Read Online Death by Black Hole: And Other Cosmic Quandaries by Neil deGrasse Tyson - Free Book Online
Authors: Neil deGrasse Tyson
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daylight lasts as long as night. Those two days are the spring and fall equinoxes (from the Latin for “equal night”). On all other days of the year the Sun rises and sets elsewhere along the horizon. So the person who invented the adage “the Sun always rises in the east and sets in the west” simply never paid attention to the sky.
If you’re in the Northern Hemisphere while tracking the rise and set points for the Sun, you’ll see that those spots creep north of the east-west line after the spring equinox, eventually stop, and then creep south for a while. After they cross the east-west line again, the southward creeping eventually slows down, stops, and gives way to the northward creeping once again. The entire cycle repeats annually.
All the while, the Sun’s trajectory is changing. On the summer solstice (Latin for “stationary Sun”), the Sun rises and sets at its northernmost point along the horizon, tracing its highest path across the sky. That makes the solstice the year’s longest day, and the stick’s noontime shadow on that day the shortest. When the Sun rises and sets at its southernmost point along the horizon, its trajectory across the sky is the lowest, creating the year’s longest noontime shadow. What else to call that day but the winter solstice?
For 60 percent of Earth’s surface and about 75 percent of its human inhabitants, the Sun is never, ever directly overhead. For the rest of our planet, a 3,200-mile-wide belt centered on the equator, the Sun climbs to the zenith only two days a year (okay, just one day a year if you’re smack on the Tropic of Cancer or the Tropic of Capricorn). I’d bet the same person who professed to know where the Sun rises and sets on the horizon also started the adage “the Sun is directly overhead at high noon.”
So far, with a single stick and profound patience, you have identified the cardinal points on the compass and the four days of the year that mark the change of seasons. Now you need to invent some way to time the interval between one day’s local noon and the next. An expensive chronometer would help here, but one or more well-made hourglasses will also do just fine. Either timer will enable you to determine, with great accuracy, how long it takes for the Sun to revolve around Earth: the solar day. Averaged over the entire year, that time interval equals 24 hours, exactly. Although this doesn’t include the leap-second added now and then to account for the slowing of Earth’s rotation by the Moon’s gravitational tug on Earth’s oceans.
Back to you and your stick. We’re not done yet. Establish a line of sight from its tip to a spot on the sky, and use your trusty timer to mark the moment a familiar star from a familiar constellation passes by. Then, still using your timer, record how long it takes for the star to realign with the stick from one night to the next. That interval, the sidereal day, lasts 23 hours, 56 minutes, and 4 seconds. The almost-four-minute mismatch between the sidereal and solar days forces the Sun to migrate across the patterns of background stars, creating the impression that the Sun visits the stars in one constellation after another throughout the year.
Of course, you can’t see stars in the daytime—other than the Sun. But the ones visible near the horizon just after sunset or just before sunrise flank the Sun’s position on the sky, and so a sharp observer with a good memory for star patterns can figure out what patterns lie behind the Sun itself.
Once again taking advantage of your timing device, you can try something different with your stick in the ground. Each day for an entire year, mark where the tip of the stick’s shadow falls at noon, as indicated by your timer. Turns out that each day’s mark will fall in a different spot, and by the end of the year you will have traced a figure eight, known to the erudite as an “analemma.”
Why? Earth tilts on its axis by 23.5 degrees from the plane of the
If you’re in the Northern Hemisphere while tracking the rise and set points for the Sun, you’ll see that those spots creep north of the east-west line after the spring equinox, eventually stop, and then creep south for a while. After they cross the east-west line again, the southward creeping eventually slows down, stops, and gives way to the northward creeping once again. The entire cycle repeats annually.
All the while, the Sun’s trajectory is changing. On the summer solstice (Latin for “stationary Sun”), the Sun rises and sets at its northernmost point along the horizon, tracing its highest path across the sky. That makes the solstice the year’s longest day, and the stick’s noontime shadow on that day the shortest. When the Sun rises and sets at its southernmost point along the horizon, its trajectory across the sky is the lowest, creating the year’s longest noontime shadow. What else to call that day but the winter solstice?
For 60 percent of Earth’s surface and about 75 percent of its human inhabitants, the Sun is never, ever directly overhead. For the rest of our planet, a 3,200-mile-wide belt centered on the equator, the Sun climbs to the zenith only two days a year (okay, just one day a year if you’re smack on the Tropic of Cancer or the Tropic of Capricorn). I’d bet the same person who professed to know where the Sun rises and sets on the horizon also started the adage “the Sun is directly overhead at high noon.”
So far, with a single stick and profound patience, you have identified the cardinal points on the compass and the four days of the year that mark the change of seasons. Now you need to invent some way to time the interval between one day’s local noon and the next. An expensive chronometer would help here, but one or more well-made hourglasses will also do just fine. Either timer will enable you to determine, with great accuracy, how long it takes for the Sun to revolve around Earth: the solar day. Averaged over the entire year, that time interval equals 24 hours, exactly. Although this doesn’t include the leap-second added now and then to account for the slowing of Earth’s rotation by the Moon’s gravitational tug on Earth’s oceans.
Back to you and your stick. We’re not done yet. Establish a line of sight from its tip to a spot on the sky, and use your trusty timer to mark the moment a familiar star from a familiar constellation passes by. Then, still using your timer, record how long it takes for the star to realign with the stick from one night to the next. That interval, the sidereal day, lasts 23 hours, 56 minutes, and 4 seconds. The almost-four-minute mismatch between the sidereal and solar days forces the Sun to migrate across the patterns of background stars, creating the impression that the Sun visits the stars in one constellation after another throughout the year.
Of course, you can’t see stars in the daytime—other than the Sun. But the ones visible near the horizon just after sunset or just before sunrise flank the Sun’s position on the sky, and so a sharp observer with a good memory for star patterns can figure out what patterns lie behind the Sun itself.
Once again taking advantage of your timing device, you can try something different with your stick in the ground. Each day for an entire year, mark where the tip of the stick’s shadow falls at noon, as indicated by your timer. Turns out that each day’s mark will fall in a different spot, and by the end of the year you will have traced a figure eight, known to the erudite as an “analemma.”
Why? Earth tilts on its axis by 23.5 degrees from the plane of the
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