The visual appearance of penumbral, partial, and total lunar eclipses differs significantly from each other. While penumbral eclipses are pale and difficult to see, partial eclipses are easy naked-eye events, while total eclipses are colorful and dramatic.
Earth's penumbral shadow forms a diverging cone that expands into space in the anti-solar direction. From within this zone, Earth blocks part but not the entire disk of the Sun. Thus, some fraction of the Sun's direct rays continues to reach the most deeply eclipsed parts of the Moon during a penumbral eclipse.
The primary penumbral contacts (P1 and P4), as well as the early and late stages of a penumbral eclipse, are completely invisible to the eye with or without optical aid. A penumbral magnitude greater than ˜0.6 is needed before skilled observers can detect faint shading across the Moon's disk.
Even when one edge of the Moon is 9/10 of the way into the penumbral shadow, approximately 10% of the Sun's rays still reach the Moon's deepest limb. Under such conditions, the Moon remains relatively bright with only a subtle gradient across its disk. The penumbral eclipse only becomes readily apparent when it is within ˜0.05 magnitudes of becoming a partial eclipse.
In comparison, partial eclipses are easy to see with the naked eye. The lunar limb extending into the umbral shadow usually appears very dark or black. This is primarily due to a contrast effect because the remaining portion of the Moon in the penumbra may be brighter by a factor of about 500x. Because the umbral shadow's diameter is typically ˜2.7x the Moon's diameter, it appears as though a semi-circular bite has been taken out of the Moon.
Aristotle (384-322 BCE) first proved that Earth was round using the curved umbral shadow seen at partial eclipses. In comparing observations of several eclipses, he noted that Earth's shadow was round no matter where the eclipse took place, whether the Moon was high in the sky or low near the horizon. Aristotle correctly reasoned that only a sphere casts a round shadow from every angle.
The total lunar eclipse is the most dramatic and visually compelling type of lunar eclipse. The Moon's appearance can vary enormously throughout the period of totality and from one eclipse to the next. Obviously, the geometry of the Moon's path through the umbra plays an important role. Not as apparent is the effect that Earth's atmosphere has on a total eclipse. Although the physical mass of Earth blocks all direct sunlight from the umbra, the planet's atmosphere filters, attenuates and bends some of the Sun's rays into the shadow.
The molecules in Earth's atmosphere scatter short wavelength light (e.g., yellow, green, blue) more than long wavelength light (e.g., orange, red). This process, which is responsible for making sunsets red, also gives total eclipses their characteristic red-orange color. However, the exact color can vary considerably in both hue and brightness.
Because the lowest layers of the atmosphere are thicker than the upper layers, they absorb more sunlight and refract it through larger angles. About 75% of the atmosphere's mass is concentrated in the bottom 10 km (troposphere) as well as most of the water vapor, which can form massive clouds that block even more light. Just above the troposphere lies the stratosphere (˜10 km to 50 km), a rarified zone above most of the planet's weather systems. The stratosphere is subject to important photochemical reactions due to the high levels of solar ultraviolet radiation that penetrates the region. The troposphere and stratosphere act together as a ring-shaped lens that refracts heavily reddened sunlight into Earth's umbral shadow. Because the higher atmospheric layers in the stratosphere contain less gas, they refract sunlight through progressively smaller angles into the outer parts of the umbra. In contrast, lower atmospheric layers containing more gas refract sunlight through larger angles to reach the inner parts of the umbra.
As a consequence of this lensing effect, the amount of light refracted into the umbra tends to increase radially from center to edge. However, inhomogeneities in the form of asymmetric amounts of cloud and dust at differing latitudes can cause significant variations in brightness throughout the umbra.
Besides water (clouds, mist, precipitation), Earth's atmosphere also contains aerosols or tiny particles of organic debris, meteoric dust, volcanic ash, and photochemical droplets. This material can significantly attenuate sunlight before it is refracted into the umbra. For instance, major volcanic eruptions in 1963 (Agung) and 1982 (El Chichon) each dumped huge quantities of gas and ash into the stratosphere and were followed by several years of very dark eclipses (Keen, 1983).
The same thing occurred after the eruption of the Philippine volcano Pinatubo in 1991. While most of the solid ash fell to Earth several days after circulating through the troposphere, a sizable volume of sulphur dioxide (SO2) and water vapor reached the stratosphere where it produced sulfuric acid (H2SO4). This high-altitude volcanic haze layer can severely dim sunlight that must travel several hundred kilometers horizontally through the layer before being refracted into the umbral shadow. Consequently, the total eclipses following large volcanic eruptions are unusually dark. For instance, the total lunar eclipse of 1992 Dec 09 (1.5 years after Pinatubo) was so dark that the Moon's dull gray disk was difficult to see with the naked eye (Espenak, 2008, personal observation).
The French astronomer A. Danjon proposed a useful five-point scale for evaluating the visual appearance and brightness of the Moon during total lunar eclipses. The L values for various luminosities are defined as follows:
The Danjon scale illustrates the range of colors and brightness the Moon can take on during a total lunar eclipse. It is also a useful tool to visual observers in characterizing the appearance of an eclipse. The evaluation of an L value is best done with the naked eye, binoculars, or a small telescope near the time of mid-totality. It is also helpful to examine the Moon's appearance just after the beginning and just before the end of totality. The Moon is then near the edge of the shadow, providing an opportunity to assign an L value to the outer umbra. In making such evaluations, the instrumentation and the time must be also recorded.
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Espenak, F, and Meeus, J. Five Millennium Canon of Lunar Eclipses: -1999 to +3000 (2000 BCE to 3000 CE), NASA Tech. Pub. 2008-214172, NASA Goddard Space Flight Center, Greenbelt, Maryland (2009).
Espenak, F, and Meeus, J. Five Millennium Catalog of Lunar Eclipses: -1999 to +3000 (2000 BCE to 3000 CE), NASA Tech. Pub. 2008-214173, NASA Goddard Space Flight Center, Greenbelt, Maryland (2009).
Keen, R.A., "Volcanic Aerosols and Lunar Eclipses," Science, 222, pp. 1011-1013 (1983).