The Moon's place in the sky

First of all, a disclaimer, this thread may appear to be more suited to astronomy, but as weather enthusiasts are often avid sky watchers, I think it's perhaps appropriate to review the basic facts about the Moon's orbit in our weather forum. At the very least, it probably means that many more Boards members will happen to notice the thread and so the information will be more widely available.

We've already had some discussions about the Sun's place in the sky, which changes mainly in the predictable seasonal way due to the fact that the earth's rotational axis is inclined at 23.4 deg to the orbital plane. That has changed very gradually over historical time frames, it used to be closer to 24 deg in the Neolithic period and is slowly on its way to a smaller value of about 22.6 deg in the distant future. Those changes have produced small but significant changes in the position of sunrise and sunset at solstice times of the year (the Neolithic observers would have seen the Sun rising very slightly further north at the summer northeast sunrise point, and also a bit further south than we see it nowadays at the winter sunrise).

The Moon is unique among large satellites in the solar system in that it rotates in the ecliptic plane and not the earth's equatorial plane (which is where all the other major satellites orbit their planets). This is no doubt due to the larger solar gravitational force on our Moon compared to similar forces out five to thirty times further at the various other planets with large satellites. But the Moon does not orbit in the ecliptic perfectly, it is inclined to it by 5.1 deg, and that causes some interesting changes over relatively short cycles. Before getting to those, I will mention that the most familiar lunar cycle is "synodic" or related to the earth-Sun axis, so that we observe a cycle of lunar phases with 29.53 days between them, on average. Most laypersons are probably only familiar with this one lunar cycle. It basically forms the foundation of our calendar which had to be adapted to the differential (amounting to about 11 days) between twelve lunar months and a solar year. Cultures which use a lunar calendar adapt by adding periodic thirteenth months, which needs to be done roughly every third year and more precisely seven times every nineteen years. That is because the Moon almost returns to the same dates for full and new moon every 19 years (this does drift forward in the calendar by about a half day per century so it's not quite a perfect rule). The Jewish calendar, as one example, has an added month known as "second (twelfth month)" that is added when needed.

Our calendar left behind the lunar connection when the Romans decided they wanted to have a solar-based calendar but the fact that months average just over 30 days and weeks are seven days (like the four phases of the moon) shows that the severance was not complete.

There are several other lunar cycles of interest, and perhaps of particular interest now as we approach the peak of one of them. Some explanation is required.

The fact that the Moon orbits in the ecliptic plane with a 5.1 deg inclination means that twice each lunation, it is that far above or below the ecliptic plane (the path in the sky where the Sun would be at various times of year). Now this cycle is not linked to the synodic month but to the sidereal month, defined as the period of time between the Moon's passage of fixed star reference points. That cycle is shorter by 2.2 days, and in more precise numbers, is 27.32166 days long. The Moon usually travels near two bright red stars that happen to be almost opposite one another in the sky, Aldebaran (in Taurus) and Antares (in Scorpio). So as one example, it would be 27.32 days between near conjunctions of these stars and the Moon. The Milky Way galaxy can be seen in dark skies to the left of both of these, and the Moon crosses at two points very close to its highest and lowest declination points in each cycle (declination is the difference between the ecliptic plane and the earth's equatorial plane along which declination is defined to be zero deg). Latitude, on the other hand, is the difference between the ecliptic plane and any given point.

So on the average, the Moon crosses the galactic structure we call the Milky Way at about 23.4 North (full moon in late December) and 23.4 deg South (full moon in late June). However, that 5.1 deg differential is always moving, fairly rapidly by astronomical standards, in a retrograde direction (opposite to the Moon's forward motion along the ecliptic), and it takes only 18.6 years to perform one full rotation of the path. At the present time, we are approaching a peak in declinations where the 5.1 deg will be superimposed on the declination extremes. This will peak in summer 2024. The dates of recent declination peaks came in 1913.0, 1931.6, 1950.2, 1968.8, 1987.4, 2006.0 and then due again 2024.6. So already we are seeing quite a range in lunar declination that will be quite apparent at the winter full moons with the Moon riding very high, at almost 28 deg. I just happened to see the moon at this same position early this morning (not full because we are at a much different angle to the Sun) and it was quite close to the lower of the two stars Castor and Pollux. In the two following winters the full moons will be even slightly higher. And in the next three summers you'll really notice how low the full moons will be appearing as they transit in the southern skies at midnight.

At the opposite point of the cycle, the latitude counteracts the declination and the Moon crosses the Milky Way at around 18 deg N and then 18 deg S, making those full moons noticeably lower in winter and higher in summer. That was last the case around 2015.

So the fact that the latitude cycle is a bit faster than the declination cycle gives ride to another period, the "tropical" period of 27.2122 days. That one does leave an imprint on temperatures, I have found in some research a small but repeating over long intervals signal where temperatures peak slightly after the Moon passes its lowest (-5.1 deg) latitude point, with a second smaller peak after passing +5.1 deg. I am still working out what physical cause and effect might account for this. The slightly longer declination cycle has some predictable temperature associations, most notably, a bit of a peak around winter full moons which are at the declination maximum (which I call northern max in my research). That peak could be the signal of weak tidal interactions in the atmosphere. Why are they weak when they are strong in the oceans? Most probably, it is because oceans have shorelines therefore tidal influences have to ebb and flow. The atmosphere only has partial constraining boundaries like high mountain ranges in a few cases, so whatever tidal effects may be ongoing do not hit boundaries and therefore would ripple out endlessly while dissipating. That might however cause an interference pattern which is basically the foundation of some research I have done on "timing lines" where lunar effects seem more concentrated around the hemisphere.

I would present some of this data but as the claim is basically that the effects are small, and probably outside the range useful to forecasting questions, there would be little value in it. The longer 18.6 year cycle might also leave some faint signatures in the data, the claim was made as early as the mid-20th century by noted climatologists Bryson and Lamb that some predictable temperature and circulation results might follow the 18.6 year cycle. What I found doing my own investigation was that the zonal index seems to peak at around 3-6 years after the declination peak, at which point there was a blocking index peak. Then when the moon reached its lowest declination range, it seemed to be associated with a growing tendency for colder winters (peaking around 2-4 years after the declination minimum). Here again these signatures are interesting but not statistically large enough or consistent enough to use them very much in forecasting.

A final lunar cycle of note is the "anomalistic" month which is the period between perigees in the lunar orbit. The range of the Moon's distance is something like 12% above and below its mean distance. At perigee, the Moon is closest to the earth. This happens every 27.55 days, which is 0.23 days later in each sidereal month. So unlike the precession of the nodes (which is retrograde), perigee is prograde relative to the declination cycle. It takes 8.86 years to rotate around the orbital path, and is currently set to a perigee near southern declination maximum. One practical effect of lunar perigee is that when it occurs at a total eclipse of the Sun, that eclipse has a longer duration in the path of totality. When the Moon is at apogee at a total eclipse, it will not fully cover the Sun's disk and the result is an "annular" eclipse. The forthcoming April 2024 total eclipse will be fairly close to lunar perigee so totality will last about twice as long as the 2 min 30 sec that we had in the recent August 2017 event. At perigee the Moon is moving somewhat faster through the sky. These variations can account for cases where the actual period between full or new moons is a bit different from the long-term average of 29.53 days. That can vary by at least 0.25 days in specific cases.

So the bottom line here is that the Moon is subject to four observable cycles, and there are other minor wobbles that mean that while basically we only see the half of the moon facing the earth, we actually see closer to 57% of its total surface due to these "librations."

The Moon is currently approaching its new phase, so I will use this thread to draw attention to interesting events in the Moon's orbital cycle(s) later in the autumn and winter. Because outer planets Mars to Neptune are moving slowly forward in the ecliptic plane, the Moon requires a bit of added time to reach them after each sidereal orbit. For Saturn the differential is about .07 days and for Jupiter it is closer to .18 days, so roughly every 27.5 days the Moon passes Jupiter (for Mars the period is closer to 28 days). Each planet has its own inclination to the ecliptic plane, most are above it when off to the April side of our orbit, and below it when over towards the October side, as Jupiter is now. There are times when the Moon passes in front of each of the bright planets, and this usually leads to a sequence lasting about a year while the geometries are close enough for an "occultation" to take place. Jupiter is approaching a point where occultations will take place, Saturn will reach a similar point about a year later. The bright stars Aldebaran, Antares, Regulus and Spica, all have periods in the 18.6 year cycle with occultations. Antares will have its occultations shortly after the declination maximum as the Moon needs to reach almost its lowest latitude when passing Antares. Aldebaran on the other hand gets its occultations near declination minimum as it is in the winter sky and nearly five degrees south of the ecliptic plane. Spica and Regulus are a bit more complicated as they happen to be close to the ecliptic. They can have two periods each 18.6 years with occultations. No other well known bright stars are ever occulted by the Moon. Asteroids can also be occulted, not an event visible to the naked eye, but a time for astronomers to get useful data concerning the probable dimensions of asteroids and whether or not they have fainter satellites (which might briefly remain in view while the parent asteroid disappears).