The Pull of the Moon. The Moon's pull is strongest when it is nearest, and that happens when it is straight up or overhead (Moon up). This point is called the zenith. A second strong point occurs when the Moon is at the opposite point or beneath us at a point called the nadir (Moon down). The Moon is weakest in radial upward force when it is on either horizon, rising or setting. At these times the radial force is directed downwards toward the center of the Earth.
When the Moon is at the zenith, or overhead, gravitational force is at its strongest and it pulls us up, ever so slightly. When the Moon is at the nadir (on the other side of the Earth from us), a special form of centrifugal force, stronger than gravity, and pushes us out or away from the surface of the Earth. In other words, the effect of the Moon at zenith or nadir is to lift us up or away from the surface of the Earth, but for different reasons.
The two points during the day when the Moon is up or down are when the radial lunar force is at a maximum. However, sometimes the pull of Moon up is greater than that for Moon down, and vice versa. This variation depends upon what is called the diurnal inequality, which varies during the course of a month. This diurnal inequality is responsible for the difference in the height of successive high tides and depends upon which part of the eclipitic the Moon is located. When the Moon is in the equinoctial signs, Aries and Libra, the pull of Moon up is the same as that of Moon down for a given day. However, when the Moon is in the solsticial signs, Cancer and Capricorn, the pull is unequal. When the Moon is above the equator and in the sign Cancer, the pull at Moon up is always stronger than the pull at Moon down. When the Moon is below the equator and in the sign Capricorn, the pull at Moon down is always stronger than the pull at Moon up.
Your geographic latitude will affect how unequal the Moon Up and down can be. If I am here in Big Rapids at almost 44 degrees of latitude North and the Moon has a declination of minus 28 degrees (which it can reach), then at Moon up, the angle between my zenith (Moon up) and the Moon is some 72 hours. However, some 12 hours later, when the Moon is at my nadir (Moon down), the angle between my nadir's latitude (40 degrees South) and the declination of the Moon at - 28 degrees is only some 16 degrees. At this time, the Moon down pull will be much stronger than the Moon up pull.
The Moon is weakest, as mentioned, when it is on the horizon, either rising or setting, each day. However, this too varies during the month depending on the declination of the Moon. The closer the Moon comes to your own geographic latitude, the stronger the effect. Therefore, if you are residing in a northern latitude, the Moon will be closer to you in the ecliptic sign Cancer and this will cause the Moon to be somewhat stronger at Moonrise and set.
Both the Sun and Moon exert a gravitational pull on the Earth. Although the Sun is much more massive, its greater distance results in the gravitational pull of the Moon being almost twice that of the Sun. In any case, we experience their combined effect rather than each singly. This effect varies with the monthly lunar cycle.
At New and Full Moons, the combined pull of the Sun and the Moon is greatest. This pull is weakest at the lunar quarters. Therefore, this pull waxes and wanes with the month. It is strongest at new moon, grows weaker at First Quarter, is strong again at the Full Moon and then weak at Fourth Quarter, and on around. At New and Full Moon, the Moon's tidal effect is, in effect, added to the solar effect and the resultant tractive force is in increased in the ration 3:2 , the tide-generating force of the Sun being one half that of the Moon. During the First and Last Quarters, when the Moon and Sun are some 90 degrees apart, the resultant tractive force is roughly one half of the lunar force alone.
This combined solar/lunar force is subject to some variation (other than that already pointed out) due to the fact that the Moon can have latitude above or below the ecliptic. The Moon's orbit can reach some 5 degrees above or below the plane of the Earth's orbit, the ecliptic. Where the Moon crosses the ecliptic are what are called the ascending and descending nodes of the Moon. At these points (twice a month), the combined force of the Sun and Moon is greatest.
So far we have discussed something of the effects of the Moon as it transits overhead or beneath our feet each day. Yet it is the combined vector force of the Sun and Moon that produces the strongest pull that we feel during any 24 hour period. Keeping track of this vector force is a little complicated and that is where the Natural Cycles program can be a real lifesaver. It does it for us. In fact the program will keep track of the Sun, Moon singly or their combined vector. In any case, here are the various components that the program will calculate and graph:
Radial component. This is the tidal component that lifts us away from the face of the Earth at zenith and nadir passage. You will note that their are two periods each day (zenith & nadir) when this component reaches a maximum value and that, depending on your geographic latitude, these are often unequal in magnitude. At the rising and setting points in the daily cycle, the effect is to push us down towards the center of the Earth. At all other points, aside from the above mentioned four, the effect is transverse or horizontal:
Horizontal component. In addition to the vertical or radial tidal components, there are horizontal or transverse forces that push and pull us across the surface of the Earth in various directions. The earths rotation produces semidiurnal changes in the tide-generating forces both in direction and magnitude.
East/West horizontal component. These forces reach zero values at zenith, nadir, rising and setting times and become strongest at the intermediate times (45 degree points) between the above four events. These horizontal components vary depending upon the geographic latitude. In a 24 hour period, the effect of the horizontal component is as follows:
Starting from Moon up, the transverse pull grows stronger to the West, reaches maximum magnitude some 45 degrees (3 hours) after Moon up, and fades until we reach the point at which the Moon is setting at which time the horizontal force has again dropped to zero. After this we are pulled to the East, dropping off again at Moon down. At this point, a Westerly pull is again felt, diminishing to zero at Moonrise. After Moonrise, we experience an Easterly pull, reaching a peak some three hours before the Moon is at our zenith, and dropping to zero at the zenith point.
North/South horizontal component. These force also have a North South component that varies on a 24 hour basis. It is much like the East/West component, and functions as follows. There is no North/South component for places located along the equator. In other latitudes, the force vector describes an ellipse. At Moon up and Moon down, it is directed toward the South, while at Moonrise and Moonset it is directed toward the North. The North/South component is of the same order of magnitude as the East/West component.
Declination Cycle. The monthly cycle (tropical month of 27.32 days) of lunar declination contributes to the overall tidal effects. The closer the Moon comes to being overhead, the more powerfull are its effects. If we live in the northern hemisphere, then when the Moon rides high above the celestial equator, when it is in the sign Cancer, it will comes closest to our own geographic latitude, and to being overhead. This effect can further be enhanced when the latitude of the Moon reaches its maximum value of some 5 degrees. Thus the total declination of the Moon can reach some 28 1/2 degrees above and below the ecliptic. This happens (North or South) once in about 18.6 years.
Perigee/Apogee. These are the points when the Moon, due to its non-circular orbit, is closest and furthest (respectively) to the Earth. The Moon moves at its greatest speed when it is at perigee and at its slowest when furthest from the Earth at apogee. The gravitational pull of the Moon is much stronger at perigee than at apogee.
The apogee/perigee points (the line of apsides that connects them) are not fixed along the ecliptic, but move slowly foward along the ecliptic over a nine year period.
Lunar Speed. In addition, this line of apsides also flucuates backwards and forwards in the ecliptic slightly with a period of 31.81 days. This is due to the eccentricity of the Moon's orbit, and this fluctuation is called evection. The resulting effect is the the Moon speeds up and slows down at different rates in the four weeks from one perigee to the next.
The Moon's speed is also affected by the lunar phases, since the Sun's pull on the Moon is different in the various lunar quadrants. The Moon moves faster from the Last Quarter to the New Moon, and slower from the New Moon to the First Quarter. It also speeds up from the First Quarter to the Full Moon, and slows down from the Full Moon to the Last Quarter.
The greatest possible astronomical tide-generating force occurs when, at the same time, the Sun is a perigee, the Sun and Moon are at Full or New Moon and both the Sun and Moon have zero declination. This happens about once in 1600 years, 250 B.C., 1400 A.D, and it will happen around 3300 A.D.
Major Tide-related Phenomena
Semi-Diurnal (12 hr., 25 min.) Time between Moon up and Moon down caused by the rotation of the Earth.
Diurnal (24 hr., 50 min.), time between succeeding upper and lower transits of the Moon caused by rotation of the Earth and declinat of Sun and Moon.
Interval between spring tides (14.76 days average), time from New Moon to Full Moon or vice versa caused by the phase relation between the Sun and Moon.
Lunar fortnightly (13.66 days), time for moon to change declination from zero to maximum and back to zero caused by the varying declination of the Moon.
Anomalistic month (17.55 days), time for moon to go from perigee to perigee caused by the ellipticity of the Moon's orbit.
Solar semi-annual (182.6 days), time for Sun to change declination from zero to maximum and back to zero caused by the varying declination of the Sun.
Anomalistic year (365.26 days), time for the Earth to go from perigee to perigee caused by the ellipticity of the Earth's orbit.
In recent year, the phenomenon of the solar wind has become of more interest to researchers. In brief, the Sun spews forth an endless stream of charged particles in all directions -- the solar wind. This solar wind blows far out into the solar system and beyond. The Earth's magnetic fields serve to shield us from direct contact with the solar wind's charged particles. The Earth's magnetic field is rounded torwards the Suns, and stretches out in a long tail away from the Sun, just like a comet. The solar wind rushes around and past the Earth and on out into space. The Moon passes thru the different sections of the Earths magnetic sphere in its monthly orbit. At New Moon it is always in the upstream portion of the magnetosphere, facing the Sun and downstream in the Earth's tail at Full Moon. At First Quarter, the Moon is to the dusk side of Earth and at Fourth Quarter, the Moon is in the dawn side of the magnetosphere.
The interelationship of the solar wind and the Earth's magnetosphere is receiving considerable attention in recent years. It has been suggested that the passage of the Moon thru the Earths magnetic shield may serve to trigger various weather and magnetic activity. The interelationship of indicators like the geomagnetic index, solar flux and other measures of solar activity with the Moon is just now in the process of being researched and understood.
Copyright: Michael Erlewine