellar Parallax (Click on the image to view at full scale [Size: 7Kb]) In the picture above, the line of sight to the star in December is different than that in June, when the Earth is on the other side of its orbit. As seen from the Earth, the nearby star appears to sweep through the angle shown. Half of this angle, is the parallax

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