Luna, our 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 as seen from Earth.
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
Moon’s orbit around the Sun
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. Then we’d see Luna, our moon, revolving in this pattern around the Earth:
We can really see more than half of the moon’s surface from the Earth. About 59% of the Moon’s surface is visible, thanks to
Lunar libration in latitude – due to the Moon’s axis being slightly inclined relative to the Earth’s axis. From our angle we can at one time peek over the north pole of the Moon, and then later in the lunar month we peek over the south pole. Over the entire four week cycle it gives the the effect of the Moon slowly “nodding its head yes.”
Diurnal (daily) libration – due to the observer first viewing from the western edge of the Earth as the Moon is rising, and then later from up to four thousand miles away to the east as the Moon is setting. This is due to the rotation of the Earth. The difference in perspective between the rising and setting of the Moon appears as a slight turning of the Moon first to west and then to east, as though “shaking its head no.”
Libration of longitude – an effect of the Moon’s varying rate of travel along its slightly elliptical orbit around the Earth. The Moon travels faster when it is at its closest to Earth, and its slowest when it is farthest away. Its rotation on its own axis is more regular, the difference appearing again as a slight east-west “no” oscillation.
— Skywise Unlimited, Astronomy 101
Over the course of a month, these tree types of motion reveal the following.
Eclipses depend on shadows
This photo shows the three types of shadows
Three types of shadows that occur in space.
What are the conditions for a lunar eclipse?
Types of lunar eclipses
Let’s look at the geometry:
Shouldn’t the moon be pitch black during the peak of a lunar eclipse? It would seem so, so why does it appear red?
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.
Detail of a solar eclipse
Solar and Lunar Eclipses. Vector Image ID: 166954466. Copyright: Alhovik
Seeing the “dark side of the moon”
“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.”
The origin of the moon
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
Next Generation Science Standards
MS-ESS1-1. Develop and use a model of the Earth-sun-moon system to describe the cyclic patterns of lunar phases, eclipses of the sun and moon, and seasons. [Clarification Statement: Examples of models can be physical, graphical, or conceptual.]
ESS1.A: The Universe and Its Stars
Patterns of the apparent motion of the sun, the moon, and stars in the sky can be observed, described, predicted, and explained with models. (MS-ESS1-1)
ESS1.B: Earth and the Solar System
The solar system consists of the sun and a collection of objects, including planets, their moons, and asteroids that are held in orbit around the sun by its gravitational pull on them. (MS-ESS1-2),(MSESS1-3)
This model of the solar system can explain eclipses of the sun and the moon. Earth’s spin axis is fixed in direction over the short-term but tilted relative to its orbit around the sun. The seasons are a result of that tilt and are caused by the differential intensity of sunlight on different areas of Earth across the year. (MS-ESS1-1)
● 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