Our Fate Of Longer Days And Nights

You stayed up until 2:00 a.m. to finish your physics lab report. If only there were more hours in the day… your wish could one day come true since Earth’s rotation is gradually becoming slower, prolonging daytime and nighttime. 

The Earth takes about 24 hours to complete a full rotation. Thus, we have 12 hours of daylight and 12 hours of nighttime. However, one Earth day hasn’t always been 24 hours long (the term “day” refers to the entire duration of Earth’s rotation). Approximately 4.5 billion years ago, when the moon had just formed, one Earth day was less than 10 hours long. Over time, the gravitational force exerted by the newly-formed moon attracted Earth’s ocean water. This created tides that bulged outwards from the planet. The gravitational pull, combined with the friction produced by the tides, countered Earth’s rotation. These forces slowed it down, contributing to a steady reduction in Earth’s rotation speed each century.

The sun, though 93 million miles away, still exerts a gravitational force on Earth. It pulls on Earth’s atmosphere, causing atmospheric bulges. The sun’s gravity pulls on the atmosphere unevenly, resulting in a torque (the amount of force that causes a body to rotate about its axis), which increases Earth’s speed of rotation. Because the moon is closer to Earth, its gravitational force on the tides is 10 times stronger than the sun’s gravitational force on the atmosphere. Think of the moon as a strong teenager spinning a globe in one direction while the sun is a weak child trying to spin it in the other direction. In this way, the sun’s countering force reduces the force of the moon, slowing down Earth’s rotation. As a result, our planet’s days continued to grow longer, but not very quickly.

After about two billion years, the rate of increase in Earth’s daylight hours became irregular. If the rate had remained, one day in the 21st century would have been more than 60 hours long. Scientists determined that between 2.2 billion and 600 million years ago, our Earth experienced a “pause” in the slowing of its rotation. During this time, the atmosphere became significantly warmer due to the end of the Huronian Ice Age. The significant increase in global temperatures produced variations in atmospheric pressure—thermal (atmospheric) tides. Thermal tides drove air away from warmer regions on Earth, generating more movement of the atmosphere. As a result, the atmosphere moved at a higher speed, which sped up Earth’s rotation as well. This led to resonant stabilization on Earth. 

 Most objects vibrate because they are not in a state of equilibrium but are trying to achieve one. Resonance, a key player in Earth’s rotation, is a phenomenon that causes objects to vibrate at their natural frequency—the frequency as it oscillates—without any external driving force. As a result, the object vibrates more strongly when it is subjected to vibration from an external source equal to the object’s vibration. For example, microwaves in a microwave oven oscillate close to the frequency of water molecules (2.45 gigahertz). This amplifies the frequencies of the water molecules inside foods, causing them to gain sufficient kinetic energy (movement energy) and increase the food’s temperature. In a similar manner, the accelerated thermal tides became just as fast as Earth’s rotation, achieving the same frequency. Since the thermal tides were resonant with Earth’s rotation, the speed of this rotation increased. At some point, the speed of thermal tides caught up to the speed of ocean tides. Since the thermal tides and ocean tides moved in opposite directions, there was no net increase or decrease in the Earth’s rotation speed. For over a billion years, one Earth day was consistently 19.5 hours long. 

Eventually, 600 million years ago, the atmospheric tides and Earth’s rotation no longer matched speeds. Large-scale Earth events like earthquakes and the end of an ice age decreased Earth’s speed of rotation. Since the thermal tides and Earth’s rotation were no longer moving in sync, their resonance was gone. Without resonance, the Earth became subject to greater influence from the moon than from thermal tides, causing Earth days to continue increasing in length. In today’s 24-hour Earth days, the atmospheric tides travel around the world in only 22.8 hours. This is out of sync with the Earth’s rotation rate. Thus, Earth rotates 1.7-milliseconds slower each century.

Global warming also plays a role in the shortening of day and night. As global temperatures continue to rise, the atmospheric tides travel faster, “outrunning” the Earth’s rotation. In other words, the atmospheric tides and Earth’s rotation become even more out of sync. The more out of sync these two forces become, the less influential thermal tides become on Earth’s rotation speed. As a result, the moon’s tidal forces are felt more strongly and Earth spins slower, prolonging daytime and nighttime.

Unless global warming is halted or another major event (a massive earthquake, another ice age, etc.) disturbs Earth’s rotation, the days will only grow longer. Though the Earth’s rotation is slowing at a faster rate than it has historically, a dramatic increase in day length won’t be noticeable for millions of years. Perhaps in a million years, future Stuyvesant students will finally get enough sleep during the long nights. Or maybe not, if they are assigned more homework during their equally long days!