how far? measuring distance in the universe. how? gian domenico cassini, in 1672, used a technique...
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How Far?
Measuring Distance in the Universe
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How?
• Gian Domenico Cassini, in 1672, used a technique called parallax to measure the distance to Mars.
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Cassini figured it out!
• The Sun is about 93 million miles from
• Earth. As Earth and Mars never come closer than 35 million miles to each other.
• Saturn, the most distant planet known when Cassini was alive, is around 900 million miles away.
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How?
• Parallax!
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What is Parallax?
• Try this out:– Hold 1 finger out at
arm’s length
– Close one eye, and then the other
– Notice how your finger “jumps?
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Measuring Stars
• Stars are billions of light years away – winking won’t help
• Our orbit is 186 million miles wide (93 million + 93 million)– Star A and B are the
same star, as they appear 6 months apart
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How to:
• The parallax angle p is illustrated in the above figure.
• We can determine they adjacent (d) using trigonometry.
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Trigonometry
• Our adjacent line (d in the prior picture) is the distance from Earth’s Orbit (BC) to the star (A)
• Adjacent length= opposite length tangent angle A
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Trigonometry
• Measure the angle of the star to our central point of orbit
• Simple calculation• Even the Hubble Telescope limited to 1000 LY
Earth’s Orbit
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Parsec
• The method of parallax gives rise to a natural distance unit that astronomers call the parsec. – defined as the distance at
which a star would have a parallax angle p equal to one second of arc.
– using trigonometry distance equal to 206,265 astronomical units or 3.26 LY
• AU is the average distance of Earth to Sun
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Further away?
• The universe is expanding
• Galaxies furthest away are moving away faster than galaxies closer to us
• We can us Doppler Effect
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Visible Spectrum
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What is Red Shift?
• As light moves away, the waves stretch out• Longer waves are shifted towards the red• As light moves towards us, the waves condense• Shorter waves are shifted towards the blue
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Spectral Signature
• Each element has a unique spectral signature• Two elements combine
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