image: Wikimedia commons (composite of two images -- foreground and background).
The point of New Moon takes place when our moon passes the point in its orbit which places it most directly between our planet and the sun.
From an "overhead" or "top-down-looking" viewpoint, this would mean that the sun, moon, and our planet Earth would be perfectly aligned, with the moon in between the earth and the sun. At the point of Full Moon, they are aligned again, and from the same top-down viewpoint would be perfectly aligned, but this time with the earth in between the sun and the moon.
As you can envision from the above description, a solar eclipse can only occur at the point of New Moon, because a solar eclipse takes place when the moon blocks out the sun from the point of view of observers on earth. Likewise, a lunar eclipse can only occur at the point of Full Moon, because a lunar eclipse takes place when the shadow of the earth passes across the face of the moon (said another way, a lunar eclipse occurs when the earth blocks out the sun from the point of view of an observer on the moon).
However, we all know that solar eclipses and lunar eclipses do not occur at every single New Moon and Full Moon. The reason they don't is that, if we shift our point of view from a "top-down" viewpoint to a "side-on" viewpoint, the plane of the moon's orbit around the earth will be seen to be at an angle relative to the plane of the earth's orbit around the sun. That is to say, if the plane of earth's orbit around the sun is envisioned as a glass disc or plate, the plane of the moon's orbit around the earth would be a smaller glass disc or plate which intersects the larger plate at an angle (an "oblique angle," in one sense, which is why this divergence between the two planes is referred to as an "obliquity").
The moon, in its path around the smaller "plate," will only pass through the larger "plate" (the plane of earth's orbit around the sun) at two points -- one of them going "up" (towards the north pole, if we imagine north to be "up" in this case, although there is no real "up" in space) and one of them going "down."
If you are able to envision all of this, you will realize that an eclipse of either kind (with the moon obscuring the sun for those on earth, or the earth obscuring the sun for those on the moon) can only occur when the moon is passing through one of those two points of intersection with the plane or "plate" of earth's orbit around the sun. For this reason, the plane of earth's orbit around the sun is known as the "ecliptic" (because eclipses can only occur when the moon crosses this particular plane).
In fact, an eclipse of either the sun or the moon can only take place when the earth, sun, and moon are aligned from the perspective of both the "top-down" view and the "side-on" view.
The two points at which the moon's path crosses the ecliptic plane (one going "up" and the other going "down") are known as the lunar "nodes." An eclipse occurs when the moon is passing through one of its two nodes at the same time that it is lined up from the "top-down" perspective -- either when it is passing through the top-down point of being directly between the sun and the earth (a solar eclipse) or when it is passing through the top-down point of having earth directly between the moon and the sun (a lunar eclipse).
Previous posts have examined these lunar nodes and their role in creating eclipses, and have included helpful diagrams and videos to help envision all of the celestial mechanics described in the above discussion -- see for instance here and here.
As most readers who have spent any time looking at news sites on the web this past week are undoubtedly aware, the moon's orbit is presently aligned such that the lunar nodes are in-line with the earth and the sun, such that tomorrow's New Moon will create a solar eclipse when the moon passes between the earth and the sun at the same time it is passing through a lunar node -- in this case, the South Node (moon is crossing "down" towards the southern hemisphere, also known as the "descending node").
Because the earth is so much larger than the moon, a earth's shadow completely covers the face of the moon during a lunar eclipse -- but because the moon is smaller than the earth, the shadow of the moon only covers a portion of our planet's surface during a solar eclipse. Thus, the eclipse can only be seen along a certain path created by the "line" between the sun and the moon where it "impacts" the surface of the earth. In this case, that line of the shadow's path will be entirely in the southern hemisphere, and will spend most of its time speeding across the Atlantic Ocean, crossing the continent of South America from Chile to Argentina, then crossing the vast Atlantic between South America and Africa, and making "landfall" in Angola, crossing a bit of Zambia, and finishing up in the Democratic Republic of Congo. Numerous websites can show you the path of this solar eclipse -- one such site can be found here.
Those who do not live near this line of the shadow's path and unable to make the trip to see it will have another opportunity in August of this year, when a highly-anticipated total solar eclipse is set to take place, during the point of New Moon when the moon is again crossing the ecliptic plane, this time at an "ascending node," on August 21. (Note that the moon, of course, passes through each of the two nodes once a month -- but that, as discussed earlier, the nodes do not always line up with the points of New Moon and Full Moon).
If you are at all able to do so, you should begin to make your plans now to find a point along the line of the moon's shadow in order to experience the total eclipse of the sun in August 2017. You can find numerous maps and descriptions of the path of totality on the web.
As part of your preparations, you may want to obtain a copy of Death of Gods in Ancient Egypt, by Jane B. Sellers (1992). The book's descriptions of the experience of a total eclipse should put to rest any question of whether or not it is worth making the effort to travel to a point underneath the line of the moon's shadow during the total eclipse of August 2017, if it is at all possible for you to do so.
Additionally, Jane Sellers makes a strong case that the cycle of myths surrounding the "Contendings of Set and Horus" in ancient Egypt are related to the effects visible from earth during solar eclipses, whether they are annular (such as the eclipse of February 26, 2017) or total (such as the eclipse of August 21, 2017).
As an aside, the difference between and annular and a total eclipse is caused by the distance that the moon is from the earth at the point of eclipse -- if it is further away, it appears slightly smaller and does not cover the entire face of the sun, but if it is closer it will "just fit" over the sun's face. Many observers down through the centuries have marveled at the amazing serendipity or Providence of the fact that the moon's size and orbital distance are so perfectly calibrated to create such incredible effects during solar eclipses (much more incredible information about the harmonies between the size of the earth and the moon are available in the in the mind-blowing work and discoveries of Scott Onstott, at Secrets In Plain Sight).
Jane Sellers in Death of Gods in Ancient Egypt also makes a strong case that many aspects of the cycle of myths involving Isis and Osiris, Set and Horus, originate in the constellations and the cycles of their motions throughout the year -- and throughout the "Great Year" of the precessional cycle. In doing so, she follows along ground broken by Hertha von Dechend and Giorgio de Santillana in their seminal work Hamlet's Mill (1969), whose work Sellers acknowledges and references throughout her own book.
In fact, she begins her book with an epigraph from Hamlet's Mill --
It is not the beliefs and religions which circle around and fight each other restlessly; what changes is the celestial situation.
In Death of Gods in Ancient Egypt, Jane Sellers makes an argument that solar eclipses were seen and allegorized as a battle between Set (or Seth) and the sun-god Horus. She also makes the case, particularly in the discussion on pages 106 through 108, that when the trial of Set by the gods declares that he shall be put "under Osiris," this refers to the placement of the constellation Lepus underneath Orion in the celestial realm. The outline of Lepus, as Sellers shows on page 107, very much resembles the "long-eared Seth animal" found in ancient Egyptian artwork depicting the god Set or Seth.
Right now happens to be a particularly good time of year for stargazers to observe the glorious constellation Orion, and the outline of Lepus directly below Orion. Orion is still very high in the sky during the "prime-time" viewing hours after sunset and before midnight, as earth's daily rotation on its axis causes the stars to move from east to west across the night sky.
As we continue along our orbital path, however, we "pass up" the different constellations, causing them to move just a bit further "towards the west" each day (that is to say, stars along the ecliptic will be located about four degrees further towards the west at the exact same time on successive nights). Thus, each night at midnight (for example), the stars of Orion (and Lepus, and nearby Canis Major, which contains the brilliant star Sirius, associated with the goddess Isis) will all be four degrees further towards the west than they were at midnight the night before.
As they get further and further towards the west, they will become less and less easy to observe, until the day when they will be so far to the west at sunset that they will not be visible in the sky at all during the hours of darkness (until they reappear in the east at sunrise, about seventy days later).
Thus, as you prepare for the eclipse in August, you might want to be reading Jane Sellers' book now -- and also going out at night to observe the stars of Orion, Lepus, and Canis Major, if it is at all possible to do so.
As the eclipse takes place tomorrow, you can consider these ideas, and make your plans -- and if you are in a part of the globe that allows you to see tomorrow's eclipse, I wish you fair skies and happy (and safe) viewing!
Below are two diagrams showing the relative locations of Orion, Lepus, and Canis Major in the night sky (first with the outlines and labels, and then without):
and, the exact same stars but without the labels and outlines: