Transcript Day 11

Announcements
•Next exam is scheduled for Thursday March
24. Will cover Chapters 6 & 7 and possibly
some of Chapter 8. Sample questions will be
posted this week.
•Dark Sky Observing Night Monday. Forecast
doesn’t look good for it. If we go, set-up starts
at 6:30pm. If needed, I’ll post a cancellation
notice on APSU Astronomy by 5:00pm on the
day of the observing.
The problem of Longitude
Finding your latitude is easy, just measure the
altitude of Polaris to get close. To be precise you
need to know how far Polaris is from the NCP
Measuring
longitude
requires
determining
your angle
from a
reference
line on
Earth
The Longitude Act of 1714
offered a £20,000 prize for an
accurate method of determining
longitude at sea
The matter was
brought to a head
after a particularly
disastrous shipwreck
in 1707 in which four
large ships and over
1400 sailors of the
Royal Navy perished
Early methods involved measuring
the moons of Jupiter
Once their orbits were determined, ephemeredes
could be generated to show their location at any time
in the future. Then all you have to do is accurately
measure your local time.
Accurate ephemeredes of Jupiter’s
moons became available in 1668
Gian Domenico Cassini
The problem
was it isn’t
possible to
accurately
measure the
moons of
Jupiter from
the deck of a
rolling ship
If you could accurately measure the
difference between true north and
magnetic north, that might work
Problem was the magnetic pole isn’t stable, it moves over time
Next came attempts to use the
Moon to measure time
An eclipse is seen by everyone on Earth at the same time
The location of the Moon with
respect to the background stars
could be used if…
Nevil Maskelyne, Astronomer
Royal, published his first Nautical
Almanac in 1766
Again, the
problem is
making
astronomical
observations
from the deck
of a rolling
ship at sea
John
Harrison
came up with
the solution:
make an
accurate and
stable clock
Harrisons
1st attempt
in 1735
used a
double
pendulum
design
He continues with several other
models
H3 front and back
H4 tested in 1764
H4 passes several sea trials and meets all the requirements
but isn’t awarded the prize due to “land trials” at Greenwich
His final model, H5 built in 1770, has
better than the required accuracy
James Cook took one of the copies
on a voyage to the Pacific
It took many
years and a
Royal Decree
before
Harrison was
finally awarded
£8,750 by act
of Parliament
in 1773
Watch NOVA: Lost at Sea