Moon Phases and Eclipses Moon Phases and Eclipses Chapter 3

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.

3-1 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

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

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.

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 29.53 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 29.53 days.

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.

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.

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

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

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.

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.

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.

Lunar Eclipses: 2015-2024 There are typically one or two lunar eclipses per year

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

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

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.

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.

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.

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

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.

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.

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.

Solar Eclipses: 2015-2024 There are on average about 1 – 2 total solar eclipses per year

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.

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.

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

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.

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.

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.

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