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Lunar Eclipses

The Science Behind These Events


Lunar Eclipses

Diagram showing when a Lunar Eclipse will occur.

What Is A Lunar Eclipse?

Simply put, a lunar eclipse occurs when the Earth passes directly between the Sun and the Moon. (This means that lunar eclipses, by definition, can only occur during a full Moon.) The result is that direct sunlight from the Sun does not strike the lunar surface. However, this does not mean that no light reaches the surface.

Under certain types of eclipses, light from the Sun can actually be bent by Earth's atmosphere, casting the Moon in a red or orangish color. Other types of eclipses will simply block a percentage of the Sun's rays, making the Moon appear darker. And of course, some eclipses are a combination of the two phenomenon (see below).

How Does This Differ From The Normal Lunar Cycle?

While the Moon does go around the Earth, it's path is actually normally tilted with respect to the Sun. Therefore the different phases of the Moon that we observe during the normal lunar cycle are a result of only half of the Moon getting sunlight at any given time. Then depending on where you are on Earth, you see the Moon at an angle (see image).

The Different Types of Lunar Eclipses

Boiling lunar eclipse down to the Earth's shadow being cast over the surface of the Moon is quite an over simplification. As already stated, some of the Sun's light can still reach the surface because of the bending of the light around the Earth. But to complicate the matter further, the Earth's shadow is actually broken up into two distinct parts (see image).

The umbra is the portion of the Earth's shadow that does not contain any direct radiation from the Sun. The focus of the umbra is the point at which all three celestial bodies are properly aligned. Despite this, the eclipse does not completely drape the Moon in darkness because light from the Sun can actually be refracted through the Earth's atmosphere and find its way to the Moon.

The result of this refraction is that the Sun's light will be separated into individual colors. The more directly aligned the Earth, Moon and Sun are the Moon's color will transition to more of an orange hue, then red. This is actually the same phenomenon that causes the sky to appear reddish at sunset and sunrise.

When the Moon is fully contained within the umbra, the Moon is said to be in total eclipse. This event can last nearly two hours, while the Moon can be in at least partial eclipse for nearly four hours.

The penumbra is the region of space where the Earth is only partially blocking the light from the Sun. As the Moon moves from outside the shadow toward the umbra the Moon will begin to appear darker and darker. Again, the Moon never appears in complete darkness because some of the light from the Sun is always reaching the surface up until the Moon reaches the umbra.

Normally the Moon will lie only partially in the penumbra area (known as a penumbral eclipse), but occasionally, the Moon will find itself completely in the penumbra. These events, called total penumbral eclipses, are rare. And they are typically characterized by one side of the Moon appearing darker than the other due to one side approaching or receding from the umbra.

These total penumbral eclipses can immediately precede or follow a partial eclipse where the Moon is partially in each of the umbra and penumbra regions.

The Danjon Scale For Lunar Eclipse Brightness

There are obviously a variety of lunar eclipses, causing a variety of different Lunar appearances. In order to classify what kind of lunar eclipse is occurring a scale known as the Danjon scale was developed by André-Louis Danjon.

Essentially an L value is determined based solely on the appearance of the Moon. Using only the naked eye, the observer estimates which category the eclipse falls into to.

  • L=0, This represents the darkest eclipse, and is probably what most people imagine when they think of a lunar eclipse.

  • L=1, While still very dark, there is a grey or brown hue to the Moon. However, details of the Moon are still difficult to identify.

  • L=2, During this type of eclipse the Moon will appear dark red, possibly with a slight hint of orange. The Moon still appears very dark at this value.

  • L=3, The Moon is now brick-red and noticeably lighter than the previous value. Also, the edges can appear lighter, possibly with a yellowish hue.

  • L=4, The Moon now appears bright red or orange, while the edge of the Moon appears almost bluish

The problem with the Danjon scale is that it is not based on an absolute flux or color level, meaning that scientific instruments aren't used to decide what L value the eclipse represents. Therefore, different people observing the same eclipse can arrive at different L values. So, it's not very precise, but typically it yields a fairly good idea of what kind of eclipse you are observing.

When Is the Next Lunar Eclipse?

There are always at least two lunar eclipses per year. However, these are sometimes penumbral eclipses which can be difficult to see because the Moon simply appears slightly darker. And given atmospheric conditions, no noticeable different may be apparent.

Total and partial eclipses are far more exciting however, although these are more rare. Typically there are anywhere from zero to three total or partial eclipses each year. To determine when the next eclipse will occur, NASA has put together a handy online tool, which will tell you the date and time of the next lunar eclipse given your location on the Earth.

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