Free Zoom: From Atoms and Galaxies to Blizzards and Bees: How Everything Moves by Bob Berman Page A

care? Then our clocks would almost never need leap seconds. But we didn’t. So every year or two now, the little daily error accumulates enough so that we must take care of the accrued discrepancy.
    To sum up, the real problem is not that Earth is slowing, which happens too gradually to matter much. It’s that each of our current days is longer than a day was in 1820, upon which our timekeeping system is, bewilderingly, based.
    Because we foolishly designed the “second” around the 1820 data, we now need to compensate for the difference between a day now and a day when James Monroe was president. That means adding a second every five hundred days or so. It’s a “patch” to keep Earth-spin time and atomic-seconds time in agreement.4
    As the Earth slows, the sun moves more leisurely across the sky. Its gradual slowdown is of course unnoticed in human lifetimes. Instead the dominant rhythm that affects us is its position in the sky. The sun is low and feeble during winter and high and fierce in summer. The daily light-darkness ratio—winter’s short days and long nights—is also critical. Other than that, most folks are oblivious to the sun’s motion. How many people even realize that throughout the Northern Hemisphere, in the United States, Europe, China, and so on, the sun always moves to the right? Meaning the sun rises diagonally upward to the right, then moves directly rightward at midday, and sets by slinking rightward into the western horizon.
    Equatorial residents view something different. There the sun rises straight up until it gets overhead. Then, through the afternoon, it drops straight down like a lead ball. Because of this, it quickly buries itself below the horizon after sunset. Twilight in the tropics is always short. In the Southern Hemisphere, the sun moves leftward during the day. It’s a quick way of knowing where you are in case you’re ever shanghaied and wake up on another continent.
    Can you handle one more solar oddity? Over the course of a year, day and night are not balanced. Thanks to our atmosphere, which bends light, the sun seems to sit on the horizon when it’s actually already set. At that point we see a ghost, a solar phantom. This air trickery, refraction, grants most locations seven minutes of extra daily sunlight. It’s why days and nights are not equal at the equinoxes: sun dominates.
    This undeserved sunshine adds up. We enjoy forty extra hours of sunlight annually. The year is not even close to a fifty-fifty day-night mix.
    On top of that, as we all know, sunset is never followed by sudden blackness. On the moon, yes, but not here. Refraction delivers its enchanting gift of twilight. Its brightest portion bestows yet another hour of useful light split between dawn and dusk.
    The brightest afterglow is called civil twilight. Although it sounds vague, the term twilight is precisely, legally defined, dictated by the sun’s unseen motion below the horizon. In the evening it’s the interval between sunset and the time when the sun has sunk six degrees, or a dozen sun widths. Civil twilight lasts about a half hour in most places. At its conclusion, according to many municipal ordinances, streetlights must be on.5
    But the bottom-line sun motion is its speed as it crosses the sky. Most people don’t know about angles or degrees, so let’s simply use the sun’s own width as a measuring tool. Think about all the sunsets you’ve watched. How long does it take the sun to move a distance equivalent to its own diameter? Or ponder the moon instead, which moves at the same visible speed. The answer:
    Crossing the sky, the sun traverses its own width in exactly two minutes.
    During a sunset, because the sun slides into the horizon at an angle, the interval from first contact to complete disappearance is about three minutes. This is right on the borderline of perceptible motion. The sun appears to move at the same speed as the minute hand of a kitchen clock when viewed from a few feet