Parallax measurement. Schematic illustration of the parallax measurement for a star, determining its distance through basic geometry, using the Earth orbit (of diameter two astronomical units (a.u.)) as a baseline.
The Earth describes a yearly orbit around the Sun (shown as the blue ellipse in the figure). Observing the stars from Earth creates therefore some very useful peculiarities. Consider, for example, the measurement of the position of a star. With respect to the Sun, most stars only show a linear motion, but when observed from Earth, the direction in which we see the star depends on where in its orbit the Earth is, or, in other words, the time of the year we measure it, and the direction of the star as seen from the Sun (the dashed line in the figure above). As seen from Earth, the position at which we observe any star will, over the period of a year, describe a small ellipse on the sky (the white ellipse in the figure). This ellipse is the projection of the Earth orbit as seen from the Sun, at the distance of the star. Therefore, the further away a star is, the smaller will be this ellipse. This effect is called the stellar parallax. Determining the size of the major axis of the ellipse provides the value of the parallax (see figure above), which is the most direct method of measuring distances to nearby stars. It uses basic geometry in which the Earth orbit forms the baseline. One serious problem in the determination is the fact that the typical distances between stars in the solar neighbourhood are very large compared to the available baseline, the diameter of the Earth orbit around the Sun. The nearest star is about 270 000 times further away from us than the Sun, which is already at a respectable distance of 149 million km.
@ESA vodcast: Episode 6: Charting the Galaxy - from Hipparcos to Gaia to learn more about parallax measurements, and the Hipparcos and Gaia missions (download the vodcast from ESA's website).
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Page last updated: 02 June 2014