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Eclipses - Why Not Every Month?
by Jay Ryan

When I was a kid in school, we were taught very little practical astronomy, and what we were taught was mostly confusing. I remember hearing that a Full Moon occurred when the Moon was "behind" the Earth, as seen from the Sun. I also remember learning that a lunar eclipse occurred when the Moon was "behind" the Earth so as to fall into the Earth's shadow. 

Similarly, I was taught that a New Moon occurred when the Moon was near the Sun, but a solar eclipse was caused by the Moon passing in front of the Sun so that the Moon's shadow passed over the surface of the Earth. Hearing these explanations, I was puzzled as to why we didn't have eclipses every month. The New Moon and Full Moon are monthly events, yet eclipses were relatively rare and made the news. 

What I did not learn as a kid was the orbit of the Moon is slightly "tilted." The Moon's orbit is inclined 5.2 degrees to the Plane of the Ecliptic, which is the plane of the Earth's orbit around the Sun. As a result of this inclination, the Moon wanders over a large area of the sky, sometimes passing through the plane of the Earth's orbit, other times passing high above. This creates many interesting phenomena of the Moon, from changes in its appearance to variations in the the tides. It also affects the timing and circumstances of lunar and solar eclipses.


Most of the time, the New Moon passes high above the Sun or far below, and does not cross over the face of the Sun.  Similarly, the Full Moon usually passes above or below the Earth's shadow, and does not pass directly into it.  However, at two times in each lunar month, the Moon does cross the plane of the Earth's orbit.  At these times, the Moon appears to cross the ecliptic, which the apparent path of the Sun's motion through the constellations of the zodiac.

The Nodes
The points where the Moon's orbit crosses the ecliptic are called the nodes of the Moon's orbit.  The ascending node is the point where the Moon is moving north of the ecliptic, while the descending node is the point where the Moon is moving south of the ecliptic.

In most months, the Moon crosses the ecliptic in any old waxing or waning phase, and is far from the Sun.  However, a couple times a year, the Sun, Earth, and Moon will line up in such a way that one of the Moon's nodes is very near the Sun.  At such times, the New Moon crosses the ecliptic in proximity to the Sun so that an eclipse occurs.  Similarly, the Full Moon crosses the ecliptic within the Earth's shadow.  It is for this reason that the path of the Sun through the stars is called the ecliptic, since this is where eclipses occur.


The Moon's nodes are not always seen in the same zodiac constellations at all times.  In fact, the nodes are constantly moving toward the west in a cycle that takes about 18.6 years.  This is called the regression of the nodes.  So if a lunar eclipse is seen in the constellation Gemini, a year and a half later another lunar eclipse will be seen in Taurus.  For this reason, over a long enough period of time, solar and lunar eclipses can be seen on just about any date of the year from any place in the world.

The phenomena of eclipses and nodal regression was a baffling mystery down through all history.  However, following the revolutionary law of universal gravitation of Isaac Newton, the science of celestial mechanics showed that the complex motions of the Moon were the result of the combined gravities of the Sun and Earth tugging on the Moon and continually diverting its orbit, but allowing very precise prediction of future eclipses. 

As a result, we understand that a solar eclipse is followed by a lunar eclipse and that eclipse pairs are found about six months apart.  In 2008 eclipses are seen at the New and Full Moons of February and August.  However, such eclipse pairs do not always result in total eclipses, nor eclipses that can be seen in the same parts of the world.  But they can be predicted for centuries forward and backward in time. 

Using these techniques, calculations of known historical eclipses have been used to assign exact dates to many events in history where no actual dates have been recorded.  For example, a lunar eclipse over Jerusalem reported by the historian Flavius Josephus was used to estimate the birth of Jesus as being in 4 B.C.

The Names of the Nodes
For much of history, Classical Astronomy was extensively used by sailors to navigate their way from port to port and eventually around the world.  Anyone who has read any books on the Age of Exploration or who has seen any of these pirate movies will know that the seafarers of old were a very colorful bunch and used a lot of quaint jargon of the sea.  This also applies to their use of astronomy.  Everyone out there worried about "pagan influences" in astronomy will be interested to know that the old mariners referred to the Moon's  ascending node as "the head of the Dragon" and the descending node as "the tail of the Dragon"!

The head of the Dragon is the place where that the Moone dothe come over the line Ecliptick, from the South part, unto the North part: and the Tayle of the Dragon is, where the Moone passeth over the line ecliptick from, the Northe part unto the South part.  The use of the head and tayle of the Dragon, is to know, when that there is any eclipse of the Sunne or Mone: and of what quantitie or greatnesse the eclipse is.  -- William Bourne, A Regiment For the Sea (A.D. 1574)

This colorful perspective allows us to interpret the following old woodcut, which is from an Austrian publication in A.D. 1534. It shows the Dragon's head engulfing an eclipsed Moon in the constellation Leo. The Sun is shown at the Dragon's tail with a black circular edge indicating a partial eclipse, in the constellation Aquarius, on the opposite side of the sky from Leo.

The modern techniques of eclipse calculation reveal that there was in fact an eclipse pair that meet this exact description. A partial solar eclipse at the descending node was visible over Europe on January 14, 1534. This would have occurred in the part of the sky traditionally associated with the constellation Aquarius. Later that month, on the night of January 29-30, a total eclipse of the Moon near the ascending node was seen over Europe.  Compare the picture of the Sun in the woodcut above to this animation of the solar eclipse (http://astro.ukho.gov.uk/eclipse/0121534/Vienna_Austria_1534Jan14_anim.gif) showing the appearance as seen from Vienna in 1534.   

This woodcut is quite interesting as being carved nine years before Mikolaj Kopernik published his heliocentric theory, and more than 150 years before Isaac Newton's famous work was published.  It might have been carved based on an eyewitness account, since the astronomy of eclipse prediction was very rough at that time.  But this should illustrate the precision available today to modern science. 

I find it amazing that the LORD created such a remarkable order in the heavens and that He gave Man the mind to observe and develop the mathematics to make such a precise model.  Compare this sophisticated piece of practical, quantitative "here and now" science with the speculative, inferential explanations used to prop up evolutionary beliefs.

Also by Jay Ryan:  
Signs & Seasons Book, Field Journal and Test Manual Pack
Signs & Seasons Book, Field Journal and Test Manual Pack


Signs & Seasons: Understanding the Elements of Classical Astronomy
Signs & Seasons: Understanding the Elements of Classical Astronomy

Signs & Seasons Field Journal and Test Manual
Signs & Seasons Field Journal and Test Manual


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