Navigation and Astronomy

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Navigation and Astronomy

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A First for Astronomy!

• The first good measurement of c was by Ole
  Roemer, studying the moons of Jupiter.

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Motivated by Navigation

• Pole Star tells us how far north or south we are.
• How do we tell how far east or west we have
  travelled?

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No Problem, if

• if the world was not covered with oceans!
• (Merely pace out the distances, put up markers,
  build roads, etc.)
• Oceans complicate issues. The water moves!

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Ocean Currents ( storms, winds)
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Tragic Consequences
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A Thought Experiment

• You are drugged and kidnapped.
• You wake up lying in a grassy meadow.
• The sun is just rising.
• You ring a friend in London England, using a cell
  phone.
• She tells you that it is noon in London (the sun
  is high in the sky).
• Where are you?

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What You Need

• A way of figuring out the local time. This is
  straightforward just observe the sun.
• A way of knowing the current time in London (or
  some other global reference point). But how? No
  cell phones!

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Keeping London Time Unlikely Approaches

• Fleets of stationary signal ships
• Kenelm Digbys Powders of Sympathy
• (Read Longitude, by Dava Sobel)

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Two Sensible Possibilities

• Build a clock which you leave set on London
  time while you travel across the oceans.
• Problem early clocks were not precise enough.

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The Necessary Precision

• If your clock is one minute slow or fast, your
  navigational error is about 10 miles a real
  problem when nearing a rocky shore!

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The Problem Eventually SolvedThe Harrison Clocks
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Alternatively

• Use a clock in the sky.
• For example, predict exactly where the moon will
  be relative to the background stars for all
  future times.

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Hard to Do!

• Motions are complex! No computers then.
• Modern astronomy (Greenwich Observatory) was
  invented to solve this problem!

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Another Perfect Clock in the Sky

• Use Jupiters moons as indicators of time, like
  the hands of a clock.
• Io orbits Jupiter
• every 1.769 days,
• like clockwork.

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Not Ideal!

• 1. Not always visible. (Sometimes behind the
  sun, on the far side of its orbit.)
• 2. Need a telescope to see the moons at all
  hard to do this on a tossing ship. (Galileo had
  a solution)

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Moreover

• The Jupiter clock seemed to be gaining and
  losing time!
• Why???

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The Ideal

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Surprise!
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Implication

• Light travels at a finite, measureable speed.
• Thus we do not see the whole universe as it is
  all at once!
• The farther out we look, the farther back in
  time we are seeing.