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 = Show your work for credit!