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Understanding Day Length Fluctuations – What They Are and What Causes Them

BY Sonal Panse September 18, 2019
Day length fluctuations.

A graph showing deviation of day length. (Ⅱ Ⅶ Ⅻ / Wikimedia Commons)

Most of us have learned in science class that a day consists of 24 hours. The time taken for Earth to turn once its axis is 23 hours and 56 minutes and 4.0916 seconds, and this can fluctuate too according to how the Earth’s atmospheric and oceanic processes affect its rotation.

Understanding day length fluctuations

The Earth takes to complete one rotation around its axis, but the length of time for this rotation can vary. So, while we think of a day as having 24 hours, this isn’t strictly the case. Some 300 million ago, the spin of the Earth was faster than it is now. If you took a time machine back, you would encounter days that were 21 hours long.

The Earth’s rotation has been gradually slowing over the millennia, and, consequently, the length of day has increased. However, there are other factors like climate, winds, oceanic processes, and atmospheric pressure systems that also affect the rotation of Earth on its axis. According to researchers, winds blowing against mountain ranges can be so strong that they affect the Earth’s rotation and cause day length fluctuations over a period of one year. These fluctuations may increase or decrease the length of the day by a millisecond.

If a day is 23 hours, 56 minutes and 4 seconds long, then how do we correct time?

We don’t. There are two different types of days: a sidereal day and a solar day.

A sidereal day is less than 24 hours, and that is the amount of time that the Earth takes to rotate 360 degrees on its axis. However, that is not the day we measure.

What we measure is the solar day, which is almost 24 hours long. It is the amount of time that the sun takes to move through the sky and end up roughly in the same spot in the sky. Why is the solar day length different than the length of the sidereal day? Well, because the sidereal day only considers the Earth moving around its axis, while the solar day also takes into account the Earth’s rotation around the sun.

Day length research by the University of Liverpool

Researchers from the University of Liverpool, led by Professor Richard Holme from the School of Environmental Sciences, studied fluctuations in day length from 1962 and 2012. Their research covered the fluctuations over one year and fluctuations over 10 years. Along with discovering that the variations in the length of the day were caused by processes in the Earth’s core, they were able to produce a model of the variations in the length of the day. For this model, they took into account the effects of atmospheric and oceanic processes on the Earth’s rotation to come up with time scales longer than a year.

Prior to Professor Holme’s study, which, incidentally, was conducted together with the Universite Paris Diderot, the general explanation about the fluctuations in length of the day was far from satisfactory. Thanks to the study, we now know that there are two key signals generated by the Earth’s core that characterise the variations. First, there is a steady 5.9-year oscillation, and, secondly, there are episodic jumps that occur simultaneously with abrupt changes in the Earth’s magnetic field. The study was published by Nature.

Day length research by NASA

The Atmospheric and Environmental Research Inc.’s scientist David A. Salstein studied wind and satellite data to collate information about day length fluctuations. He discovered that the Earth’s rotation signal is affected by strong changes in the Earth’s atmosphere. These include changes in the atmospheric pressure around the world and the motions of the winds that may be caused by climate cycles such as El Niño that affect global weather patterns.

The winds and air pressure patterns change from year to year, and these annual changes mean that the Earth is subjected to different forces. Now, El Niño can occur every two to seven years, and during the years it does, we experience stronger winds. The Earth slows its rotation around its axis in this period due to these strong winds. Of course, it is a very slight slowing that increases the length of a day by a thousandth of a second.

Newton’s laws of motion and the concept of angular momentum

We can use Newton’s Laws of Motion to understand day length fluctuations. Let’s paraphrase these laws: The first law states that if an object is in constant motion, it will remain in motion like that unless an outside force applies to it. The second explains that force is created from mass and acceleration. The third proclaims that for every action there will be an equal and opposite reaction.

The law of conservation of momentum resulted from the third law. According to this law, when two or more bodies act on each other in a separate system, their total momentum will remain constant. That is, unless an outside force acts on them.

Now, when the Earth spins around its axis, its overall mass and its rotation confer a certain amount of angular momentum on it. An additional force known as torque arises away from the Earth’s rotational axis as a result of surface wind changes and distribution changes in high and low-pressure patterns. The torque affects and changes the Earth’s rotational rate and also the direction of the Earth’s rotational axis.

Since the law of conservation of momentum states that the total momentum remains constant unless an external torque acts on them, the changes in the Earth’s rotation and the atmosphere’s rotation remain linked and the sum of the angular momentum or push of the Earth remains constant.

However, if the atmosphere speeds up due to strong westerly winds, then the Earth’s rotation has to slow down and the length of day will increase. Also, if more atmosphere moves to a lower latitude further from the Earth’s axis of rotation, and the atmospheric pressure increases, it also gains angular momentum and the Earth will slow down as well.

If the total angular momentum is to remain constant, there has to be a balance between the changes in the angular momentum in different regions. Let’s say there is a large atmospheric mass in one hemisphere and a comparatively smaller atmospheric mass in the other hemisphere, then there will be wobble. As per the law of conservation of momentum, the poles will then shift.

Exchange of angular momentum

Regular evaporation and precipitation occurs between the atmosphere and the non-gaseous parts of the Earth and leads to an exchange in angular momentum. A mass of water vapour arises into the atmosphere from the oceans and, in doing so, slows down the Earth’s speed of rotation. When the water vapour returns to the oceans in the form of rain, there is an increase in the Earth’s rotational speed. These changes in rotational speed cause corresponding day length fluctuations.

Fact Analysis:
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