Our Moon’s Contribution to Natural Variability:
The Moon’s gravitational pull (along with the gravitational pull of the sun, of course) has shaped much of Earth’s past and present. The moon impacts the Earth’s Tidal Patters. Tides are one of the more observable results of the moon’s gravitational pull.
We have the moon’s gravity to thank for the Earth’s steady axis and it’s one factor among several that influence Earth’s consistent rotation in the same direction. Earth’s unique and favorable axis of rotation determines the seasons and keeps our climate amenable to the development of life. Our moon also stabilizes Earth on its axis, so it’s less wobbly than it would be otherwise. You might recall we reviewed the Earth’s Wobble in Chapter One.
Not only does the moon light our night skies, inspire wonderment, and dictate the schedules of werewolves, it also makes life, as we know it possible. From its influences on the tides, to the regulation of the seasons, and its influence on our atmosphere, we have many reasons to thank our celestial neighbor.
The Moon is Earth’s nearest astronomical partner and we all know that it is the principal cause of the ocean tides. It has a large effect on the oceans and it also affects the atmosphere.
Figure 10. Shows the impact of the Sun and Moon’s gravitational pull on Planet Earth. It should be noted that although the Moon is mush smaller than the Sun, it is also much closer therefore that gravitational pull is greater that what might be expected.
The Moon’s smaller gravitational exertion in comparison to the Sun’s is contravened by its proximity to Earth. For instance, the moon is 2.17 times more effective than the massive sun in raising tides on Earth.
The moon’s gravity pulls at the Earth causes predictable rises and falls in sea levels tides. Tides also occur in lakes, the atmosphere, and within Earth’s crust. Since the atmosphere is also a fluid, the Moon generates gravitational tides in the skies above us.
Figure 11. Atmosphere Tides are also produced through the gravitational effects of the Moon. Lunar gravitational tides are weaker than solar thermal tides and are generated by the effect of the Moon’s gravitational attraction on the atmosphere.
The Moon’s gravitational related weather and climate impact on the Earth is smaller than the Sun’s thermally generated daily “tides” which heat and expand the air diurnally mainly through absorption by water vapor.
The Sun’s heating then also drives huge convection cells thereby generating the earth’s main wind systems like the “trade winds,” westerlies, etc. The Earth’s rotation turns these winds westerly in the northern hemisphere and easterly in the southern hemisphere.
So, the Moon’s gravitational pull impacts Planet Earth’s Ocean tides, as well as our Atmosphere.
Figure 12. As the water moves along the Earth, the combined forces of the Earth’s rotation and the gravitational pull from interstellar bodies such as the moon cause ocean levels to fluctuate continuously. A spring tide, which occurs when the moon is full or new, combines with the gravitational pull of the sun and causes a large difference between high and low tides. A neap tide occurs during quarter phases and works to cancel out the tidal effect from the sun, resulting in commensurate waves.
One could ask; why is it that the Moon’s gravity has a larger effect on the ocean’s tides than the Sun? As mentioned before, the Sun has a far larger direct gravitational force than the Moon, however the Moon orbits the earth much closer and in turn, the moon and earth orbit the Sun together.
The tides are a subtle effect caused by traction (lateral attraction) of the oceans by moon’s gravity and because the Earth – Moon system rotates about their center of gravity which is a point some 4000 km from the Earth’s center. The moon’s tidal force is proportionally greater and that explains why the lunar tidal force is stronger than the solar tidal force because it is over 500 times closer to the earth.
Our moon does have a gravitational pull on the atmosphere as well, but like the sun’s impact on our ocean tides, it’s a much weaker effect than the heating provided by the sun. If the moon were the main cause behind this atmospheric stretching, it would work the same way as the ocean tides; high tide would mean that you also had the most atmosphere above you, instead of what we see; a 24 hour cycle of our atmosphere heating and cooling under the sun’s rays.
As in the Ocean, the atmosphere also has tides and the moon impacts the earth’s atmospheric tides, as well as the oceans tides.
Figure 13. The moon contribute to fluctuations known as atmospheric tides, which are bulges and oscillations in the atmosphere similar to the way in which tides occur on water. Understanding atmospheric tides is important for understanding energy flow from the upper to lower atmosphere. However, the moon’s effect, which is the combined force of oceanic motion and the moon’s gravitational pull on the atmosphere, is much smaller than the sun’s effect, which is mostly a product of solar heating rather than gravitational pull.
We would ne missing some other important comments, if we neglected to mention Kim Long’s; “The Moon Book.” He states the ocean tides we experience on earth are caused by the sum of the moon’s gravitational gradient and the sun’s gravitational gradient. When the sun and the moon are aligned, or nearly aligned, their gravitational gradient fields add together constructively, leading to extra strong tides (high tide is extra high and low tide is extra low). This alignment happens when the moon is a new moon or a full moon, which occurs about every two weeks.
The moon takes about a month to orbit the earth; hence strong tides occur about twice a month. In contrast, when the sun and the moon are unaligned, their gravitational gradients tend to cancel out, leading to weak tides (high tide is not very high and low tide is not very low).
We have an active moon and its gravitational pull combined with that of our sun make our Planet Earth livable for all inhabitants. Here we can see our Sun and Moon working together.
Figure 11. The ocean tides on earth are caused by both the moon’s gravity and the sun’s gravity. In general, ocean tides are not generated by the overall strength of gravity, but instead by the differences in gravity from one spot to the next (the gravitational gradient). Because ocean tides are the effect of ocean water responding to a gravitational gradient, the moon plays a larger role in creating tides than does the sun. But the sun’s gravitational gradient across the earth is significant and it does contribute to tides as well.
Two tidal bulges cause the tides, one nearest to the Moon and the other on the opposite side to the Moon. The one on the opposite side is actually the result of the centrifugal effect of the Earth Moon rotation. In fact the ocean on the side opposite to the Moon is bulged outward due to this centrifugal force. Tides are greater at higher latitudes, which is why the strongest tides in the world are found in Canada and Northern Europe whereas relatively weak tides are found at the equator.
We often hear the remark that weather systems are chaotic and the flapping of butterflies’ wings can trigger a hurricane. Therefore it seems a reasonable hypothesis that the lunar tides may be one trigger for stormy weather, especially as tides are stronger nearer the poles. During the lunar month there are 2 so-called spring tides when the Moon and Sun align so their tidal effects sum up. The above hypothesis would imply more stormy rainy weather with the spring tides. Another effect is the precession of the lunar orbit with a timescale of about 18 years. This actually changes the inclination of the Moon in the sky, effectively moving the tidal bulge to a more or less northerly position. Folklore about the Moon’s effect abounds in many cultures, so one could ask; what other ways can the moon affect the weather?
There are other direct astronomical effects that the moon has on the Earth’s radiation balance and there seems to be one very important long-term benefit.
One, the center of mass of the Earth Moon system is actually about 4000 km from the center of the Earth. As the Moon orbits this point so too does the Earth “orbit” the center of mass point. In other words the Earth changes its position relative to the Sun by about 8000 km. The mean Earth-Sun distance is about 150 Million km so the monthly change in solar radiation caused by this leads the to an approximate 0.03 degrees change on the earth.
Two, there is reflected sunlight from the Sun. So-called moonshine is simply light reflected or re-emitted from the moon and this radiation serves to heat the earth slightly. This is an extra radiation of the direct solar energy and this also gives a roughly 0.01-0.02-degree effect on planet earth’s temperature.
Three, the moon stabilizes the Earth’s axis. It seems likely that without the Moon’s stabilizing gyroscopic effect; the earth’s axis would be more chaotic. The seasons rely on the axis being tilted to the orbital plane of the earth-sun by about 23 degrees. Computer simulations show that the moon’s tidal effect has probably stabilized this tilt over billions of years. Clearly maintaining summer, winter conditions for each hemisphere has had a massive impact on the earth’s climate and this could also be the key to life developing.
In conclusion, the Moon does affect the Earth’s climate. It is even fundamental to maintaining and stabilizing the seasons. There is also growing evidence that the Moon can influence generations of storms and rainfall. Eventually energy loss by tidal forces will slow the length of the day to say 25 hours and the Moon’s distance from the Earth is also slowly increasing. Luckily any noticeable effects will take 10s of millions of years and way before, then the next Ice Age will be far more of a problem for mankind.
In chapter 4, we will review the impact of Polar Conditions on Natural Variability