Is the Earth’s Rotation Speeding Up?

A complex tidal action of the moon on the earth slows the earth’s rotation. We call this tidal braking. Since the earth and moon represent a closed system in this interaction, by Newton’s third law of motion, the earth pulls back on the moon, accelerating its orbital motion around the earth. This causes the moon’s orbit to increase in size, so the moon is slowly spiraling away from the earth. The current rate of lunar recession is measured as nearly four centimeters per year. That is less than two inches per year, so that may not seem like much, but over time, it can add up.

How can we measure this rate of lunar recession so accurately? As I pointed out in a recent article, Apollo astronauts left special reflectors on the moon. Frequently, astronomers use large telescopes to bounce brief laser light pulses off those reflectors and record the reflected beams. The difference in time between the transmitted and received beams gives a very accurate distance to the moon.

Apollo 14’s laser ranging retro reflector (LR3). Image via Wikimedia Commons.

Where this gets interesting is that we know the physics of this interaction very well, and the math reveals that the rate of lunar recession is not constant. Rather, the closer the earth and moon are to one another, the faster the rate of lunar recession is. Since the moon is getting farther away, in the past, the moon must have been moving away faster than the four centimeters per year that we measure today.

The maximum age that the earth-moon system can have is less than one-third of the 4.5 billion years.

Proper modeling allows us to trace the moon’s distance from the earth as a function of time in the past, using the four centimeters per year as our starting point. Over a few thousand years, the earth-moon distance would not have changed much. But what about over 4.5 billion years that most scientists think is the age of the earth-moon system? The maximum age that the earth-moon system can have is less than one-third of the 4.5 billion years. Of course, that does not automatically mean that the system is only 6,000 years old, but it does seem to eliminate any age greater than 1.5 billion years.

Have We Seen the Last of Leap Seconds?

Many creationists are aware that the related phenomenon of the earth’s rotation slowing is the need of leap seconds from time to time, and so many creationists confidently point to those occasional leap seconds as evidence that the earth and moon are young. Consequently, many recent creationists are probably confused by recent press statements that we may have to start subtracting seconds rather than adding seconds. It’s as if the earth has started increasing its speed of rotation, making the day slightly shorter rather than longer. If so, then has the moon stopped receding from earth?

No, the moon is still receding from earth, even though the earth’s rotation is speeding up, as it has been for half a century. What? Then what are all those leap seconds all about? I have a more technical discussion of what is going on in an upcoming Answers Research Journal article, but here, I’ll give a very brief description of what is going on.

One fundamental purpose of astronomy is for timekeeping (Genesis 1:14). Time of day is based upon the sun’s location in the sky and hence is tied to the earth’s rate of rotation. For nearly two centuries after Newton, physicists and astronomers assumed that the earth’s rotation rate was fixed. That seemed reasonable, and it was consistent with what we observed. However, by the latter nineteenth century, technology of time measurement had greatly improved, and it was obvious that the earth’s rotation was slowing. This caused a problem because this meant our time standard was changing, but dynamic processes, such as gravity keeping planets in orbit, do not depend upon varying time. This called for a new time standard that is not varying. This non-varying time standard is called ephemeris time. Meanwhile, the slowly varying time standard that we use in everyday life is based upon Universal Time, which is closely related to Greenwich Mean Time.

Tourists at the Royal Observatory, Greenwich, in line to take pictures with the Prime Meridian monument. Image via Wikimedia Commons.

More than a century ago, astronomers decided to define the second (in terms of ephemeris time) as a precise fraction of the year in 1901. Since this was based upon a dynamic process (the earth’s orbit around the sun), this was a good standard for ephemeris time. Even then astronomers noted the drift between the second as defined by observing the sun’s position in the sky from the second defined by dynamic processes.

Over the years, advancements in technology allowed measuring these quantities with greater accuracy, which required some tweaks in how we define time. In 1967, time was redefined in terms of the speed of light. Atomic clocks became very accurate, so in 1972, the time standard was once again refined, with Universal Time now fixed by atomic clocks, not by where the sun is in the sky. This introduced a problem because this new definition of Universal Time would slowly lead to a deviation from time as told by where the sun is in the sky. The solution agreed upon then was to add or delete seconds as needed to bring Universal Time back into close agreement with time as we have always expressed it in terms of where the sun is in the sky.

As it turns out, there are effects other than tidal braking that change the earth’s rotation rate. Some of these effects act over the short term of a few years, some effects are on the midterm of a few decades, while others are on the long term of a century or more. Some of these factors slow the earth’s rotation, while others increase the earth’s rotation. These other effects are cyclical, going from slowing the earth’s rotation to speeding the earth’s rotation. Only tidal braking is always slowing the earth’s rotation, which places an upper limit in the age of their creation.

Remember that the original standard for the second was tied to the earth’s rotation in the year 1901? That year happened to be near the peak of the fastest rotation of the earth over the past two centuries. As the standard for time measurement changed over the years since, astronomers kept that original definition of ephemeris time by renormalizing each new definition to the original definition. Between 1901 and 1972 when atomic clocks became involved, the earth’s rotation sped up but then slowed down to what its rate was early in the twentieth century.

The first successful atomic clock, built in England in 1955. Image via Science Museum Group, CC BY-NC-SA 4.0.

Since dynamic time was fixed to a shorter second than what we have experienced since 1972, leap seconds were added, not because the earth’s rotation was slowing that much but because the standard second was too short. Indeed, in the five decades since 1972, there has been an increase in the earth’s rotation rate, which is why leap seconds were required. It’s very counterintuitive that leap seconds were required because the earth’s rotation has been speeding up, when all along most of us assumed that it was because the earth’s rotation was slowing.

Indeed, in the five decades since 1972, there has been an increase in the earth’s rotation rate, which is why leap seconds were required.

But the rate that we have added leap seconds has decreased in recent years. That is because the rate of the earth’s rotation is nearly back to what it was in 1901. If and when the earth’s rotation reaches and exceeds the 1901 rotation rate, then we will have to subtract seconds rather than add them. There is discussion to scrap the addition and subtraction of seconds. If that happens, Universal Time and ephemeris time will drift. Also, our time standard for everyday use will slowly drift from the time as told by the sun. I don’t like that.

In case you are wondering what is responsible for the earth’s rotation rate to be decreasing now, it appears to be due to motions in the molten part of the earth’s core. That process is not known well at all. The best we can do is observe the variations in the earth’s rotation, account for all the known causes, and infer that the remainder of the change is due to changes in the earth’s core. On a related front, the earth’s magnetic poles field has been moving very quickly in recent years. Since the earth’s magnetic field is due to motion in the core, this indicates some unusual things transpiring in the earth’s core.

Conclusion

So is the moon receding at a rate that demonstrates that the earth-moon system cannot be any older than 1.5 billion years? Yes. Is the addition of leap seconds evidence that supports that argument? Until recently, I would have said yes, but now I don’t think so. This whole issue of time measurement is very tricky. If my brief description here has left your mind in a muddle, don’t feel like the Lone Ranger—this had me banging my head against the wall for a while too. Welcome to the club.