Download The Ever Expanding Universe: Part II

The Ever Expanding Universe: Part II
By the 19th century good working absolute distances to the Planets had been made by astronomers
from all around the world using the parallax method. Much of the inspiration to make these
measurements came from Edmond Halley, England’s 18th century Royal Astronomer, who urged
the next generation of astronomers to measure the upcoming transits of Venus (he would die
before these events would occur) in order to get some of the best possible parallax data on the
closest and easiest to measure planet. Recall that Kepler had determined the relative distances to
the planets and that once one planets absolute distance was known all the planets absolute
distances would be known. With this planetary success the next challenge was to measure the
actual distance to a Star for the first time in history.
But finding the parallax to a star would require very precise optics. The parallax of the closest
planets, Mars and Venus are both within a thousandth of a degree so the challenge to find the
parallax to even the nearest Star was indeed great. Recall how the parallax method works to find
distance. Two surveyors measure the angles to an object along a baseline and scale the triangle to
find its distance. The same technique can be used for stars with one modification. When the
parallax for a Star is found the baseline is the diameter of the Earth’s orbit along its major axis.
This larger distance increases the chance of detecting the tiny parallax of a Star. And the triangle
drawn is used to draw a right triangle that describes the distance from the Earth to the Sun to the
Star. Look at the pictures below.
The Earth in the picture is shown
twice, once now and once six months
from now. This is the longest baseline
astronomers can use without venturing
into space! One angle measurement is
made now to the star and another six
months later. Notice that the stars
position will move relative to
background stars when angle
measurements are made six months
apart. Friedrich Bessel succeeded in
measuring a star by parallax in the 19th
century. This task took him 8 years
and required the finest optical
telescope of the day. The star, 61
Cygni, was measured accurately to
within 10% of its actual distance of 11
light years to Earth. This achievement
was equivalent to measuring the two
edges of a dime from sixty miles!
Finally, after 2000 years, the problem
of measuring star distances that had convinced the Greeks that they must be rather close and
revolved around a stationary Earth ironically proved that the stars were far away and that the
Earth revolved around a star. Copernicus had finally been vindicated.
Try a parallax problem: If the parallax to a star is 0.38 how far is the star in parsecs? How far
is the star in light years? A parsec is a distance of 3.26 light years.
Recall parallax is the apparent change in position of an object (a star) when measured from two
ends of a baseline and is measured as an angle. The formula for finding the distance to a star is:
Distance in parsecs =
1
parallax
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