a1_13_mercury

a1_13_mercury - Mercury: A Moon-Like Planet Some basic...

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Mercury: A Moon-Like Planet
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Some basic numbers Semi-major axis: 0.387 AU (35.9 million miles) Orbital Eccentricity: 0.206 Orbital Inclination: 7.0 degrees Orbital Period: 88 days Sidereal rotation period: 58.6 days Diameter 4878 km (3031 miles) Density: 5.43 g/cc Mass (Earth = 1): 0.055 Obliquity: 0.0 degrees Number of satellite: 0
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Some basic numbers, cont’d Daytime temperature: 740 K Nighttime temperature: 90 K Because of spin-orbit coupling (produced by Solar tides), there is a simple 2:3 ratio between Mercury’s rotation period of 58.6 days and its orbital 88 day period. The interval from sunrise to sunset at a given location on Mercury is 88 days and the night lasts 88 days, meaning a “day” really last 176 Earth days – twice Mercury’s year.
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First, Mercury always lies close to the Sun…making it a very dif±cult planet to see, no less study in detail.
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…same problem in the morning. Its orbit puts it always close to the Sun.
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Mercury is hard to study.
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In the late 1800 ʼ s, Giovanni Schiaparelli tried to map out the surface features of Mercury. He derived a rotation period of 88 days… i.e., equal to its orbital period . He got it wrong. But for a simple reason.
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Schiaparelli kept seeing the same features every two years, so he concluded it must have a rotation period equal to 88 days, just like its orbital period. The truth is a 3:2 ratio. Spin-Orbit “coupling”
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How did it get to this 3:2 spin orbit coupling? It probably was spinning faster than it does today and got “stuck” at the 3:2 spin/orbit ratio.
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a1_13_mercury - Mercury: A Moon-Like Planet Some basic...

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