Presentation on theme: "Announcements Next exam is Monday April 7. It will cover the rest of Chapter 5 (from Kepler) and all of Chapters 6 and 7. Sample questions have been posted."— Presentation transcript:
Announcements Next exam is Monday April 7. It will cover the rest of Chapter 5 (from Kepler) and all of Chapters 6 and 7. Sample questions have been posted. First Quarter Observing Night Monday. Starts at 8:30pm so set-up will begin at 7:30 if clear, 7:45 if cloudy
Based on the positions calculated by Bessel, attempts to measure proper motion began
Bradley pointed out that the motion was a combination of the stars motion and the motion of the solar system
Tobias Mayer proposed a method of untangling the two motions but couldn’t see it in the available data Mayer looked for a pattern in the measured proper motions of stars
It took the clever William Herschel to find the direction we are moving: towards Hercules His calculations were based on only a few stars but they gave the correct answer. When F. W. A. Argelander redid the calculation using almost 400 stars 50 years later he got almost the same answer.
Back to the problem of measuring stellar parallax Galileo had proposed using double stars to measure relative parallax shifts. The problem was, many of these “double stars” are actually binary star systems so both stars are at the same distance.
Others proposed using the brightness as a measure of distance This assumes that all stars have the same luminosity. The double star data was starting to show that was an incorrect assumption
Estimates for stellar distances were being reported and they were large James Gregory estimated the distance to Sirius at 83,190 AU Newton had used a similar method and come up with a distance of 1 million AU to Sirius. He simply couldn’t believe it was that large so he didn’t publish his results.
Several took up the technique of Hooke: use a zenith telescope Samuel Molyneux used a zenith sector designed by George Graham along with help from James Bradley
Bradley’s best measurements, accounting for the aberration of light and nutation could not show any parallax greater than 1 arcsec
Wilhelm Struve proposed a way to choose the best candidates for parallax measurements 1.The star should have a large proper motion 2.The star should be bright 3.The star should be a widely separated binary
Struve had the best telescope maker produce an equatorial telescope for his observations Joseph von Fraunhofer The Dorpat telescope
Struve came up with a parallax of 1/8 of an arcsec for Vega Unfortunately, a second round of observations gave a different number.
Using a similar instrument, Bessel observed 61 Cygni In 1838, Bessel came up with a parallax angle of 1/3 arcseconds. That gives a distance of 10.4 ly. The currently accepted value is 11.4 ly.
The Birth of Cosmology Newton’s Principia contained almost nothing on the stars or the structure of the universe
Richard Bentley was the first to get Newton thinking about cosmology Bentley asked Newton what would happen if the universe were “uniform”. Newton though he meant a truly uniform distribution of mass but he corrected Newton in that he meant a uniform distribution of stars.
Newton’s idea was a symmetrical, uniform distribution of stars
There were problems with the uniform distribution of stars The most obvious problem is that it ignores the Milky Way
The other problem was a local gravitational collapse Newton’s solution: God reaches in and puts things right
To test his model Bentley arranged the stars in shells of 1 st magnitude, 2 nd, 3 rd magnitude, …
The problem was, the numbers didn’t match reality
A Newton supporter, Samuel Clarke, defended these ideas against Gottfried Leibniz Newton had a previous run-in with Leibniz about the invention of calculus
Another voice contrary to Newton was William Stukeley Stukeley’s idea was that the universe was something like Saturn, a central bulge (where we are) surrounded by a ring (the Milky Way)
Stukeley was one of the first to ask “Why isn’t the night sky brilliant?”
J. P. L. de Cheseaux proposed that the interstellar medium absorbed most of the light
By the mid-1800’s it was realized that the ISM would glow like a star
Thomas Wright proposed a spherical universe with God’s Abode at the center
Immanuel Kant didn’t like the idea of multiple “Divine Centers”
William Herschel decided to settle the question by observation
To assist him, he brought his sister Caroline from Germany He gave her a small reflector to look for comets. She discovered 8
Herschel undertook a survey of the stars using large telescopes
Using a statistical analysis he proposed a new shape for the universe
Eventually, his son John takes up the challenge
William Parson finally ends the Herschel domination of large telescopes with The Leviathan