Chapter 3. Lunar Eclipses

3.1 Introduction

Total Lunar Eclipse Over Maui (July 16, 2000)

The goals of this module: After completing this exercise, you should be able to:

1. Explain the conditions required for lunar eclipses.
2. Describe the three types of lunar eclipses.
3. Explain why the Moon appears red during a total lunar eclipse.

In this module you will explore:

1. How the line of nodes determines whether an eclipse will occur.
2. How the umbra and penumbra of the Earth's shadow affect the type of lunar eclipse observed.
3. The role the Earth's atmosphere plays in lunar eclipses.

Why you are doing it: Lunar eclipses are an intriguing phenomenon. You may have observed one or two already and it is likely that you will have future opportunities to see one. Understanding how and why they occur will help you understand one part of astronomy that directly affects your world.

3.2 Background

From the earliest records of human history down to the present time, man has wondered about, and been amazed by, eclipses. As the following quotes show, eclipses have captured our interest and imagination as we have sought to understand and predict them. (All quotations are courtesy of mreclipse.com.)

"But about the first watch the Moon in eclipse, hid at first the brilliance of her heavenly body, then all her light was sullied and suffused with the hue of blood."
(From: Curtius, History of Alexander. These quotations refer to a lunar eclipse at moonrise in Sicily and at Arbela, of 20/21 September 331 BC. Quoted in Historical Eclipses and Earth's Rotation, by F. Richard Stephenson, Cambridge University Press, 1997, page 372.)

"And the moon in haste eclipsed her,
and the Sun in anger swore
He would curl his wick within him
and give light to you no more"
(Said to refer to a lunar eclipse of 425 BC, and an annular solar eclipse of 424 BC. Aristophanese (Greek, c450-385 BC) Chorus of Clouds (423BC))

"His defeat is like an eclipse of the Sun or Moon; does it harm the brightness (of these bodies)?"
(Extract from a speech made in 597 BC, in support of a general who had recently suffered defeat, and was in danger of execution. He was reinstated! From: Tso-chuan (Chinese, about 300 BC). Quoted in Historical Eclipses and Earth's Rotation, by F. Richard Stephenson, Cambridge University Press, 1997, page 223.)

"I will show portents in the sky and on earth,
blood and fire and columns of smoke;
the sun shall be turned into darkness
and the moon into blood before the great and terrible day of the Lord comes." (Joel, Chapter 2, verses 30, 31 (Old Testament))

People in ancient times were mystified and captivated by eclipses of the Sun and the Moon, but they failed to understand them at a basic level. In this activity, we will study lunar eclipses from a modern scientific perspective to gain understanding of how and why they occur.

3.3 When do lunar eclipses occur?

Look at the animation below to find out during which phase lunar eclipses occur.

3.4 Line Of Nodes

The reason that the Moon and the Earth are not always aligned is that the Moon's orbit is tilted by 5° to the plane of the Earth's orbit. This causes the Moon to sometimes miss the Earth's shadow because it is above or below it. The intersection of the plane of the Moon's orbit and the plane of the Earth's orbit is called the line of nodes. Lunar eclipses occur only when the Moon is on the line of nodes during a full phase, as shown in the next animation.

The line of nodes is the blue line representing the intersection of the planes described by the red ellipse and the blue ellipse. The animation will pause at every full Moon. You may check "Side view" to better view if there is an alignment between the Sun, Earth, and the Moon.

3.5 What does an eclipsed Moon look like?

The Moon doesn't disappear completely during a total eclipse. Instead, when it moves completely into the umbra of the Earth's shadow, the Moon turns distinctly reddish. Why does this happen? Look at the figure below to find out.

Summary

Why does the bending of the Sun's rays turn the Moon red instead of white? The particles that make up Earth's atmosphere are about the size of the wavelengths of blue light, so they absorb blue light efficiently. These particles then re-emit the absorbed light randomly, so that the blue wavelengths no longer move uniformly forward. This process is called scattering. Longer wavelengths of light (like red light) pass through Earth?s atmosphere without being absorbed, so red light is not scattered. Instead, as the red light enters the Earth's atmosphere it bends or is refracted. Refraction occurs when light passes from one medium into another (here, when light moves from outer space into Earth's atmosphere), and the light changes direction. Earth's atmosphere bends the rays of red light such that they illuminate the Moon, making it appear red. This is shown dramatically below.

Notice that the shadow of the Earth appears curved as it crosses the lunar surface. This happens because the Earth itself if round, and thus casts a circular shadow on the Moon.

3.6 What kinds of eclipses can be seen?

Earlier in this activity, we found that the tilt of the Moon's orbit causes it to sometimes miss the Earth's shadow, resulting in a full phase with no eclipse. When the Moon and the Earth are exactly aligned, then a total lunar eclipse occurs. If the alignment is close but not exact, then a partial or penumbral eclipse can occur. A partial eclipse occurs when only part of the Moon passes through the umbra (or darkest part) of the Earth's shadow. A penumbral eclipse occurs when the Moon passes through the penumbra of the shadow instead of the umbra. In the penumbra, only part of the Sun is blocked by the Earth, so the shadow is less dark than the umbra where the Sun is completely blocked by the Earth. Use the figure to investigate how the Moon's position affects what happens.

Look at the figure below which shows the Moon passing through the center of the Earth's shadow, the umbra.

3.7 Views Of The Lunar Eclipse

Now that we know the conditions needed for a lunar eclipse to occur, and what eclipses look like, we can ask: who gets to see a lunar eclipse? Are they only visible over a small region of the Earth or are they accessible to everyone? The figure below helps to answer these questions.

Click on the red dots to see what we see from earth at those points.

3.8 Quick Check Quiz

Indepth Activity: Lunar Eclipses