Lunar Eclipses for Beginners

©2009 by Fred Espenak

Introduction

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? You'll learn the answers to these questions and more in MrEclipse's 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 Moon > New Crescent > First Quarter > Waxing Gibbous> Full Moon >
        Waning Gibbous > Last Quarter > Old Crescent > New Moon (again)

Phases of the Moon
Phases of the Moon.

The phase known as New Moon can 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 word month is derived from the Moon's 29.5 day period?

To many of us, Full Moon is 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 Moon's 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.

Lunar Eclipse Geometry
Geometry of the Sun, Earth and Moon During an Eclipse of the Moon
Earth's two shadows are the penumbra and the umbra.
(Sizes and distances not to scale)

Types of Lunar Eclipses

An eclipse of the Moon (or lunar eclipse) can only occur at Full Moon, and only if the Moon passes through some portion of Earth's 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 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 because they are subtle and hard 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 due to the Moon's 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 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.

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 is only possible when the Moon is in the New Moon phase. For more information, see Solar Eclipses for Beginners.

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

Why is the Moon Red During a Total Lunar 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 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 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.

2000 Total Lunar Eclipse Sequence
Total Lunar Eclipse of 2000 Jan 20-21
Beginning (right), middle (center) and end (left) of totality
(click to see more photos)


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. See MrEclipse's Picks for camera, lens and tripod recommendations. For more photographs taken during previous lunar eclipses, be sure to visit Lunar Eclipse Photo Gallery.


Lunar Eclipse Frequency and Future Eclipses

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 eclipse Date to 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.

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 Eclipse Duration column 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 (in bold) is the length of the total eclipse.



Eclipses of the Moon: 2014 - 2020
Date Eclipse Type Umbral Magnitude Eclipse Duration Geographic Region of Eclipse Visibility
2014 Apr 15 Total 1.291 03h35m
01h18m
Aus., Pacific, Americas
2014 Oct 08 Total 1.166 03h20m
00h59m
Asia, Aus., Pacific, Americas
2015 Apr 04 Total 1.001 03h29m
00h05m
Asia, Aus., Pacific, Americas
2015 Sep 28 Total 1.276 03h20m
01h12m
e Pacific, Americas, Europe, Africa, w Asia
2016 Mar 23 Penumbral -0.312 - Asia, Aus., Pacific, w Americas
2016 Sep 16 Penumbral -0.064 - Europe, Africa, Asia, Aus., w Pacific
2017 Feb 11 Penumbral -0.035 - Americas, Europe, Africa, Asia
2017 Aug 07 Partial 0.246 01h55m Europe, Africa, Asia, Aus.
2018 Jan 31 Total 1.315 03h23m
01h16m
Asia, Aus., Pacific, w N.America
2018 Jul 27 Total 1.609 03h55m
01h43m
S.America, Europe, Africa, Asia, Aus.
2019 Jan 21 Total 1.195 03h17m
01h02m
c Pacific, Americas, Europe, Africa
2019 Jul 16 Partial 0.653 02h58m S.America, Europe, Africa, Asia, Aus.
2020 Jan 10 Penumbral -0.116 - Europe, Africa, Asia, Aus.
2020 Jun 05 Penumbral -0.405 - Europe, Africa, Asia, Aus.
2020 Jul 05 Penumbral -0.644 - Americas, sw Europe, Africa
2020 Nov 30 Penumbral -0.262 - Asia, Aus., Pacific, Americas

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.

Upcoming lunar eclipses visible from the U.S.A. include Apr. 15, 2014 (total) and Oct. 08, 2014 (total).


Total Lunar Eclipse Sequence
Total Lunar Eclipse Over Maui
A Nikon 8008 was used in multiple exposure mode to capture the entire eclipse
on one frame of film. A second exposure captures morning twilight.
Total Lunar Eclipse of 2000 July 16 (Lahaina, Maui)
(click to see larger image)