Digital Diatribes

A presentation of data on climate and other stuff

Full Eclipse Coming, and The Lunar Cycle (2nd in a Series)

Posted by The Diatribe Guy on February 17, 2008

As I was taking a look at the weather site this evening with my 6-year old (trying to find out whether the 8 inches of snow we’d already received today was going to stop sometime before our house was buried) he saw a little video box that he asked me to play. Of course, as soon as I did, he lost interest and fled away to his next pursuit, but I was glad he had drawn my attention to it. Little did I know that this coming Wednesday, February 20, there will be a full lunar eclipse! This doesn’t come around all that often, and as luck would have it it’s supposed to be below zero degrees Fahrenheit when it happens, but you can bet I’m going to be checking that out.

Well, this got me thinking about something I had put together some time ago. Almost a year ago, when I was on one of my Global Warming/Climate Change kicks, I started doing research into the concept of cycles. I had intended to post a series about my research, but soon life took me in another direction and I failed to follow through. As a concept, it is my contention that Climate Change is effectively driven by a complex array of cycles. These cycles range from the daily cycle of a rotation of the earth, to monthly cycles relating to the moon, to the annual revolution of the earth, and so on and so forth. There are earthly cycles, solar cycles, axis oscillation cycles, and even cycles in our solar system and galaxy that can impact our planet. The contention I had was that everyone understands and expects the short cycles. By simple experience, we observe them, and while there are scientific explanations to everything, we don’t need a climatologist to explain to us that earth is warmer during the day and cooler at night, or that summer is hotter than winter. But I think we too easily dismiss other cycles that we do not understand as well, or those that go on for centuries or millennia, since we do not ever observe a full cycle. Instead, we convince ourselves that there is something unusual about changes we see going on today because it is different from a few decades ago, and the conclusion is that we are somehow causing it. I think we give ourselves too much credit.

Well, anyway, My “first in a series” post (almost a year ago) is here. It deals with the “one day cycle.”

After I made that post, I did some research on the lunar cycles. The moon makes a revolution, depending on the cycle measured, once every 27-30 days. Reading about the lunar eclipse reminded me of this research. Well, I actually found my notes and decided to share them here. The one day cycle addresses the tides, so I will not address that here. This is more an examination of the lunar cycles and whether or not there is any impact on the earth due to that cycle.

Well, I looked at a few things, and was actually surprised at how many ways they measure the lunar cycles. There are five cycles.

The Synodical Cycle is the length of time from New Moon to New Moon. It takes 29.531 days. It is the longest of the lunar cycles because it not only requires a full revolution of the moon around the earth in order to get to the same spot such that the sun’s light does not reflect towards earth, but in order for the moon to get in that same spot, it must actually travel more than one revolution relative to position above the earth. That is because the earth is revolving around the sun. Thus, in the 27+ days it takes the moon to revolve, there is another couple days of catching up to do because the sun’s light is now hitting the moon from a different angle than the month before, relative to our perspective.

The Aspidal Cycle is the 28 day rotation commonly referred to when people talk about how long the lunar cycle is. It’s actually the least observable cycle, however. It reflects the amount of time from the point where the moon’s axis is tilted furthest away from the earth, to the next time this occurs. The moon doesn’t appear to be rotating on its axis, but it is. It rotates at the same speed as it revolves around the earth, which is why we always the same side. It’s easy enough to see this by – oh – using your kids as a prop. Have a kid face one wall, and have another kid stand still. Have a kid walk in a circle around the other one, but continue facing the wall. As the kid goes in a circle, at some point, all sides can be seen at some point or another. If the kid now does a circle while looking at the other kid all the way around, the kid will have spun himself/herself around one time. Anyway, what we don’t see is that the moon is tilted on its axis just like earth is. While it is rotating, the axis is fixed. So, the moon is tilting towards us at one point and away from us at another. It takes 28 days to complete this cycle.

The Anomalistic Month Cycle represents the point at which the moon is at its closest point to earth in its orbit, and how long it takes to get to that point again. This takes 27.55 days. The reason this is shorter than 28 days also has to do with the fact that the Apsidal Cycle is also dependent on the earth’s revolution round the sun. Not to the extent of the Synodical Cycle, but there is still a catch-up required to get to the same axis-tilt point. The Anomalistic Month, however, is only dependent on where the moon is in its orbit, and this would be the same whether the earth was revolving around the sun or just sitting there.

The Sidereal Cycle is the amount of time it takes for the moon to appear in the same place in the sky. This takes 27.33 days. So, why would this be less than 27.55 days? One might think that the same spot in orbit = same spot in the sky. Well, the moon’s orbit is also rotating. It makes a full rotation once every 9 years. Because of this movement, it acts to “fling” the moon ahead a little bit, at least as we see it in its place in the sky. So, if we see the spot of the moon at apogee, the Sidereal month will occur just short of the next apogee, because the moon’s orbit rotated a bit. Making this a bit more complicated is that the orbit of the moon does not stay the same elliptical shape at all times. So, the Anomalistic and Sidereal Cycles are averages over the course of entire precessions.

Finally, the Nodical Month Cycle is the time it takes the moon to cross the equator traveling in the same direction as the previous crossing. This takes 27.21 days. The reason for this lesser time has to do with the same reasons for the shorter time frame in the Sidereal Cycle, as well as one additional factor – the plane of the moon’s orbit relative to the earth also precesses up and down. One full precession takes 18.6 years. Over this period, it varies from 18 to 28 degrees. This movement acts to shave a little time off the time needed to pass the equatorial line.

As interesting as this all, is, my research did indicate that the lunar cycles impact temperature and other geographic events, albeit not in a huge way. Basically, the moon is infinitely important by its mere presence. But its particular location at any one time, apart from the tides, don’t seem to be a driving factor in climate conditions, but following are the effects that I did find. It is estimated that the temperature difference during a full moon versus a new moon is 0.2 degrees Celsius. Temperature changes at the poles have been estimated to be impacted by the lunar month by a range of 0.55 degrees Celcius, according to a study by Shaffer, Cervery, and Balley. While not huge, it still indicates that cycles matter. All these cycles intertwine, and a lot of small impacts can lead to big ones on the occasions that they line up together.

It is also interesting to note that there are not just aquatic tides. There is an “earth tide.” It is estimated that land is “pulled” 2 centimeters by the moon when the rotation of the earth moves into moon territory. It is a reasonable question to ask, then, if the moon can cause seismic shifts. John Bellini, a geophysicist, studied this and found no evidence. Other studies in New Zealand agreed. The US Geological Survey also agreed, but in studies they did report a “small but significant” correlation between rate of aftershocks in volcanic regions and the semi-diurnal tides.

There does appear to be an effect in conjunction with solar minimums when the moon is within 4 degrees of the ecliptic plane, however, on weather and other phenomena, particularly in the mid-latitudes. But now we’re moving beyond the monthly lunar cycles, so maybe we’ll come back to that some day.

So, I’m not sure this post really says a whole lot about cycles as it relates to Global Warming and Climate Change. But the topic did interest me, and I learned quite a bit in the research. Hopefully, it’s interesting to the reader, as well.

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