| If we assume
that the earth moves, that is, that it rotates daily and revolves annually
around the sun, one might reasonably expect the apparent position of a
star to shift its relative position with other stars. Put differently,
if the earth were fixed and stable at the center of the cosmos, then its
distance to the stars would remain fixed and unchanged. But if the earth
in fact moved (if the diameter of its annual orbit around the sun was many
millions of miles) then the apparent position of stars might be expected
to change. Other conclusions follow from the same assumption.
For example, a close star would have a large apparent shift in relative
position (it would have a large angle of parallax) while a more distant
star would have a smaller angle of parallax. But what could this
mean? A number of reasonable inferences seem to follow. If
there was no observable semi-annual stellar parallax, would it be reasonable
to conclude that the stars are disproportionately further away than the
planets? Would it be reasonable to infer that the earth did not move?
Historically, as it happens, the absence of observable stellar parallax
was a strong argument against the motion of the earth. Quite simply,
putting the earth in motion caused a thousand inconveniences. The
absence of stellar parallax seemed good evidence that the earth did not
move. As it turns out, stellar parallax was not observed until 1838.
It required a superior telescope. The angle of stellar parallax observed
about 150 years ago was extremely small, about one-half of one minute.
The button below will take you to two illustrations of Stellar Parallax.
One shows the large parallax angle for a nearby star, the second shows
a small parallax angle for a star further away. |
