1. Moon Phases and Eclipses
Moon Phases and Eclipses
Chapter 3

2. Guidepost
In this chapter, you will consider four important questions about the Moon
Why does the moon go through phases?
What causes a lunar eclipse?
What causes a solar eclipse?
How can eclipses be predicted?
Editors note: Capitalizing “Moon” here makes this consistent with the text.

3. 3-1 The Changeable Moon
Starting this evening, begin looking for the Moon in the sky
As you watch the Moon on successive evenings, you will see it:
Following its orbit around Earth
Cycling through its phases as it has done for billions of years

4. The Moon’s Orbital Motion
From Earth, we see different portions of the Moon’s surface lit by the sun, causing the phases of the Moon

5. The Lunar Phase Cycle
We always see the same side of the Moon facing Earth
In this sequence of lunar phase snapshots taken at intervals of 1 day, the Moon cycles through its phases from crescent to full to crescent. From Earth you see the same face of the Moon, the same mountains, craters, and plains, at all times, but the changing direction of sunlight changes what part is illuminated and produces the lunar phases.
Figure 3-1 In this sequence of lunar phase snapshots taken at intervals of 1 day, the Moon cycles through its phases from crescent
to full to crescent. From Earth you see the same face of the Moon, the same mountains, craters, and plains, at all times, but the
changing direction of sunlight changes what part is illuminated and produces the lunar phases.

6. The Moon’s Sidereal and Synodic Periods
The Moon orbits eastward around Earth in 27.3 days, the Moon’s sidereal period. This is how long the Moon takes to circle the sky once and return to the same position relative to the stars.
The Moon’s synodic period (to reach the same position relative to the sun, i.e., the same lunar phase) is days.
The Moon orbits eastward around Earth in 27.3 days, the Moon’s sidereal period. This is how long the Moon
takes to circle the sky once and return to the same position relative to the stars.
The Moon’s synodic period (to reach the same position relative to the sun, i.e., the same lunar phase) is days.

7. The Waxing Phases of the Moon
From night to night, the moon appears to move from west to east (right to left) - opposite to the motion within one night!
The waxing phases of the Moon can be seen primarily in the evening sky.
From night to night, the moon appears to move from west to east (right to left) - opposite to the motion within one night!
The waxing phases of the Moon can be seen primarily in the evening sky.

8. The Waning Phases of the Moon
As the illuminated portion of the Moon shrinks from the time of full moon to new moon, the Moon is said to “wane,” an
old-fashioned word for decrease.
As the illuminated portion of the Moon shrinks from the time of full moon to new moon, the Moon is said to “wane,” an old- fashioned word for decrease.

9. Common Misconceptions
Misconception: The Moon has a dark side that is never lit by the Sun
Truth: Any location on the Moon is sunlit for two weeks and in darkness for two weeks as the Moon rotates
Misconception: People tend to act up at the full Moon
Truth: Careful statistical studies of records from schools, prisons, hospitals, and so on, show that it isn’t true

10. 3-2 Lunar Eclipse
In cultures all around the world, the Moon is a regular counter of the passing days and months
Sometimes unusual events of the Moon arise

11. Earth’s Shadow
The shadow cast by a map tack can be used to understand how shadows produce eclipses
Umbra = The region of total shadow
Penumbra = The region of partial shadow
The shadow cast by a map tack can be used to understand the shadows of Earth and the Moon. The umbra is the region of total shadow; the penumbra is the region of partial shadow.
Figure 3-3 The shadow cast by a map tack can be used to understand the shadows of Earth and the Moon. The umbra is the
region of total shadow; the penumbra is the region of partial shadow.

12. Total Lunar Eclipse
Figure 3-4 (a) In this diagram of a total lunar eclipse, the Moon passes from right to left through Earth’s shadow.
(a) In this diagram of a total lunar eclipse, the Moon passes from right to left through Earth’s shadow.

13. Total Lunar Eclipse (cont’d.)
Lasts up to 1 hour and 40 min
Reflects sunlight scattered by Earth’s atmosphere
During a total lunar eclipse, the Moon turns a coppery-red color. In this photo, the Moon is darkest toward the lower right, the direction toward the center of the umbra. The edge of the Moon at upper left is brighter because it is near the edge of the umbra.
Figure 3-5 During a total lunar eclipse, the Moon turns a coppery-red color. In this photo, the Moon is darkest toward the lower right, the direction toward the center of the umbra. The edge of the Moon at upper left is brighter because it is near the edge of the umbra.

14. Lunar Eclipses:
There are typically one or two lunar eclipses per year

15. Partial and Penumbral Lunar Eclipses
The Moon’s orbit is inclined by a bit more than 5º relative to Earth’s orbital plane
Moon does not always pass through center of the Umbra
Partial lunar eclipse
Rest of the Moon enters the penumbra only
Penumbral lunar eclipse
Moon only enters the penumbra
Moon is only partially dimmed

16. 3-3 Solar Eclipses
For millennia, cultures worldwide have understood that the Sun is the source of life
People explained the terrifying sight of the Sun gradually disappearing with stories

17. The Angular Diameters of the Sun & Moon
The angular diameter of an object is related to both its linear diameter and its distance.
The sun appears approx. as large in the sky (same angular diameter ≈ 0.50) as the moon.
Figure 3-8 The angular diameter of an object is related to both its linear diameter and its distance.
The sun appears approx. as large in the sky (same angular diameter ≈ 0.50) as the moon.

18. Common Misconception
Misconception: The Moon is larger when it is on the horizon
Truth: The rising full Moon looks big when you see it on the horizon, but that is an optical illusion. In reality, the Moon is the same size on the horizon as when it is high overhead.
Apogee = the orbital point of greatest distance from Earth.
Perigee= the orbital point of closest distance to Earth.

19. The Moon’s Shadow: A Total Solar Eclipse
(a) The umbra of the Moon’s shadow sweeps from west to
east across Earth, and observers in the path of totality see a total solar
eclipse. Those outside the umbra but inside the penumbra see a partial eclipse.
(b) Eight photos made by a weather satellite have been combined
to show the Moon’s shadow moving across the eastern Pacific, Mexico,
Central America, and Brazil during an eclipse in 1991.
Figure 3-9 (a) The umbra of the Moon’s shadow sweeps from west to
east across Earth, and observers in the path of totality see a total solar
eclipse. Those outside the umbra but inside the penumbra see a partial
eclipse. (b) Eight photos made by a weather satellite have been combined
to show the Moon’s shadow moving across the eastern Pacific, Mexico,
Central America, and Brazil during an eclipse in 1991.

20. Annular Solar Eclipse
The angular sizes of the moon and the sun vary, depending on their distance from Earth.
When Earth is near perihelion and the moon is near apogee, we see an annular solar eclipse.
Figure 3-10 Because the angular diameter of the Moon and the Sun vary slightly, the disk of the Moon is sometimes too small to cover the disk of the Sun. That means the umbra of the Moon does not reach Earth, and the eclipse is annular, meaning a ring (“annulus”) of the Sun’s disk can be seen around the Moon. In this photograph of an annular eclipse in 1994, the dark disk of the Moon is almost exactly centered on the bright disk of the Sun
The angular sizes of the moon and the sun vary, depending on their distance from Earth. When Earth is near perihelion and the moon is near apogee, we see an annular solar eclipse. Because the angular diameter of the Moon and the Sun vary slightly, the disk of the Moon is sometimes too small to cover the disk of the Sun. That means the umbra of the Moon does not reach Earth, and the eclipse is annular, meaning a ring (“annulus”) of the Sun’s disk can be seen around the Moon. In this photograph of an annular eclipse in 1994, the dark disk of the Moon is almost exactly centered on the bright disk of the Sun

21. Features of the Solar Eclipse
This sequence of photos shows the first half of a total
solar eclipse.
Figure 3-11 This sequence of photos shows the first half of a total
solar eclipse.

22. Prominences
a) During a total solar eclipse, the Moon covers the photosphere, and the ruby-colored chromosphere and prominences are visible. Only the lower corona is visible in this image.
Figure 3-12 a) During a total solar eclipse, the Moon covers the photosphere,
and the ruby-colored chromosphere and prominences are visible.
Only the lower corona is visible in this image.

23. Diamond Ring Effect
(b) The diamond ring effect can sometimes occur momentarily at the beginning or end of totality if a small segment of the photosphere peeks out through a valley at the edge of the lunar disk.
Figure 3-12 (b) The diamond ring effect can sometimes occur momentarily at the beginning or end of totality if a
small segment of the photosphere peeks out through a valley at the edge of
the lunar disk.

24. Solar Eclipses:
There are on average about 1 – 2 total solar eclipses per year

25. Observing an Eclipse
A safe way to view the partial phases of a solar eclipse. Use
a pinhole in a card to project an image of the Sun on a second card. The greater the distance between the cards, the larger (and fainter) the image will be.
Figure 3-13 A safe way to view the partial phases of a solar eclipse. Use
a pinhole in a card to project an image of the Sun on a second card. The
greater the distance between the cards, the larger (and fainter) the image
will be.

26. Common Misconception
Misconception: Sunlight is somehow more dangerous during an eclipse
Truth: It is always dangerous to look at the Sun. The danger posed by an eclipse is that people are tempted to ignore common sense and look at the Sun directly, which can burn their eyes even when the Sun is almost totally eclipsed.

27. Predicting Eclipses
Making exact eclipse predictions requires a computer and proper software
Astronomers in early civilizations could make educated guesses as to which full moons and which new moons might result in eclipses

28. Conditions for an Eclipse
Figure 3-14 Eclipses can occur only when the Sun is appears from Earth to be near one of the nodes of the Moon’s orbit. (a) A solar eclipse occurs when the Moon meets the Sun near a node. (b) A lunar eclipse occurs when the Sun and Moon are near opposite nodes. Partial eclipses are shown here for clarity.
Eclipses can occur only when the Sun is appears from Earth to be near one of the nodes of the Moon’s orbit. (a) A solar eclipse occurs when the Moon meets the Sun near a node. (b) A lunar eclipse occurs when the Sun and Moon are near opposite nodes. Partial eclipses are shown here for clarity.

29. Eclipses Occur in a Cyclic Pattern
Figure 3-15 The Moon’s orbit is tipped a bit more than 5 degrees to Earth’s orbit. The nodes N and N' are the points where the Moon passes through the plane of Earth’s orbit. At those parts of Earth’s orbit where the line of nodes points toward the Sun, eclipses are possible at new moon and full moon.
The Moon’s orbit is tipped a bit more than 5 degrees to Earth’s orbit. The nodes N and N' are the points where the Moon passes through the plane of Earth’s orbit. At those parts of Earth’s orbit where the line of nodes points toward the Sun, eclipses are possible at new moon and full moon.

30. The Saros Cycle
Figure 3-16 A calendar of eclipse seasons: Each year the eclipse
seasons begin about 19 days earlier than in the previous year. Any
new moon or full moon that occurs during an eclipse season results
in an eclipse. Only total and annular eclipses are shown here.
Figure 3-17 The Saros cycle at work: An eclipse with a track having
nearly the same shape as that of the total solar eclipse of March 7, 1970,
recurred 18 years 11 1 / 3 days later over the Pacific Ocean. After another
interval of 18 years 11 1 / 3 days, an eclipse with a similar track was visible
from Asia and Africa. After another 18 years 11 1 / 3 days, in the year 2024,
an eclipse like the 1970 eclipse will again be visible from the United States.
A calendar of eclipse seasons: Each year the eclipse seasons begin about 19 days earlier than in the previous year. Any new moon or full moon that occurs during an eclipse season results in an eclipse. Only total and annular eclipses are shown here.

31. Discussion Questions
If a planet has a moon, must that moon go through the same phases that Earth’s Moon displays?
Hint: What characteristics might the moon’s orbit have to display the same phases?
Which lunar phases would be visible in the sky at dawn? At midnight?
Hint: Think about where the Sun is relative to the Moon