Cycles in the Sky

Essential Questions What causes the seasons? What causes the seasons? Why does the Moon go through phases? Why does the Moon go through phases? What is a lunar eclipse? What is a lunar eclipse? What is a solar eclipse? What is a solar eclipse?

The Seasons Many people believe we have cool winters and warm summers because the Earth gets closer and farther away from the Sun over the year. Many people believe we have cool winters and warm summers because the Earth gets closer and farther away from the Sun over the year. While the Earth’s orbit is elliptical, and we do get about 1.7% closer/farther to the Sun than average, this is not the cause of the seasons. While the Earth’s orbit is elliptical, and we do get about 1.7% closer/farther to the Sun than average, this is not the cause of the seasons.

Perihelion & Aphelion In fact, the Earth is at perihelion, its closest point to the Sun, around January 4 th when we have winter. (~147,500,000 km) In fact, the Earth is at perihelion, its closest point to the Sun, around January 4 th when we have winter. (~147,500,000 km) The Earth is at aphelion, its farthest point from the Sun, around July 4 th during our summer. (~152,500,000km) The Earth is at aphelion, its farthest point from the Sun, around July 4 th during our summer. (~152,500,000km)

What causes the seasons then? The tilt of the Earth’s axis. The tilt of the Earth’s axis. Throughout the year, the Earth’s North Pole always points towards Polaris. Throughout the year, the Earth’s North Pole always points towards Polaris. Sunlight hits the Earth at different angles in the four seasons. Sunlight hits the Earth at different angles in the four seasons.

The Seasons In the summer, the Sun is above the horizon longer because it rises north of east and sets north of west. (see diagram) In the summer, the Sun is above the horizon longer because it rises north of east and sets north of west. (see diagram) It also shines more directly down on the ground. (think of flashlight shining straight down or at an angle) It also shines more directly down on the ground. (think of flashlight shining straight down or at an angle) Both effects cause warmer weather. Both effects cause warmer weather.

Solstices The Sun is highest in the sky on the Summer Solstice, June 21 st or so. On this day, the Sun rises the farthest N of E and there are the most daylight hours. The Sun is highest in the sky on the Summer Solstice, June 21 st or so. On this day, the Sun rises the farthest N of E and there are the most daylight hours. The Sun is lowest in the sky on the Winter Solstice, December 21 st or so. On this day, the Sun rises the farthest S of E and there are the least daylight hours. The Sun is lowest in the sky on the Winter Solstice, December 21 st or so. On this day, the Sun rises the farthest S of E and there are the least daylight hours.

Equinoxes On March 21 st or so, the Vernal (Spring) Equinox occurs. The Sun rises due East and we have equal hours of daylight and night. On March 21 st or so, the Vernal (Spring) Equinox occurs. The Sun rises due East and we have equal hours of daylight and night. On September 21 st or so, the Autumnal (Fall) Equinox occurs. The Sun again rises due East and we have equal hours of day and night. On September 21 st or so, the Autumnal (Fall) Equinox occurs. The Sun again rises due East and we have equal hours of day and night.

Direct Sunlight on Solstices and Equinoxes On the Equinoxes, the Sun’s rays shine straight down on the Earth’s Equator. On the Equinoxes, the Sun’s rays shine straight down on the Earth’s Equator. On the Summer Solstice, the Sun shines straight down on the ‘Tropic of Cancer’ (23.5° North). On the Summer Solstice, the Sun shines straight down on the ‘Tropic of Cancer’ (23.5° North). On the Winter Solstice, the Sun shines straight down on the ‘Tropic of Capricorn’ (23.5° South). On the Winter Solstice, the Sun shines straight down on the ‘Tropic of Capricorn’ (23.5° South).

How does the Moon move? First, two definitions: Rotation-spin about an internal axis. (ex. Earth’s daily rotation) Rotation-spin about an internal axis. (ex. Earth’s daily rotation) Revolution-spin about an external axis. (Earth’s yearly revolution) Revolution-spin about an external axis. (Earth’s yearly revolution)

The motion of the Moon If you watch the Moon night after night you notice three things: 1. It moves eastward against the constellations. 2. The markings on its face don’t change. 3. Different parts of it are illuminated each night. (Phases)

Revolution and Rotation The first motion, eastward against the background stars, is explained by the Moon’s orbit around the Earth. The first motion, eastward against the background stars, is explained by the Moon’s orbit around the Earth. We call that a… We call that a… Revolution! Good job! Revolution! Good job! What about the surface features not changing? Does the Moon rotate? What about the surface features not changing? Does the Moon rotate?

Modeling the Moon A volunteer with a textbook to model the Moon please… A volunteer with a textbook to model the Moon please… A volunteer to play the Earth please… A volunteer to play the Earth please… “Moon guy” orbit the Earth keeping one side of the book facing “Earth guy” all the time. “Moon guy” orbit the Earth keeping one side of the book facing “Earth guy” all the time. Did “Moon guy” rotate? Try it again and don’t rotate… Did “Moon guy” rotate? Try it again and don’t rotate…

Earth’s Moon OK, so the Moon revolves around the Earth and rotates on its own axis at the same rate. OK, so the Moon revolves around the Earth and rotates on its own axis at the same rate. What’s wrong with the diagram on the left? What’s wrong with the diagram on the left?

To the Moon… Answer: The scale! Answer: The scale! The Moon is 30 Earth diameters away. The Moon is 30 Earth diameters away. If the Earth was a basketball, the Moon would be a baseball, 30 basketballs away! If the Earth was a basketball, the Moon would be a baseball, 30 basketballs away! The average distance to the Moon is 384,000 km. (Seeds, page 33) The average distance to the Moon is 384,000 km. (Seeds, page 33)

And finally, why does the Moon have Phases?

Those odd phase names: New and Full I think you get. New and Full I think you get. 1 st and 3 rd Quarter are named because they occur one quarter and three quarters of the way through the cycle. (Also we only see a quarter of the Moon’s surface at those times.) 1 st and 3 rd Quarter are named because they occur one quarter and three quarters of the way through the cycle. (Also we only see a quarter of the Moon’s surface at those times.) Wax—to gradually increase. Wax—to gradually increase. Wane—to gradually decrease. Wane—to gradually decrease. Gibbous—Latin for humpbacked. Gibbous—Latin for humpbacked.

Lunar cycles… When and where would you see each phase of the Moon? When and where would you see each phase of the Moon? How much time for 1 lunar orbit? How much time for 1 lunar orbit? The sidereal period of the Moon is about 27 days (27.32 to be exact). The sidereal period of the Moon is about 27 days (27.32 to be exact). Sidereal period is measured relative to background stars and represents the actual time to orbit Earth once. Sidereal period is measured relative to background stars and represents the actual time to orbit Earth once.

Lunar Cycles cont… The synodic period of the Moon is the time to complete a cycle of phases. The synodic period of the Moon is the time to complete a cycle of phases. This takes about 29.5 days—just over 4 weeks, a “moonth.” This takes about 29.5 days—just over 4 weeks, a “moonth.” The word synodic comes from the Greek words for together and path The word synodic comes from the Greek words for together and path Why does the phase cycle take longer than one lunar orbit? Why does the phase cycle take longer than one lunar orbit?

Sidereal vs. Synodic As the Moon orbits the Earth, the Earth orbits the Sun. As the Moon orbits the Earth, the Earth orbits the Sun. The Moon must then revolve more than 360° to reach the same alignment for New Moon. The Moon must then revolve more than 360° to reach the same alignment for New Moon. Extra 29° takes an extra 2 days. Extra 29° takes an extra 2 days.

Eclipses We will discuss various types of eclipses now. We will discuss various types of eclipses now. What does the word eclipse mean? What does the word eclipse mean? The word eclipse means… The word eclipse means… to obscure or hide. For example: “Politicians’ failures often eclipse their accomplishments.” For example: “Politicians’ failures often eclipse their accomplishments.”

Lunar Eclipses A lunar eclipse occurs at Full Moon when the Moon moves through the shadow of Earth. A lunar eclipse occurs at Full Moon when the Moon moves through the shadow of Earth. Earth’s shadow consists of two parts: Earth’s shadow consists of two parts:

The Umbra and Penumbra The umbra is the region of total shadow. The umbra is the region of total shadow. The penumbra is the region of partial shadow. The penumbra is the region of partial shadow. Seeds, page 39 Seeds, page 39

Lunar Eclipses During a total lunar eclipse, the Moon passes completely into Earth’s umbral shadow. During a total lunar eclipse, the Moon passes completely into Earth’s umbral shadow. It often appears to have a reddish cast because of sunlight refracting around the Earth. It often appears to have a reddish cast because of sunlight refracting around the Earth.

Lunar Eclipses cont. A partial lunar eclipse occurs when only part of the Moon enters the umbral shadow. A partial lunar eclipse occurs when only part of the Moon enters the umbral shadow. A penumbral lunar eclipse occurs when the Moon enters only the penumbral shadow of the Earth. They aren’t very impressive and are often not even noticed by the public. A penumbral lunar eclipse occurs when the Moon enters only the penumbral shadow of the Earth. They aren’t very impressive and are often not even noticed by the public.

Solar Eclipses

We are lucky… Earthlings can see a phenomenon that is not visible on most planets. Earthlings can see a phenomenon that is not visible on most planets. Our Moon is large enough and close enough to completely eclipse the Sun. Our Moon is large enough and close enough to completely eclipse the Sun. The Sun may be 400 times larger than the Moon, but the Moon is about 400 times closer! The Sun may be 400 times larger than the Moon, but the Moon is about 400 times closer!

Total Solar Eclipses During a total solar eclipse, the Moon completely covers the visible surface of the Sun called the photoshere. During a total solar eclipse, the Moon completely covers the visible surface of the Sun called the photoshere. Once the photosphere is covered, the outer atmosphere of the Sun, called the corona, is visible. Once the photosphere is covered, the outer atmosphere of the Sun, called the corona, is visible. Sometimes the active lower atmosphere, the chromosphere, is visible as well. Sometimes the active lower atmosphere, the chromosphere, is visible as well.

Solar Eclipses cont… A partial solar eclipse occurs when the Moon does not completely cover the Sun. A partial solar eclipse occurs when the Moon does not completely cover the Sun. Both total and partial solar eclipses take place during the New Moon phase. Both total and partial solar eclipses take place during the New Moon phase. There is a third type of solar eclipse as well… There is a third type of solar eclipse as well…

Annular Eclipse An annular eclipse is a solar eclipse in which a ring (or annulus) of the photosphere is visible around the disk of the Moon. An annular eclipse is a solar eclipse in which a ring (or annulus) of the photosphere is visible around the disk of the Moon. This occurs when the Moon is slightly farther from the Earth than average because of its elliptical orbit. This occurs when the Moon is slightly farther from the Earth than average because of its elliptical orbit.

Apogee and Perigee The Moon’s closest approach to the Earth in its elliptical orbit is called perigee. The Moon’s closest approach to the Earth in its elliptical orbit is called perigee. Apogee is the point in its orbit where it is farthest from the Earth. (annular eclipses) Apogee is the point in its orbit where it is farthest from the Earth. (annular eclipses) Apogee and perigee are also used for other satellites orbiting Earth or other planets. Apogee and perigee are also used for other satellites orbiting Earth or other planets.

Eclipse Geometry If lunar eclipses happen at Full Moon and solar eclipses happen at New Moon… If lunar eclipses happen at Full Moon and solar eclipses happen at New Moon… Why don’t we have eclipses every New and Full Moon? Why don’t we have eclipses every New and Full Moon?

The Moon’s orbit is tipped about 5° with the ecliptic.

Nodes For an eclipse to occur, the Moon must reach new or full phase just as its orbital path intersects the ecliptic. For an eclipse to occur, the Moon must reach new or full phase just as its orbital path intersects the ecliptic. Such a point is called a node. Such a point is called a node. It is more likely the Moon will be just north or south of this node and an eclipse will not occur. It is more likely the Moon will be just north or south of this node and an eclipse will not occur. See Seeds, page 47 See Seeds, page 47