Transcript Day 13

Announcements
•Next exam is Thursday March 24. Will
cover Chapters 6 & 7 and possibly some of
Chapter 8. Sample questions are posted.
•1st Quarter Observing Night Tuesday
March 15 (first Tuesday after spring break).
If clear, set-up starts at 6:30pm. If cloudy:
6:45pm.
•Project presentations today!
In 1596 a Frisian, David Fabricius,
notices a “nova” in Cetus
In 1638 another Frisian,
Johannes Holwarda
observes another “nova”
in the same place
By 1622 Johannes Hevelius figures
it out
He didn’t know why but he
figured out that the two
novas were actually a
single variable star
To aid in
quantifying the
variability of
stars, William
Herschel
published a
Catalogue of
comparative
brightness of
stars
Two York amateur astronomers
had developed the star comparison
technique while studying Algol
John Goodricke worked
with Edward Pigott
The two amateurs also discover the
variability of Beta Lyrae, Delta
Cephei and Eta Aquilae
Their original explanation of an
eclipsing star was right for Algol
Algol System
They got confused when
Delta Cephei and Eta
Aquilae behaved differently
Beta Lyrae is also an eclipsing binary
If the stars change, do they also
move? And how far away are they?
Halley determined the Proper Motion of several stars by
comparing “modern” positions to those of Ptolemy
Attempting to measure parallax
The problem required precise
measurements free of atmospheric
refractive effects
Robert Hooke had a zenith
telescope built into his house to
measure Gamma Draconis
He encountered several
problems, including a
cracked lens. Although he
reported a parallax angle
in 1669, no one believed
him.
Hooke and Wren build a zenith
telescope into the Monument to
the Great Fire
In 1725, James Bradley had a zenith
telescope built to measure parallax
While attempting to measure
parallax, he measured the
Aberration of Light
This was the first direct evidence for the motion of
the Earth around the Sun
Bradley also discovered the
Nutation of Earth
The primary cause is torque exerted by
the Moon so the period is 18.6 years
Bradley took reams of data to
correct the positions of stars but
died before reducing the data
Fredrick Wilhelm Bessel
undertook the data
reductions and published
them in Fundamenta
Astronomiae
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.