Astronomy Principles and Practice Fourth Edition.pdf

Chapter 11
The reduction of positional observations: II
11.1 Introduction
Throughout the 17th and 18th centuries, many attempts were made to measure stellar parallaxes. All
failed. The success of Newtonian science in explaining the movements of the planets (including the
Earth) about the Sun made the failure to detect the apparent shift of the brighter, and presumably
nearer, stars due to the Earth’s annual journey in its heliocentric orbit all the more exasperating. The
improvement in accuracy in measuring stellar positions in those centuries was remarkable and yet
no stellar parallax was observed. One answer, of course, was that the stars were so far away that in
comparison with the distance to even the nearest one, the diameter of the Earth’s orbit about the Sun
was minute, so minute that the apparent shift of the star was too small to be detected.
Some hope that this view was false occurred in 1718 when Halley compared modern observations
of stellar positions with those made by Hipparchus and Ptolemy. Hipparchus had observed about
140 BC while Ptolemy had been active during the 2nd century AD
Halley noticed that, even allowing for observational errors, the positions of the bright stars
Arcturus, Procyon and Sirius were now different from those they had occupied one and a half millennia
before. Halley suggested that these so-called fixed stars were not only moving through space with their
own velocities but that in all likelihood every star, if observed long enough, would be seen to be in
motion. This discovery of Halley’s encouraged other astronomers to persevere with their attempts to
measure stellar distances.
Among those astronomers was James Bradley. Quite often research with a particular aim in
science leads to the discovery of something quite unexpected but as important as, or more important
than, the original investigated effects. Bradley’s attempt to measure stellar parallax was without success
but he did make two important discoveries through it; one was aberration; the other was nutation.
11.2 Stellar aberration
By 1725 it had become obvious that any parallactic shift would be very small. Bradley, working at first
with Molyneux, set up his telescope with great care. He used a meridian circle strapped vertically to
a pillar so that it would remain rigidly fixed in position. The star he chose was γ Draconis because
it transited almost exactly in the zenith. It was also bright and, therefore, would, with any luck, be
among the nearest stars. Hence, the star entered the field of his vertically-mounted telescope each
night and crossed it to disappear off the other side. Any change in the star’s declination would be seen
as a change in the star’s path across the field, the path being ‘higher’ or ‘lower’ than usual. Moreover,
because γ Draconis transited so near to the zenith, the correction for refraction would be very small
and this was advantageous since the value of k used in the refraction formula might be in error.
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