Why is the moon not completely darkened by Earth's shadow during a total lunar eclipse?

Answer 1

the Earth's atmosphere bends light in a way where very little is actually able to still reach the moon.

Answer 2

The light comes from the Earth's atmosphere. If you were standing on the Moon during a lunar eclipse, you would see a bright red rim around the Earth, where the Earth's atmosphere is illuminated by the Sun. That light, reflected off the moon, allows us to still see the Moon during a total lunar eclipse. The moon is red because the blue light absorbed in the Earth's atmosphere, just like a sunrise or sunset.

Answer 3

It's not completely dark because of refraction of the light by the earth's atmosphere. Refraction is a fancy name for bending. The atmosphere bends some of the light that goes through it to allow some light to hit the moon.

Answer 4

The space shift bends enough of the light around the earth to provide some illumination of the moon.

Answer 5

An eclipse of the Moon (or lunar eclipse) can only occur at Full Moon, and only if the Moon passes through some portion of the Earth's shadow. The shadow is actually composed of two cone-shaped components, one nested inside the other. The outer or penumbral shadow is a zone where the Earth blocks part but not all of the Sun's rays from reaching the Moon. In contrast, the inner or umbral shadow is a region where the Earth blocks all direct sunlight from reaching the Moon.

Astronomers recognize three basic types of lunar eclipses:

1. Penumbral Lunar Eclipse

The Moon passes through Earth's penumbral shadow.
These events are of only academic interest since they are subtle and quite difficult to observe.
2. Partial Lunar Eclipse

A portion of the Moon passes through Earth's umbral shadow.
These events are easy to see, even with the unaided eye.
3. Total Lunar Eclipse

The entire Moon passes through Earth's umbral shadow.
These events are quite striking for the vibrant range of colors the Moon can take on during the total phase (i.e. - totality).
Now you might be wondering "If the Moon orbits Earth every 29.5 days and lunar eclipses only occur at Full Moon, then why don't we have an eclipse once a month during Full Moon?". I'm glad you asked! You see, the Moon's orbit around Earth is actually tipped about 5 degrees to Earth's orbit around the Sun. This means that the Moon spends most of the time either above or below the plane of Earth's orbit. And the plane of Earth's orbit around the Sun is important because Earth's shadows lie exactly in the same plane. During Full Moon, our natural satellite usually passes above or below Earth's shadows and misses them entirely. No eclipse takes place. But two to four times each year, the Moon passes through some portion of the Earth's penumbral or umbral shadows and one of the above three types of eclipses occurs.

When an eclipse of the Moon takes place, everyone on the night side of Earth can see it. About 35% of all eclipses are of the penumbral type which are very difficult to detect, even with a telescope. Another 30% are partial eclipses which are easy to see with the unaided eye. The final 35% or so are total eclipses, and these are quite extrordinary events to behold.



Beginning (right), middle (center) and end (left) of totality
during the total lunar eclipse of 2000 Jan 20-21.
(Click here to see a gallery of photos from this eclipse)
During a total lunar eclipse, the Earth blocks the Sun's light from reaching the Moon. Astronauts on the Moon would then see the Earth eclipsing the Sun. (They would see a bright red ring around the Earth as they watched all the sunrises and sunsets happening simultaneousely around the world!) While the Moon remains completely within Earth's umbral shadow, indirect sunlight still manages to reach and illuminate it. However, this sunlight must first pass deep through the Earth's atmosphere which filters out most of the blue colored light. The remaining light is a deep red or orange in color and is much dimmer than pure white sunlight. Earth's atmosphere also bends or refracts some of this light so that a small fraction of it can reach and illuminate the Moon.

The total phase of a lunar eclipse is so interesting and beautiful precisely because of the filtering and refracting effect of Earth's atmosphere. If the Earth had no atmosphere, then the Moon would be completely black during a total eclipse. Instead, the Moon can take on a range of colors from dark brown and red to bright orange and yellow. The exact appearance depends on how much dust and clouds are present in Earth's atmosphere. Total eclipses tend to be very dark after major volcanic eruptions since these events dump large amounts of volcanic ash into Earth's atmosphere. During the total lunar eclipse of December 1992, dust from Mount Pinatubo rendered the Moon nearly invisible.

All total eclipses start with a penumbral followed by a partial eclipse, and end with a partial followed by a penumbral eclipse (the total eclipse is sandwiched in the middle). The penumbral phases of the eclipse are quite difficult to see, even with a telescope. However, partial and total eclipses are easy to observe, even with the naked eye.



Phases of the Total Lunar Eclipse of 2000 Jan 20-21.
(Click here to see a gallery of photos from this eclipse)


Observing Lunar Eclipses
Unlike solar eclipses, lunar eclipses are completely safe to watch. You don't need any kind of protective filters. It isn't even necessary to use a telescope. You can watch the lunar eclipse with nothing more than your own two eyes. If you have a pair of binoculars, they will help magnify the view and will make the red coloration brighter and easier to see. A standard pair of 7x35 or 7x50 binoculars work fine. Remember to dress warmly and enjoy the spectacle!

Amateur astronomers can actually make some useful observations during total eclipses. It's impossible to predict exactly how dark the Moon will appear during totality. The color can also vary from dark gray or brown, through a range of shades of red and bright orange. The color and brightness depend on the amount of dust in Earth's atmosphere during the eclipse. Using the Danjon Brightness Scale for lunar eclipses, amateurs can categorize the Moon's color and brightness during totality.

Another useful amateur activity requires a telescope. Using a standard list lunar craters, one can careful measure the exact time when each crater enters and leaves the umbral shadow. These crater timings can be used to estimate the enlargement of Earth's atmosphere due to airborne dust and volcanic ash.

Of course, an eclipse of the Moon also presents a tempting target to photograph. Fortunately, lunar eclipse photography is easy provided that you have the right equipment and use it correctly. For more photographs taken during previous lunar eclipses, be sure to visit Lunar Eclipse Gallery 1 and Lunar Eclipse Gallery 2.



Eclipse Frequency and Future Eclipses
During the five millennium period from 2000BC through 3000 AD, there are 7,718 eclipses1 of the Moon (including both partial and total). There are anywhere from 0 to 3 lunar eclipses (not including penumbral) each year. The last time that three total lunar eclipses occurred in one calendar year was in 1982. Partial eclipses slightly outnumber total eclipses by 7 to 6.


1 Only eclipses where the Moon passes through Earth's umbral shadow are included in these values. A minor type of eclipse is the penumbral eclipse which occurs when the Moon passes through the Earth's faint penumbral shadow. Penumbral eclipses are rarely discernable to the naked eye and are of lesser importance than umbral eclipses.
The table below lists every lunar eclipse from 2000 through 2008. Click on the eclipse Date to see a map and diagram of an eclipse. Click on the Region of Eclipse Visibility to see a detailed description of an eclipse. Although penumbral lunar eclipses are included in this list, they are usually quite difficult to observe because of their subtlety. The penumbra is a partial shadow which still permits some direct sunlight to reach the Moon.

The Umbral Magnitude is the fraction on the Moon's diameter immersed in the umbra at maximum eclipse. For values greater than 1.0, it is a total eclipse. For negative values, it is a penumbral eclipse. The Total Duration is the duration of the total phase (total eclipses only).