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Meta Research Bulletin On-Line

2007 Sept. 15 issue

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Meta Research Bulletin 2007

Lunar Eclipses and Blue Moons

Tom Van Flandern /


Abstract. Total eclipses of the Moon are not really “total” because the Moon would disappear if they were. Illumination from light refracted by Earth’s atmosphere remains visible. The amount and color of such light is a function of above-cloud atmospheric conditions at the time of the eclipse. Recent major volcanic activity can fill the atmosphere with enough debris to cause the eclipsed Moon’s normal orange colors to turn dark blue, or even to cause the Moon to disappear from naked-eye view. This is a rare phenomenon, and was the origin of the expression “once in a blue moon”. A recent perversion of the original meaning allows a phenomenon now called a “blue moon” to occur in some localities about once every other year on average.


Solar eclipses

The disks of both the Sun and the Moon are about half a degree in diameter on the sky. When the Moon passes in front of the Sun, its disk may be slightly larger or slightly smaller than the Sun’s. In the former case, a ring of sunlight remains visible all around the Moon’s disk at mid-eclipse, and the event is called an “annular solar eclipse”. In the latter case, the Moon’s disk completely covers the Sun’s disk, though not its glowing atmosphere, and the event is called a “total solar eclipse”. If the disks are not well-aligned, a “partial solar eclipse” may occur.


At times of total solar eclipses, the Moon’s disk reflects no sunlight and gives off no light of its own, and is therefore almost totally black. The only significant light it reflects is sunlight that first strikes the Earth, then reflects off the Moon’s dark side and returns to Earth a second time. On average, we have about one total and one annular eclipse each year. But the path of totality is very narrow, so one must normally travel to see a total solar eclipse. However, as one of nature’s greatest spectacles, total solar eclipses are usually well worth the effort and cost.


Lunar eclipses

When the Moon passes into the Earth’s shadow, we have a lunar eclipse. These occur with a similar frequency but are visible from large areas of the Earth, and are therefore seen more commonly. The view is also considerably less spectacular than a total solar eclipse. Lunar eclipses come in a variety of flavors as well. The part of Earth’s shadow for which the Sun’s disk is only partially blocked by the Earth is called the penumbral shadow. When the Moon is in only the penumbral shadow, most people would not notice that anything was happening. This is because lots of sunlight shining into the penumbral shadow makes its boundaries indistinct, and the minor darkening of the Moon it causes is generally not noticeable.


However, as seen from locations within Earth’s shadow, the geometric disk of the Earth covers the entire disk of the Sun. These locations comprise the umbral shadow. When some part of the Moon’s disk crosses into the umbral shadow, that part of the Moon’s disk appears at first quite black, and the circular shape of Earth’s disk casting the dark shadow is plainly evident. This blackness of the umbral shadow, it turns out, is not so great as it appears. The brightness of the rest of the disk simply makes it appear black by comparison. But as the eclipse advances and the Moon’s disk goes deeper into the umbral shadow, less and less of the disk remains bright (uneclipsed). The dark umbral shadow then begins to reveal details, including bright colors – typically red, orange, and yellow. These become stronger and more apparent, and dominate the eclipse if it reaches the total stage wherein the entire lunar disk is immersed in the umbral shadow. See Figure 1.


Total lunar eclipses never happen!

Two questions naturally arises: Why doesn’t the Moon disappear when totally eclipsed? And what causes the colors in the light reflected by the Moon? The traditional answer to both questions in many older textbooks is that Earth’s atmosphere filters and scatters predominately red-orange sunlight into the umbral shadow. The atmosphere is also responsible for the umbral shadow being about 2% larger in diameter than an airless Earth would cast at the Moon’s distance. Yet at the corresponding altitude, 127 km above the Earth’s surface, the air is so thin that scattering should be minimal. So why is the dark umbra that large?


The correct explanation is that the atmosphere bends the sunlight into Earth’s shadow by refraction. Scattering is unimportant. The red coloration is then the same effect that the atmosphere has on the colors of sunlight in Earth’s sky shortly after sunset. And the shadow enlargement is refractive defocusing of sunlight, which significantly decreases the amount of light getting through even when refractive bending is quite small. But the most important effect is ordinary bending of the Sun’s direct rays by refraction, which is a banding angle of slightly over one degree at sea level. (This is an average of 34’ bending of setting sunlight on its way to the sea-level observer, and another 34’ bending from the observer back out the atmosphere on its way to the Moon.)


The bending of direct sunlight into the Earth’s umbra means two things. (1) An observer of the Moon’s surface would be able to see the Sun and its sunspots during a total eclipse, all squished into a bright ring of light surrounding the Earth’s disk. And (2) because the Sun is only half a degree in diameter, but refraction is one degree on each side of the Earth’s diameter, the Moon is too far away to ever be totally eclipsed; i.e., no direct sunlight reaches it. See Figure 2.


This means that, technically, the Moon can never be totally eclipsed. Artificial satellites of the Earth can be eclipsed, because they can enter the true umbra, where no sunlight penetrates. Again, technically, what we call the Earth’s umbral shadow out at the Moon’s distance and beyond is simply a zone where sunlight is greatly diminished by refractive defocusing. However, no terminology exists to distinguish the dark-but-colored shadow that falls on the Moon from the true shadow that reaches only to about 85% of the Moon’s distance in which no direct sunlight is present.


We therefore propose here to respect tradition and continue to call the colorful middle shadow that falls on the Moon the umbra, despite the fact that “umbra” was supposed to mean “complete”. And we further propose to use the common prefix “endo-“ meaning “inner” when we wish to refer to the true, inner shadow where no direct sunlight reaches: the endoumbra. That particular prefix is used frequently with that meaning, for example in “endogenous” to contrast with its opposite “exogenous”. The same prefix is also used in cases where it modifies a term starting with a vowel such as “endoenzyme” (an enzyme functioning inside a cell). So in accord with previous usages, no hyphen for clarity should be needed.


If the usage catches on, you saw it here first.


Really dark lunar eclipses

Most but not all lunar eclipses show the red, orange, or copper coloration. Some lunar eclipses show darker colors. In extreme cases in history, the inner umbra was blue surrounded by black, or the Moon disappeared completely to the naked eye. The cause of these special eclipses was easy to spot because they occur only after major volcanic eruptions have spread their ash into the atmosphere around the globe, changing the transparency of the atmosphere and the sunset colors everywhere. Such events have a frequency of less than one per century, although the last one was the eruption of Mount Pinatubo in the Philippines in 1991. On that occasion, the eclipsed Moon was distinctly bluish and nearly disappeared from naked-eye view.


That was the origin of the expression “once in a blue moon”, meaning an extremely rare event, the sighting of a literally blue Moon. However, in recent years, individuals impatient for more blue moons proposed a new definition. It was to be the event of a second Full Moon in a single calendar month. That happens on average about twice every five years. The most recent such occasion was in May, June, or July of this year, depending on what time zone you are located in. For a breakdown by time zone, see, which also explains why time zone matters.


Our cover photo with this issue is of the almost Full Moon on 2007 May 30, which qualified as a blue moon in the new sense for those in North America. However, for the astronomers among us, a blue moon will always be taken literally, a thrilling astronomical event so rare that many do not have the opportunity to see one in their entire lifetimes.




“The only difference between me and a madman is that I am not mad.” Salvador Dali



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