Lunar Eclipses for Beginners

The total lunar eclipse of 2004 Oct 28 was widely visible from the USA.This sequence of images captures the eclipse from start (right) to finish (left).

What is an eclipse of the Moon? What causes eclipses and why? How often do eclipses happen and when is the next eclipse of the Moon? Youll learn the answers to these questions and more in MrEclipses primer on lunar eclipses.

The Moon is a cold, rocky body about 2,160 miles (3,476 km) in diameter. It has no light of its own but shines by sunlight reflected from its surface. The Moon orbits Earth about once every 29 and a half days. As it circles our planet, the changing position of the Moon with respect to the Sun causes our natural satellite to cycle through a series of phases:

New Crescent

First Quarter

Waxing Gibbous

Full Moon

Waning Gibbous

Last Quarter

Old Crescent

The phase known asNew Mooncan not actually be seen because the illuminated side of the Moon is then pointed away from Earth. The rest of the phases are familiar to all of us as the Moon cycles through them month after month. Did you realize that the wordmonthis derived from the Moons 29.5 day period?

Many early civilizations used the Moons monthly cycle to measure the passage of time. In fact, some calendars are synchronized to the phases of the Moon. The Hebrew, Muslim and Chinese calendars are all lunar calendars. TheNew Moonphase is uniquely recognized as the beginning of each calendar month just as it is the beginning on the Moons monthly cycle. In comparison, theFull Moonphase occurs mid-way through the lunar month.

TheFull Moonis popularly known as the phase of love and romance. When the Moon is Full, it rises at sunset and is visible all night long. At the end of the night, the Full Moon sets just as the Sun rises. None of the Moons other phases have this unique characteristic. It happens because the Moon is directly opposite the Sun in the sky when the Moon is Full. Full Moon also has special significance with regard to eclipses.

Geometry of the Sun, Earth and Moon During an Eclipse of the MoonEarths two shadows are the penumbra and the umbra.

An eclipse of the Moon (or lunar eclipse) canonlyoccur at Full Moon, and only if the Moon passes through some portion of Earths shadow. That 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 Suns rays from reaching the Moon. In contrast, the inner or umbral shadow is a region where the Earth blocksalldirect sunlight from reaching the Moon.

Astronomers recognize three basic types of lunar eclipses:

The Moon passes through Earths penumbral shadow.

These events are of only academic interest because they are subtle and hard to observe.

A portion of the Moon passes through Earths umbral shadow.

These events are easy to see, even with the unaided eye.

The entire Moon passes through Earths umbral shadow.

These events are quite striking due to the Moons vibrant red color during the total phase (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 dont we have an eclipse once a month during Full Moon?. Im glad you asked! You see, the Moons orbit around Earth is actually tipped about 5 degrees to Earths orbit around the Sun. This means that the Moon spends most of the time either above or below the plane of Earths orbit. And the plane of Earths orbit around the Sun is important because Earths shadows lie exactly in the same plane. During Full Moon, our natural satellite usually passes above or below Earths shadows and misses them entirely. No eclipse takes place. But two to four times each year, the Moon passes through some portion of the Earths 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.

What is the difference between a lunar eclipse and a solar eclipse? A solar eclipse is an eclipse of the Sun. It happens when the Moon passes between the Earth and the Sun. This isonlypossible when the Moon is in theNew Moonphase. For more information, seeSolar Eclipses for Beginners.

Total Lunar Eclipse of 2004 Oct 27-28Beginning (right), middle (center) and end (left) of totality

During a total lunar eclipse, the Earth blocks the Suns light from reaching the Moon. Astronauts on the Moon would then see the Earth completely eclipse 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 Earths umbral shadow, indirect sunlight still manages to reach and illuminate it. However, this sunlight must first pass deep through the Earths 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. Earths 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 Earths 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 Earths atmosphere. Total eclipses tend to be very dark after major volcanic eruptions since these events dump large amounts of volcanic ash into Earths 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.

Total Lunar Eclipse of 2000 Jan 20-21Beginning (right), middle (center) and end (left) of totality

Unlike solar eclipses, lunar eclipses are completely safe to watch. You dont need any kind of protective filters. It isnt 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 7×35 or 7×50 binoculars work fine. Remember to dress warmly and enjoy the spectacle!

Amateur astronomers can actually make some useful observations during total eclipses. Its 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 Earths atmosphere during the eclipse. Using theDanjon Brightness Scalefor lunar eclipses, amateurs can categorize the Moons 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. Thesecrater timingscan be used to estimate the enlargement of Earths 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 photographyis easy provided that you have the right equipment and use it correctly. SeeMrEclipses Picksfor camera, lens and tripod recommendations. For more photographs taken during previous lunar eclipses, be sure to visitLunar Eclipse Photo Gallery.

Penumbral eclipses are of little interest because they are hard to see. If we consider only partial and total lunar eclipses, how often do they occur?

During the five thousand year period from 2000 BCE through 3000 CE, there are 7,718 eclipses of the Moon (partial and total). This averages out to about one and a half eclipses each year. Actually, the number of lunar eclipses in a single year can range from 0 to 3. The last time that 3 total lunar eclipses occurred in one calendar year was in 1982. Partial eclipses slightly outnumber total eclipses by 7 to 6.

The table below lists every lunar eclipse from 2014 through 2020. Click on the eclipseDateto see a diagram of the eclipse and a world map showing where it is visible from. Although penumbral lunar eclipses are included in this list, they are usually hard to see because they are faint.

TheUmbral Magnitudeis the fraction on the Moons 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. TheEclipse Durationcolumn lists the length of the partial eclipse in hours and minutes. If it is a total eclipse, two values are given. The first is the amount of time between the start and end of the partial phases while the second (inbold) is the length of the total eclipse.

Geographic Region of Eclipse Visibility

e Pacific, Americas, Europe, Africa, w Asia

Europe, Africa, Asia, Aus., w Pacific

S.America, Europe, Africa, Asia, Aus.

c Pacific, Americas, Europe, Africa

S.America, Europe, Africa, Asia, Aus.

Geographic abreviations (used above): n = north, s = south, e = east, w = west, c = central

For an extended version of this table, see:Lunar Eclipse Preview: 2011-2030.

A Nikon 8008 was used in multiple exposure mode to capture the entire eclipseon one frame of film. A second exposure captures morning twilight.Total Lunar Eclipse of 2000 July 16 (Lahaina, Maui)

Six Millennium Catalog of Lunar Eclipses: 3000 BCE to AD 3000 CE

Lunar Eclipses of Historical Interest

Danjon Brightness Scale of Lunar Eclipses

Exposure Table for Lunar Eclipse Photography

Total Lunar Eclipse of 2000 Jan 20-21

Total Lunar Eclipse of 2004 Oct 27-28

Index to Eclipse and Astronomy Photographs