The Leap Second Insertion
The last minute of June this year, Tuesday June 30th, will be an odd one. It will consist of 61 seconds due to the occasional insertion of what’s called a leap second.
I’m seeing a lot of websites that try to explain why we use leap seconds. This is often described as necessary because the earth is slowing down. A slower rotation of the earth means a longer day which means we have to add time, right?
Well, not really. The earth’s rotation is indeed slowing down regularly and relentlessly but it is minuscule. Something on the order of milliseconds per century. This is largely due to tidal breaking which goes something like this:
- The moon’s gravity causes a tidal bulge on the near-side of earth (and the far side)
- Earth’s rotation tries to spin that bulge away from the moon
- The moon tugs on the bulge trying to move it back against the rotation of the earth and the bulge tugs on the moon increasing its orbital speed
- This backwards pull on the bulge causes friction that slows the rotation of the earth
The evidence for this is clear in the geologic records. Tidal records point to a day only 23 hours long when the dinosaurs roamed the earth. Go back billions of years and it goes down to a 10 hour day and perhaps as few as a 2 or 3 hours. This loss of earth’s rotational energy can’t just disappear so it’s transferred to the moon causing it to move a couple centimeters away every year.
If earth’s tidal slowing was the primary reason for our leap-seconds then that would mean that only 118 million years ago, a day would have been just one second long. That clearly could not have been true so what the hell is the leap second doing if it’s not making up for the slowing of the earth as so many are claiming?
Think of it like a leap year which inserts one day every four years or so. We are not adding a permanent and persistent day to the calendar every leap year. We are actually re-synchronizing the calendar year with the solar year. This is because a calendar year is 365 days and our journey around the sun takes 365.25 days. Similarly, the true purpose of the leap second is to reconcile the two primary ways that we measure a day or a second so they do not diverge from each other too much.
The first way is called astronomical time. This time standard uses astronomical objects to mark the passage of time. In this scheme, signals from extremely distant quasars are used to determine with tremendous accuracy the rotation rate of the earth.
The second method is atomic time. This tracks time based on the movements of a cesium atom. When it oscillates 9,192,631,770 times, a second has passed.
The problem arises because these two time standards do not perfectly mesh. The atomic second is incredibly stable, so 60 (seconds) X 60 (minutes) X 24 (hrs) always adds up to 86,400 unchanging seconds per day. Astronomical time however is slightly variable with never the exact same amount of seconds in any given day or year. Earth’s rotation adds up to something like 86,400.002 seconds per day on average. It is this difference of 2 milliseconds that accrues every day, widening the gap between atomic time and astronomical time, that necessitates the insertion of a leap second (every 500 days or so).
The reason for this difference is the simple fact that earth’s rotation if effected by a maddeningly complex and unpredictable array of movements.
- The molten liquid and plastic-like interior disrupts consistent rotation. This is like spinning a sloshy regular egg (which falls down fast) compared to a hard-boiled egg (which spins nicely).
- Weather systems can actually subtly effect earth’s rotation
- Earthquakes can subtly redistribute the mass of the earth in ways that also impact its spin (think of a spinning ice-skater pulling in his or her arms)
- Tidal breaking, as described above, also has a measurable, if tiny, effect as well
So enjoy the extra second tonight. Anything that makes my summer a little longer is good news in my book.
Image Credit: http://www.usatoday.com/story/tech/2015/01/08/computer-chaos-feares/21433363/