Stellar Aberration and Stellar Parallax: Here Endeth the Lesson

STELLAR PARALLAX

As the earth moves, the stars appear to shift positions relative to one another as the position of the earth changed.

As shown in Figure 2-12, the angle between two stars, as seen from the earth, appears to change as the earth moved in its orbit. This effect is known as stellar parallax.


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From the time of Aristotle the absence of any observable stellar parallax had served as a major barrier against the acceptance of the heliocentric theory. The argument that the stars were too far away to exhibit parallax was hardly a convincing one; it was inconceivable that the universe could be so large. The most astute naked-eye observers from Hipparchus to Tycho Brahe were unable to find a single star that exhibited a measurable parallax, and even the invention of the telescope did not improve the situation. Eventually, however, stellar parallax was observed. The first unequivocal measurements were reported in rapid succession by Friedrich Struve in 1837, Friedrich Bessel in 1838, and Thomas Henderson in 1839. By that time there were not many supporters of the geocentric theory, so the discovery was somewhat anticlimactic.

STELLAR ABERRATION

Actually, the most convincing piece of evidence for the revolution of the earth was discovered in 1729, only two years after Newton's death, and more than a hundred years before stellar parallax was finally observed. In 1727 two English astronomers, Samuel Molyneux and James Bradley, made a series of observations of the position of the star Eltanin, in an attempt to measure the parallax of this star. They found that the star did indeed appear to move in a small circle with respect to the more distant stars over a period of a year. They realized, however, that the motion was not stellar parallax, since their observations showed that the star had its maximum shift in position at the time of year when it should exhibit no parallax at all. Observations of several other stars showed similar results. The two men found that all of these stars moved in circles of 20.5" in radius, regardless of their distances.

In 1728 Molyneux died, and Bradley was left to figure out the puzzle alone. A year later he announced his solution: the apparent shift in the position of a star is due to the motion of the earth with respect to the light coming from that star. Bradley called this effect stellar aberration.

Stellar aberration is very similar to the common phenomenon that we observe when driving in a car in a rainstorm. When the car is stationary, the rain appears to be falling vertically, but when the car is moving, the rain appears to be falling toward the car at an angle. The rain is, of course, not really falling at an angle; it only appears that way because the car is moving toward the raindrops as they are falling. Thus there is an apparent change in the direction from which the rain is seen to come because of the motion of the observer.

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The same analysis applies to light traveling down a telescope tube. If the telescope is on a moving earth, the telescope must be tilted in the direction of motion in order to receive the light from a star that is overhead. Consequently, the starlight appears to be coming from a direction that is not the true direction of the star (Figure 5-12a).

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Bradley's discovery of stellar aberration has to be considered extremely strong evidence for the heliocentric theory - for if the earth did not move with respect to the stars, there would be no stellar aberration. If one is to deny the earth's motion in the light of this discovery, one must make some rather peculiar assumptions about the behavior of light - or else propose that all stars are acted on by a force that makes them travel in elliptical paths once a year.

The Nature of Physics, Peter J. Brancazio (1975)