Tumbleweed Observatory's

Astronomy Hints



What You Need to View a Lunar Eclipse, and How To Observe

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Get Ready for the Next Lunar Eclipse

Being the closest solar system object to the Earth, the Moon is a favorite target of amateur astronomers. As this Earth And Moon Friends t-shirt design depicts, the Earth and Moon are friends forever. And one of the most exciting things to see with naked eye, binoculars, or telescope is a product of that long lasting Earth/Moon relationship, the Lunar Eclipse. Happening from 2 to 4 times per year, the Lunar Eclipse is one of the best events to use for introducing new people to the hobby of amateur astronomy.

Available to even to observers with no equipment, the Lunar eclipse is as exciting to watch as it is informative about the solar system.

An excellent site that lists the upcoming Lunar eclipses for the next several years is Mr. Eclipse.

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Lunar Eclipse Geometry

Lunar Eclipse Geometry

In case you aren't sure what a Lunar eclipse is, I've constructed the diagram you see above.

While not to scale, this geometrical drawing shows what is happening during a Lunar eclipse.

In this diagram, the Moon is shown at the left of the picture, in the shadow of the Earth.

The Earth is in the center of the diagram, and the Sun is at the right of the diagram.

As shown, the Sun is illuminating the right half of the Earth, but the Moon is passing through the Earth's shadow.

Because the Sun as seen from the Earth-Moon system is not a point source of light, but has a size of about 1/2 degree, the shadow cast by the Earth is a bit complicated.

When the Moon begins to pass behind the Earth as seen from the Sun, There's a conic shaped region that some parts of the Sun can illuminate, but some parts of the Sun are blocked by the Earth.

In this region, called the pnumbra, the sunlight is dimmed by being partially blocked. The penumbra is shown in the diagram as a gray shaded area. In fact, the region gets progressively darker toward the side of Earth opposite the Sun, as more of the Sun is progressively blocked by Earth.

Directly behind the Earth is a conic shaped region that is completely shadowed by the Earth, making a very dark shadow. This region, shown in dark gray, is called the umbra.

Penumbra Image

Penumbra Effect As Seen From The Moon

The image at left is another explanation of the penumbra. This image is a hypothetical view of the early phases of a Lunar eclipse as viewed from the Moon.

In this illustration, the Moon is in the Earth's penumbra, and as such some part of the Sun can still be seen peeking around the Earth's limb. The area behind the Earth where some but not all of the light of the Sun is blocked makes up the penumbra.

Umbra Image

Umbra Effect As Seen From The Moon

This image gives a hypothetical view of the Lunar eclipse as seen from the Moon when the Moon is in the Umbra.

When in the Umbra, no part of the Sun is visible from the Moon. However, the Earth's atmosphere refracts some of the light, and this very dim and filtered light does reach the Moon. Because of the filtering, much of the blue light is absorbed by the Earth's atmosphere, allowing only the more reddish component to reach the moon.

What Do You See During An Eclipse?

It depends upon the kind of Lunar eclipse and the part of the eclipse being viewed.

Sometimes the Moon doesn't pass through the umbra (imagine the Moon not in the exact plane of the Earth and Sun in the first diagram). In this case, the Moon only passes through the penumbra. This is called a penumbral eclipse.

If you witnessed a penumbral eclipse, you probably wouldn't even notice it. The darkening of the Moon from the partial blocking of sunlight is subtle and difficult to notice.

You might see a partial Lunar eclipse. In this type of eclipse, only part of the Moon passes through the umbra. In this case, you would see the curved shadow of the Earth clearly imposed upon the Moon.

This effect was one of the early proofs that the Earth was round. In fact, based on the curvature of the Earth's shadow, the ancient Greeks were able to determine that the Earth was roughly 4 times the size of the moon.

Every 2 or 3 years, and even sometimes a couple of times per year, you'll get the chance to see a total Lunar eclipse, where the entire Moon passes through the Earth's umbra.

When this happens, you'll be able to watch the Earth's curved shadow creep across the surface of the moon, and when the entire moon is covered, probably witness a dim and reddish-orange Moon. The reddish-orange color is caused by the Moon being illuminated by the refracted and filtered sunlight passing through the Earth's atmosphere.

If you stay with it, a few hours later the Earth's shadow will begin moving past the Moon, and the Moon will again be it's bright and inspiring self.

The entire event takes between 3 and 4 hours, so there's plenty of time to view it, and it can be enjoyed with the naked eye.

Even Better Through Binoculars

Small Binoculars

If you have a pair of binoculars, you can get even more enjoyment out of viewing a Lunar eclipse. Something like the Celestron SkyMaster Giant 15x70 Binoculars with Tripod Adapter make a great pair of binoculars for general purpose astronomical work, though by all means use any pair of binoculars at your disposal on the next Lunar Eclipse.

It might be even more enjoyable, and more relaxing, if you can mount your binoculars on a camera tripod, or some other type of stand.

With binoculars, you'll be able to see some of the major features as they get gobbled up with the Earth's shadow. You'll be able to detect the changes in color as more and more of the Moon gets covered.

You might even be able to see the color differences right at the edge of the Earth's shadow as it moves along.

If you don't have a pair of binoculars, use this handy customized search engine to find a pair.

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How About Through A Telescope?

While the event is thoroughly enjoyable with naked eye or binoculars, it's also fun to watch with a telescope. As the humorous astronomy t-shirt design titled Stargazer Weapon suggests, any astronomical weapon in your telescope arsenal will give you an enhanced eclipse experience.

Note that you don't need a large telescope for this event. I'm not even sure a large telescope would be an advantage.

I usually view Lunar eclipses with my 50mm refractor, pictured here, or my 6 inch f/5 Newtonian. Both have focal lengths of about 30 inches, and both give nice full-moon images at low power.

If you have a small telescope, train it on the moon for a real treat. As the Earth's shadow crosses over some of your favorite regions, you may want to step up the magnification a bit to see how the advancing shadow changes the look of certain features.

With a telescope, you can often see a peculiar slate-blue band on the leading edge of the advancing shadow, extending perhaps some 30 miles across the Moons surface. This peculiar region is caused by the prism effect of the Earth's refracting atmosphere, which separates the sun's light into different colors. It's quite visible with a small telescope.

The following chart relates telescope size and type to desired observing target. For observing lunar eclipses, you'd likely get the most enjoyment from the wide-field to general purpose telescopes in the table. You may also want to concentrate on the smaller instruments, as the larger ones are of no particular value for lunar observing.

Telescope/Observing Preference Table

(Small Instruments At Table Bottom)

Wide FieldGeneral PurposeNarrow Field
15" f/4.5 DOB10" f/10 DOB12" SCT
12" f/4.5 DOB10" f/6 DOB6" Refractor
6" f/5 Newt8" f/10 SCT6" Maksutov
6" f/5 DOB6" f/8 Newt6" f/10 Newt
3.5" f/8 Refractor4" f/11 Refractor4" f/15 Refractor
4.5" f/4.5 DOB5" f/10 SCT5" f/15 Maksutov
2.4" Refractor4.5" f/10 Newt3.5" f/15 Refractor
Binoculars3" f/10 Refractor3.5" Maksutov