Read Online Death by Black Hole: And Other Cosmic Quandaries by Neil deGrasse Tyson - Free Book Online Page A

solar system. This tilt not only gives rise to the familiar seasons and the wide-ranging daily path of the Sun across the sky, it’s also the dominant cause of the figure eight that emerges as the Sun migrates back and forth across the celestial equator throughout the year. Moreover, Earth’s orbit about the Sun is not a perfect circle. According to Kepler’s laws of planetary motion, its orbital speed must vary, increasing as we near the Sun and slowing down as we recede. Because the rate of Earth’s rotation remains rock-steady, something has to give: the Sun does not always reach its highest point on the sky at “clock noon.” Although the shift is slow from day to day, the Sun gets there as much as 14 minutes late at certain times of year. At other times it’s as much as 16 minutes early. On only four days a year—corresponding to the top, the bottom, and the middle crossing of the figure eight—is clock time equal to Sun time. As it happens, the days fall on or about April 15 (no relation to taxes), June 14 (no relation to flags), September 2 (no relation to labor), and December 25 (no relation to Jesus).
    Next up, clone yourself and your stick and send your twin due south to a prechosen spot far beyond your horizon. Agree in advance that you will both measure the length of your stick shadows at the same time on the same day. If the shadows are the same length, you live on a flat or a supergigantic Earth. If the shadows have different lengths, you can use simple geometry to calculate Earth’s circumference.
    The astronomer and mathematician Eratosthenes of Cyrene (276–194 B.C .) did just that. He compared shadow lengths at noon from two Egyptian cities—Syene (now called Aswan) and Alexandria, which he overestimated to be 5,000 stadia apart. Eratosthenes’ answer for Earth’s circumference was within 15 percent of the correct value. The word “geometry,” in fact, comes from the Greek for “earth measurement.”
    Although you’ve now been occupied with sticks and stones for several years, the next experiment will take only about a minute. Pound your stick into the ground at an angle other than vertical, so that it resembles a typical stick in the mud. Tie a stone to the end of a thin string and dangle it from the stick’s tip. Now you’ve got a pendulum. Measure the length of the string and then tap the bob to set the pendulum in motion. Count how many times the bob swings in 60 seconds.
    The number, you’ll find, depends very little on the width of the pendulum’s arc, and not at all on the mass of the bob. The only things that matter are the length of the string and what planet you’re on. Working with a relatively simple equation, you can deduce the acceleration of gravity on Earth’s surface, which is a direct measure of your weight. On the Moon, with only one-sixth the gravity of Earth, the same pendulum will move much more slowly, executing fewer swings per minute.
    There’s no better way to take the pulse of a planet.
     
     
    UNTIL NOW YOUR stick has offered no proof that Earth itself rotates—only that the Sun and the nighttime stars revolve at regular, predictable intervals. For the next experiment, find a stick more than 10 yards long and, once again, pound it into the ground at a tilt. Tie a heavy stone to the end of a long, thin string and dangle it from the tip. Now, just like last time, set it in motion. The long, thin string and the heavy bob will enable the pendulum to swing unencumbered for hours and hours and hours.
    If you carefully track the direction the pendulum swings, and if you’re extremely patient, you will notice that the plane of its swing slowly rotates. The most pedagogically useful place to do this experiment is at the geographic North (or, equivalently, South) Pole. At the Poles, the plane of the pendulum’s swing makes one full rotation in 24 hours—a simple measure of the direction and rotational speed of the earth beneath it. For all other positions on