Read Online The Universe Within by Neil Shubin - Free Book Online Page B
Authors: Neil Shubin
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physics and momentum tie the speed of the spinning of Earth to the rotation of the moon. The impact on our lives is as straightforward as it is profound: the length ofdays and of months, like the workings of theseasons, derive from the Earth-moon system. Every clock andcalendar, like the cells of our bodies, holds artifacts of a cataclysm that took place over 4.5 billion years ago.
The big whack. Theorigin of the moon.
KEEPING TIME
The Romans had an effective way of controlling troublesome officials in the far-flung regions of their empire. Instead of gerrymandering districts to stay in power—to help friends and get rid of foes—Caesar and his cronies found the ultimate way to retain control. They gerrymandered the calendar. Have a political friend in one region? Add a few extra days to his term. Want to get rid of a foe in another place? Lop days of his rule off the year. This was wonderfully effective; however, over time, not only did thedecentralized calendar make ruling difficult, but the year became a patchwork of political kludges, fixes, and compromises.
The nature of Earth’srotations in space makes it ripe for thesekinds of abuses. We all learn this material in school, but most of us forget the meaning of the planet’srotations by the time we are in college. A recent survey ofHarvard undergraduates asked the simple question: What causes theseasons? Over 90 percent of them got the answer totally wrong. The answer has nothing to do with the amount of light that hits Earth during winter and summer, nor with Earth rocking back and forth, nor with the planet getting closer to the sun over the course of the year.
As we’ve known since thedays ofCopernicus and his contemporaries, the moon rotates around Earth, while Earth retains its constant 23.5-degree tilt as it rotates around the sun. The angle that sunlight hits the planet changes at different parts of theorbit. Direct light generates the long days and heat of summer; tilted and less direct light gives us shorter and colder winter days. The seasons aren’t generated by Earth rocking back and forth; they derive from the planet having a constant tilt as it rotates around the sun.
Because of the different orbits that affect our lives—ours around the sun and the moon around us—there are choices to make when constructing acalendar. Of course, the length of a year is based on the rotation of Earth around the sun. If we know the longest and shortest days, we can carve up the year into months based on the seasons. Another way to do this is to base the calendar on the position of the moon as it goes from full to partial to new every twenty-nine days. The problem is that you can’t synchronize alunar calendar with aseasonal, or solar, one. The number of lunar cycles does not correspond easily to the number of seasonal ones.
So what do we do? We add fudge factors.Julius Caesar’s calendar had a leap year every threeyears to keep the months in line with the seasons. The problem with this calendar for the Catholic Church was the extent to which the date of Easter wandered. To rectify this situation,Pope Gregory XIII initiated a new calendar in 1582. Italy, Spain, and a few other countries launched itimmediately following the papal bull, resetting October 4, 1582, to October 15, 1582, losing elevendays. Other countries followed to different degrees. Britain and the colonies, for example, only accepted it in 1752. One of the most important issues to iron out, naturally, was when to collect taxes.
Years, months, and days can, at least in theory, be based on celestial realities, but minutes and seconds are mostly conventions. Our calendar has seven days because of the biblical story of a six-day creation, followed by a day of rest. Minutes and seconds are in units of 60 due to a matter of convenience. Theancient Babylonians had a number system based on 60. It turns out that 60 is a wonderful number because it is divisible by 1, 2, 3, 4, 5, and 6.
Humans are a
The big whack. Theorigin of the moon.
KEEPING TIME
The Romans had an effective way of controlling troublesome officials in the far-flung regions of their empire. Instead of gerrymandering districts to stay in power—to help friends and get rid of foes—Caesar and his cronies found the ultimate way to retain control. They gerrymandered the calendar. Have a political friend in one region? Add a few extra days to his term. Want to get rid of a foe in another place? Lop days of his rule off the year. This was wonderfully effective; however, over time, not only did thedecentralized calendar make ruling difficult, but the year became a patchwork of political kludges, fixes, and compromises.
The nature of Earth’srotations in space makes it ripe for thesekinds of abuses. We all learn this material in school, but most of us forget the meaning of the planet’srotations by the time we are in college. A recent survey ofHarvard undergraduates asked the simple question: What causes theseasons? Over 90 percent of them got the answer totally wrong. The answer has nothing to do with the amount of light that hits Earth during winter and summer, nor with Earth rocking back and forth, nor with the planet getting closer to the sun over the course of the year.
As we’ve known since thedays ofCopernicus and his contemporaries, the moon rotates around Earth, while Earth retains its constant 23.5-degree tilt as it rotates around the sun. The angle that sunlight hits the planet changes at different parts of theorbit. Direct light generates the long days and heat of summer; tilted and less direct light gives us shorter and colder winter days. The seasons aren’t generated by Earth rocking back and forth; they derive from the planet having a constant tilt as it rotates around the sun.
Because of the different orbits that affect our lives—ours around the sun and the moon around us—there are choices to make when constructing acalendar. Of course, the length of a year is based on the rotation of Earth around the sun. If we know the longest and shortest days, we can carve up the year into months based on the seasons. Another way to do this is to base the calendar on the position of the moon as it goes from full to partial to new every twenty-nine days. The problem is that you can’t synchronize alunar calendar with aseasonal, or solar, one. The number of lunar cycles does not correspond easily to the number of seasonal ones.
So what do we do? We add fudge factors.Julius Caesar’s calendar had a leap year every threeyears to keep the months in line with the seasons. The problem with this calendar for the Catholic Church was the extent to which the date of Easter wandered. To rectify this situation,Pope Gregory XIII initiated a new calendar in 1582. Italy, Spain, and a few other countries launched itimmediately following the papal bull, resetting October 4, 1582, to October 15, 1582, losing elevendays. Other countries followed to different degrees. Britain and the colonies, for example, only accepted it in 1752. One of the most important issues to iron out, naturally, was when to collect taxes.
Years, months, and days can, at least in theory, be based on celestial realities, but minutes and seconds are mostly conventions. Our calendar has seven days because of the biblical story of a six-day creation, followed by a day of rest. Minutes and seconds are in units of 60 due to a matter of convenience. Theancient Babylonians had a number system based on 60. It turns out that 60 is a wonderful number because it is divisible by 1, 2, 3, 4, 5, and 6.
Humans are a
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