Day 15

•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
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
This assumes that all
stars have the same
luminosity. The
double star data was
starting to show that
was an incorrect
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
solution: God
reaches in
and puts
things right
To test his model Bentley arranged
the stars in shells of 1st magnitude,
2nd, 3rd magnitude, …
The problem
was, the
didn’t match
A Newton supporter, Samuel
Clarke, defended these ideas
against Gottfried Leibniz
Newton had a
previous run-in
with Leibniz
about the
invention of
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
Eventually, his son John takes up
the challenge
William Parson finally ends the
Herschel domination of large
telescopes with The Leviathan

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