Chapter 3. Solar Eclipses

3.1 Introduction

The Sun in total eclipse

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

1. Determine during which lunar phase a solar eclipse can occur.
2. Explain how the three types of solar eclipses occur.
3. Determine the relative portion of the population who can observe a total and partial solar eclipse
4. Explain why the Moon exactly blocks the Sun's visible surface during a total solar eclipse.
5. Understand why solar eclipses don't occur every month.

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.
4. How the apparent sizes of the Moon and Sun affect what types of solar eclipses are observed.

Why you are doing it: Total solar eclipses are spectacular phenomena which are exciting to watch, but only viewable by a small population of Earth during any occurrence. While they occur regularly and predictably, they are visible for a small region of Earth at one time, and so people have the common misconception that they are rare. After completing this activity you will see how common they are, and understand how astronomers are able to predict when they will occur and where they will be viewable.

3.2 Background

Mayan Eclipse Representation

Solar eclipses have been observed by a number of ancient cultures, such as the Mayans, the Aztecs, and the Chinese, to name a few. The earliest recorded history of a solar eclipse was made in 2000 B.C. While these cultures did not understand what a solar eclipse was, and often feared them (offering sacrifices and performing other rituals to appease whatever was "devouring the Sun"), they were able to accurately predict their occurrences.

Today, we have an understanding of what a solar eclipse is. Simply put, an eclipse occurs when one object blocks the view of another object. In the case of a solar eclipse, the Moon blocks our view of the Sun. We even know when solar eclipses occur. Astronomers can predict where and when solar eclipses occur due to our understanding of the motions of heavenly bodies.

3.3 A Solar Eclipse

Watch the Moon in the animation as it slowly moves between the Sun and Earth.

As the Moon orbits around the Earth, every once in a while it passes directly between the Earth and Sun. At this position, marked with a red dot, the Moon casts a shadow on the Earth, completely blocking the Sun's light from being observed by anyone at that position. Observers will see the Moon pass in front of the Sun, creating a solar eclipse. From the observer's point of view, the Sun is being eclipsed (or blocked from view) by the Moon.

Check out what you would see from Earth if you were located on the 'red dot'. CLICK on "view from Earth" and the first control button to view the progress of the entire solar eclipse.

3.4 Umbra and Penumbra

Due to the fact that the Moon is roughly four times smaller than the Earth, its shadow does not completely cover the Earth. In fact, the total shadow of the Moon (or umbra) only falls on a small region of Earth (the dark circle when the shadow touches Earth in the figure above). People in this area will observe a total eclipse; the entire surface of the Sun is blocked by the Moon. A partial shadow (or penumbra) also falls over the Earth. The penumbra is the lightly shaded region in the animation above. People in this region will see a partial eclipse, or part of the Sun blocked by the Moon. The red dot on the surface of Earth represents the viewpoint from Earth. Drag your mouse vertically across the red dot in the animation and move it so the dot is in the penumbra. Click on "View from Earth" and the first control button to observe a partial solar eclipse.

3.5 Umbral Shadow's Path Across The Earth

The alignment between the Earth, Moon and Sun must be near perfect, preventing solar eclipses from occurring every New Moon. When they do happen though, only a small region of Earth will be able to observe a total solar eclipse. The umbral region of the Moon's shadow on Earth can cover an area up to 270 km in diameter, which is only a small area on Earth's surface. Due to the Moon's orbit around the Earth, the umbral shadow appears to move across the Earth from West to East, in the figure above. Because of this, solar eclipses last for only a few minutes at a time, for a given observer. The path of the umbral shadow across the Earth is known as the eclipse path, and describes the locations on Earth where a total solar eclipse is viewable on a given day.

3.6 Apparent Sizes

When the Moon completely blocks the Sun, creating a total solar eclipse, it blocks the visible surface of the Sun, known as the photosphere. Due to a unique coincidence, the Moon's apparent size in the sky is almost exactly the same as the Sun's apparent size in the sky as well. The result is that the Moon almost exactly blocks the photosphere of the Sun, but not its outer layer, the corona, which can extend many solar radii away from the Sun. As you observe a total solar eclipse (using protective eyewear, of course) you will see the photosphere slowly disappear, and then, just as it completely disappears, the corona will suddenly spring into view, as the much brighter photosphere is completely blocked (as seen in the central frame of the figure shown to your right).

Question 3.3

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3

If two people are the same distance away from you, who appears bigger: Someone who is 5 feet tall or someone who is 6 feet tall? If two people are both 6 feet tall, but one is 10 feet away from you while another is 100 feet away from you, who appears taller? Try again.

Correct.

Bigger objects will of course appear bigger, but the farther away an object is, the smaller it appears to an observer.

As you determined, the apparent size of an object is directly related to its physical size, and indirectly related to its distance. We can write this as a formula:

Below is a table that contains the physical size and average distance of the Moon and Sun from Earth, as well as a rough calculation of their apparent sizes as seen from Earth:

As you can see from the table, the Moon and the Sun are roughly the same apparent size. While the Sun is about 400 times farther from us than the Moon, it is also about 400 times as big. Therefore, the two objects appear the same size as seen from Earth. The Moon just covers the Sun during a solar eclipse!

Incorrect.

Bigger objects will of course appear bigger, but the farther away an object is, the smaller it appears to an observer.

As you determined, the apparent size of an object is directly related to its physical size, and indirectly related to its distance. We can write this as a formula:

Below is a table that contains the physical size and average distance of the Moon and Sun from Earth, as well as a rough calculation of their apparent sizes as seen from Earth:

As you can see from the table, the Moon and the Sun are roughly the same apparent size. While the Sun is about 400 times farther from us than the Moon, it is also about 400 times as big. Therefore, the two objects appear the same size as seen from Earth. The Moon just covers the Sun during a solar eclipse!

3.7 Apparent Sizes and Earth-Moon Orbit

Time-lapse photography of an annular eclipse

While the table on the previous page contained the average distance of the Moon and the Sun from Earth, the actual distances of the Moon and the Sun from Earth vary. The Earth's orbit around the Sun is slightly elliptical, as is the Moon's orbit around the Earth. Because of this, the apparent sizes of the Sun and Moon change slightly over time. The figure to the right shows the case when the Moon's apparent size is smaller than the Sun's apparent size, such that when a total eclipse occurs, the Moon doesn't completely block the photosphere of the Sun, and we get an annular solar eclipse.

3.8 Perfect Alignment and The Ecliptic

So why don't solar eclipses occur every New Moon? It has to do with the fact that the Earth, Moon and Sun have to be near-perfectly aligned in order for it to occur. It turns out that the Moon's orbit is not aligned with the ecliptic (the Earth's orbital plane about the Sun). It is inclined at only a 5 degree angle, but that's enough to prevent an eclipse most months. Many times, when the Moon is in the New Moon position, it is not in the ecliptic plane, but rather above it or below it. This exact alignment only occurs when the Moon passes through the ecliptic plane during the New Moon phase. Start the animation to observe when the alignment occurs for a solar eclipse. The animation will pause at each New Moon. Continue the moon's motion by clicking on the play button.

3.9 Quick Check Quiz

Indepth Activity: Solar Eclipses