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Chapter Two — Welcome to the Solar System

A Short History of Nearly Everything – by Bill Bryson

Chapter 2 –Welcome to the Solar System

  • It wasn’t until 1978 that Pluto’s moon was discovered.  Relative to the size of its planet, the moon of Pluto is the largest moon in the solar system.
  • Pluto was named “Pluto” partly because the first two letters of the name form a monogram of Percival Lowell’s name, the man who first proposed the existence of the planet. It was discovered by Clyde Tombaugh in 1930.
  • Pluto is tiny. It is just one-quarter of 1% as massive as Earth. If you set Pluto on top of the lower 48 United States, it would only cover half of the states. Seven of the moons of other planets in our solar system are larger than Pluto, including Earth’s moon.
  • Pluto’s orbital path is 17 degrees out of alignment relative to the orbit of the other planets. During much of the 20th century, it was closer to the Sun than was Neptune.
  • All the visible “stuff” in our solar system (Sun, planets, moons, asteroids, comets, etc.) fills less than one-trillionth of the available space.
  • It is 5 times farther from Jupiter to Neptune than it is from Earth to Jupiter. Neptune receives only 3% as much sunlight as Jupiter.
  • On the scale of the Earth being reduced to the size of a pea, Jupiter would be over 1000 feet away and Pluto would be 1.5 miles away, the size of a bacterium. Proxima Centauri, our next closest star, would be 10,000 miles away.
  • On the scale of Jupiter being reduced to the size of a period at the end of a sentence, and Pluto no bigger than a molecule, Pluto would still be over 35 feet away from Jupiter.
  • From Pluto, our Sun would appear no larger than a pinhead, just barely a bright star.
  • Forty years ago, it was thought that there were about 30 moons in our solar system. Now there are “at least ninety”, a third of which have been found in the last 10 years.
  • Our solar system is unbelievably vast. Pluto is barely one-fifty-thousandth of the way to the edge of our solar system. The Oort cloud starts somewhere beyond Pluto and stretches for 2 light-years out into the cosmos. Pluto is about 40 AU (1 AU is the distance from the Sun to Earth) from us, and the heart of the Oort cloud is about 50,000 AU from us.
  • About 4.6 billion years ago, the gas and dust that formed our solar system began to aggregate. 99.9% of the mass of the solar system went to make the sun. About 4.5 billion years ago, an object the size of Mars crashed into Earth, blowing out enough material to form our Moon.
  • The Sun is about 5 billion years old, and will burn for another 5 billion years
  • The Sun is 75% hydrogen and 24% helium.  It contains 99.86% of all the mass in our solar system.
  • The surface temperature of the Sun is 10,000 degrees Fahrenheit.  The temperature at the Sun’s core is about 30 million degrees Fahrenheit.
  • Every minute, the Sun converts 564 million tons of hydrogen into 560 million tons of helium and 4 tons of radiant energy.  The process is nuclear fusion. However, during its 10 billion year life cycle, it will only convert 1% of its mass into into energy.
  • The Sun takes 250 million years to complete a cycle of the Milky Way galaxy.  Thus, it has completed 20 cycles of the galaxy thus far in its lifetime.
  • The Sun rotates once on it axis of rotation every 25 days.
  • The cone of the Moon’s shadow is just long enough to reach the Earth, so we see solar eclipses in a very narrow band.  The umbra moves across the surface of the Earth at about 1000 mph.
  • The diameter of the Sun is about 400 times (389 exactly) larger than the diameter of the Moon, and the distance from us to the Sun is about 400 times (432 exactly) greater than the distance from us to the Moon, so their angular diameter in the sky is about equal.
  • The angle of the Moon’s orbit around Earth is inclined about 5 degrees relative to the orbit of the Earth around the Sun, thus we do not have eclipses every month.
  • Diameters: Sun is 865,000 miles, Earth is 8,000 miles, and the Moon is 2,000 miles.
  • There are generally two solar eclipses each year.  About half of them are partial, and the remaining ones are divided between full and annular eclipses.  A solar eclipse is visible only across a narrow strip across the Earth. Since the Earth is 3/4 covered by water, there is a much higher probability that an eclipse will be visible at sea.
  • Sunspots are caused by disturbances and twists in the Sun’s strong magnetic fields.  They are 2000 degrees cooler than the surrounding area. Sunspots wax and wane in an 11-year cycle.  The average sunspot is twice the diameter of the Earth. Sunspots tend to form in groups.
  • The 1761 transit of Venus —
    •  The 1639 transit of Venus was observed by only two humans – Horrocks in Liverpool and Crabtree in Manchester.
    •  However, the 1761 transit was observed by 100’s of astronomers across the globe.
    •  Edmond Halley sparked the interest in the transit, suggesting that by timing the transit and using the data to determine the astronomical unit.
    •  Kepler’s 3rd Law enabled astronomers to determine relative distances of planets, but absolute distances were unknown.
    •  Halley’s idea was to have observers all over the globe, timing precisely the 2nd and 3rd contacts of the transit.  Because of the parallax effect,   different observers would record slightly different durations.  The distance could then be triangulated from the data.

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