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Earth Moon system

the Earth moon system

For the most part, as we will see in the diagram below, people on Earth can only see 1/2 of the moon’s surface. The moon orbits around the Earth in the same time it takes to rotate, so one face is locked towards us, the other face always away from us. Without spaceships we would never have seen the other side of the moon.

phases of the moon

Why does the moon appear to have phases?
David Rose here, developer of MoonConnection.com, writes:

Sunlight is shown coming in from the right. …The dotted line from the earth to the moon represents your line of sight when looking at the moon.

The large moon image shows what you would see at that point in the cycle.

For the waning gibbous, third quarter, and waning crescent phases you have to mentally turn yourself upside down when imagining the line of sight. When you do this, you’ll “see” that the illuminated portion is on your left, just as you see in the large image.

One important thing to notice is that exactly one half of the moon is always illuminated by the sun.

Of course that is perfectly logical, but you need to visualize it in order to understand the phases. At certain times we see both the sunlit portion and the shadowed portion — and that creates the various moon phase shapes we are all familiar with.

Also note that the shadowed part of the moon is invisible (*) to the naked eye; in the diagram above, it is only shown for clarification purposes. Finally, please realize this diagram is only meant to demonstrate how the phases work; the small inner moons in the diagram do not show the fact that the same side of the moon always faces Earth.

So the basic explanation is that the lunar phases are created by changing angles (relative positions) of the earth, the moon and the sun, as the moon orbits the earth.
Phases of the moon, at MoonConnection.com

(*) Mostly invisible – if the only light coming to us from the moon was from the Sun, this was would entirely true. But some light reflects off of the Earth, into space, and onto the moon’s dark surface – and a small amount of that light reflects back to the Earth where we may weakly see it. – RK

and of course, while the moon is orbiting the Earth, the Earth itself is orbiting the Sun, right?
So if we flew high above the plane of the ecliptic, and steadied ourselves with respect to the Sun, then we could look down and see the Earth slowly revolving around the Sun – and we’d then see Luna, our moon, revolving in this pattern around the Earth:

Phases of the Moon, as seen looking southward from the Northern Hemisphere. The Southern Hemisphere will see each phase rotated through 180°. The upper part of the diagram is not to scale, as the Moon is much farther from the Earth than shown here. https://en.wikipedia.org/wiki/Lunar_phase

Phases of the Moon, as seen looking southward from the Northern Hemisphere. The Southern Hemisphere will see each phase rotated through 180°. The upper part of the diagram is not to scale, as the Moon is much farther from the Earth than shown here.
https://en.wikipedia.org/wiki/Lunar_phase

Lunar libration

We can really see a little more than half of the moon’s surface from the Earth. The Wikipedia article explains:

The Moon generally has one hemisphere facing the Earth, due to tidal locking. Therefore, humans’ first view of the far side of the Moonresulted from lunar exploration in the 1960s; however, this simple picture is only approximately true: over time, slightly more than half (about 59%) of the Moon’s surface is seen from Earth due to libration.

Libration is manifested as a slow rocking back and forth of the Moon as viewed from Earth, permitting an observer to see slightly different halves of the surface at different times.

There are three types of lunar libration:

  • Libration in longitude results from the eccentricity of the Moon’s orbit around Earth; the Moon’s rotation sometimes leads and sometimes lags its orbital position.
  • Libration in latitude results from a slight inclination between the Moon’s axis of rotation and the normal to the plane of its orbitaround Earth. Its origin is analogous to how the seasons arise from Earth’s revolution about the Sun.
  • Diurnal libration is a small daily oscillation due to the Earth’s rotation, which carries an observer first to one side and then to the other side of the straight line joining Earth’s and the Moon’s centers, allowing the observer to look first around one side of the Moon and then around the other—because the observer is on the surface of the Earth, not at its center.
Lunar libration with phase

Lunar libration with phase

eclipses depend on shadows: the three types of shadows

the three types of shadows you’ll find in space

lunar eclipses

What are the conditions for a lunar eclipse?

what are the types of lunar eclipses

lunar-eclipse-types
let’s look at the geometry:

shouldn’t the moon be pitch black during an eclipse? Why does it appear red?

Sky & Telescope illustration

Sky & Telescope illustration

text

solar eclipses

when does a solar eclipse occur?

Solar Eclipse

The article from Astronomy Online explains:

The ecliptic is the a virtual plane that splits the Sun in half. Along this plane orbits the 9 planets, their moons and the asteroids. ..

It is important to realize that this ecliptic is only an apparent plane – as the orbits are inclinated by various degrees.

In other words, the planet orbits are not required to rotate on this plane.

Same is true for our Moon – in fact, our Moon’s orbit has an inclination of 5°. This inclination is important because this is the reason for having a variety of eclipse types – i.e. partial or total.

In addition to an inclination, all orbits also have a slight elongation. It is very rare, and safe to say impossible, for an orbit of a body about another body to be an perfect circle. According to Newton’s Law, objects with mass will affect other objects with mass. As Earth orbits the Sun because of the gravity attraction by the Sun, Earth’s mass also affects the Sun (although very little, but this can still be measured). The result is an elliptical orbit. This measured result is called eccentricity.

Because of the inclination and elongation of Moon’s orbit about Earth, there are three variations of a Solar Eclipse.

If the umbra is focused to a point on the Earth’s surface, a total solar eclipse will occur.

If the umbra is focused above the Earth’s surface, an annular eclipse will occur.

If the Moon orbits just above or below the Line of Nodes, a partial solar eclipse occurs. Notice the absence of a defined Umbra.
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Detail

 Solar Eclipse via Shutterstock. Solar and Lunar Eclipses. Vector.  Image ID: 166954466 Copyright: Alhovik

Solar Eclipse via Shutterstock.

Solar and Lunar Eclipses. Vector Image ID: 166954466. Copyright: Alhovik

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Seeing the “dark side of the moon”, from a satellite

“A NASA camera aboard the Deep Space Climate Observatory (DSCOVR) satellite captured a unique view of the moon as it moved in front of the sunlit side of Earth last month. The series of test images shows the fully illuminated “dark side” of the moon that is never visible from Earth.

The images were captured by NASA’s Earth Polychromatic Imaging Camera (EPIC), a four megapixel CCD camera and telescope on the DSCOVR satellite orbiting 1 million miles from Earth. From its position between the sun and Earth, DSCOVR conducts its primary mission of real-time solar wind monitoring for the National Oceanic and Atmospheric Administration (NOAA).

EPIC maintains a constant view of the fully illuminated Earth as it rotates, providing scientific observations of ozone, vegetation, cloud height and aerosols in the atmosphere. Once EPIC begins regular observations next month, the camera will provide a series of Earth images allowing study of daily variations over the entire globe. About twice a year the camera will capture the moon and Earth together as the orbit of DSCOVR crosses the orbital plane of the moon.

These images were taken between 3:50 p.m. and 8:45 p.m. EDT on July 16, showing the moon moving over the Pacific Ocean near North America. The North Pole is in the upper left corner of the image, reflecting the orbital tilt of Earth from the vantage point of the spacecraft.”

A NASA camera aboard the Deep Space Climate Observatory (DSCOVR) satellite captured a unique view of the moon as it moved in front of the sunlit side of Earth last month. The series of test images shows the fully illuminated “dark side” of the moon that is never visible from Earth. The images were captured by NASA’s Earth Polychromatic Imaging Camera (EPIC), a four megapixel CCD camera and telescope on the DSCOVR satellite orbiting 1 million miles from Earth. From its position between the sun and Earth, DSCOVR conducts its primary mission of real-time solar wind monitoring for the National Oceanic and Atmospheric Administration (NOAA). https://www.nasa.gov/feature/goddard/from-a-million-miles-away-nasa-camera-shows-moon-crossing-face-of-earth

A NASA camera aboard the Deep Space Climate Observatory (DSCOVR) satellite captured a unique view of the moon as it moved in front of the sunlit side of Earth last month. The series of test images shows the fully illuminated “dark side” of the moon that is never visible from Earth.
The images were captured by NASA’s Earth Polychromatic Imaging Camera (EPIC), a four megapixel CCD camera and telescope on the DSCOVR satellite orbiting 1 million miles from Earth. From its position between the sun and Earth, DSCOVR conducts its primary mission of real-time solar wind monitoring for the National Oceanic and Atmospheric Administration (NOAA).
https://www.nasa.gov/feature/goddard/from-a-million-miles-away-nasa-camera-shows-moon-crossing-face-of-earth

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Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework
ESS1. Earth’s Place in the Universe
8.MS-ESS1-1b. Develop and use a model of the Earth-Sun system to explain the cyclical pattern of seasons, which includes Earth’s tilt and differential intensity of sunlight on different areas of Earth across the year.
8.MS-ESS1-2. Explain the role of gravity in ocean tides, the orbital motions of planets, their moons, and asteroids in the solar system

Next Generation Science Standards: Science & Engineering Practices
● Ask questions that arise from careful observation of phenomena, or unexpected results, to clarify and/or seek additional information.
● Ask questions that arise from examining models or a theory, to clarify and/or seek additional information and relationships.

SAT Subject Test in Physics
Circular motion, such as uniform circular motion and centripetal force


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