UCSB Science Line
Sponge Spicules Nerve Cells Galaxy Abalone Shell Nickel Succinate X-ray Lens Lupine
UCSB Science Line
Home
How it Works
Ask a Question
Search Topics
Webcasts
Our Scientists
Science Links
Contact Information
Why are there phases of the Moon?
Answer 1:

Phases of the moon are actually pretty easy to understand once you see the right picture of the Moon's orbit and the Earth's orbit. I'll try to describe some of it but the best thing for you to do is to go to the library (or the internet) and find the diagrams that contain the Sun, Earth, and Moon in different parts of the Moon's orbit. I don't think you will have any trouble finding them.

Just as the planets orbit the sun in ellipses, the moon orbits the Earth in an ellipse, about once every month. Another thing to keep in mind is that the Moon's orbit takes it slightly out of the plane of the Earth's orbit around the sun. To understand the phases, you need to keep track of the location of the sun, Earth, and Moon together.

If the Earth is between the sun and Moon, the sun's light reflects off the moon and it looks to folks on Earth as if the moon is full. The Earth's shadow typically does not fall on the moon because the moon is slightly above or below the plane of the Earth's orbit. Every now and then, the Earth's shadow does fall on the moon - this is a lunar eclipse. Keep in mind that a full moon lasts for a few days but an eclipse lasts for just a few hours.

A week later in the Moon's orbit, the Moon and Earth are about equally far from the sun. In other words, if you drew a line from the sun to the Earth and a line from the Earth to the Moon, they lines would cross at a 90 degree angle. You see this as a half moon - the lit up half is the part facing the sun and the dark half is the part farther away from the sun.

Anyway, from this line of reasoning, you can also figure out what approximate time of day (or night) the moon will have risen and, if you're careful, even where on the horizon it will appear.

Answer 2:

Phases of the moon are caused by the alignment of the earth, the moon, and the sun. Like the earth, 1/2 of the moon's surface is always exposed to direct sunlight. However, depending on the moon's orientation with to the earth during it's 28 day orbit of the planet, a person on the surface of the earth will only be able to see a fraction of that exposed area, and the rest of the moon will appear to be dark or in shadow.
The three most easily identifiable phases are the full moon, quarter moon, and new moon.
When the moon is full, the moon is located almost directly opposite the earth from the sun, and a person standing on the surface of the earth looking up at the moon will see the entire half of the moon facing the earth lit by sunlight. The 1/2 of the moon facing away from the earth will be in shadow.


(Shadow)Moon(Sunlight)
Earth <--------Sunlight------------ The Sun

During the quarter moon phase, the moon is oriented at a right angle (or 90 degrees) to the earth from the sun. While 1/2 of the moon remains exposed to sunlight, a person on the earth looking up at the moon will only see half of this lit area, while the other portion of the moon exposed to sunlight will be facing away from the earth.

(Shadow)Moon (Sunlight)

(Shadow)Moon (Sunlight)

Earth <--------Sunlight------------ The Sun
During the new moon phase, the moon is oriented almost in a direct line between the sun and the earth, so that the half of the moon that is exposed to sunlight is facing away from the earth. A person looking up at the moon from the earth's surface would see only the half of the moon that is in shadow.

Answer 3:

The moon appears to us to have different phases because as it rotates around the Earth, different parts of it are lit up by the sun. We only see the part of the moon that the sun is shining on. When the moon is between the sun and the Earth, we don't see the moon (new moon) because only its backside is lit up by the sun. The full moon occurs when the Earth is between the moon and the sun. In this orientation, we see one whole side of the moon that is lit up by the sun.

Answer 4:

As the Moon travels in it's 29.5 day orbit around the Earth, we see the Moon in different phases depending on where it is in relation to the Sun. During a full moon,the Earth is between the Sun and the Moon and we see the side of the Moon that the Sun is shining on. As the Moon continues in its orbit it reaches a point about seven days later in which the angle between the Sun and the Moon is about ninety degrees and thus we see only half of part of the Moon that the Sun is shining on (3rd Quarter). Then about another week later the Moon is between the Earth and the Sun and we can't see the Moon at all since it is only up during the day and the Sun is shining on the back side of the Moon. This is called New Moon. After that, the Moon continues along with more and more of it being visible in the early evening until finally it is full again

Answer 5:

Draw a picture showing the Earth at center with moon on her monthly cycle. Show the earth in the center fixed and the sun way to the right and then draw the moons location during a monthly cycle. if you start the moon at 3 o'clock, you will see that the sun is shining on the far side of the moon. !!the side of the moon facing Earth is NOT getting any sunlight! we call this NEW MOON (you cant see the moon )
Then a week later , note that from the Earth we would see 1/2 of the moon light up. Then after another week (two weeks into the cycle) the moon will be on the far side of Earth (at 9 o,clock) and the sun will illuminate the entire face of the moon...this is a FULL MOON. Again there are picture s of this in any elementary astronomy book that illustrate it very well.

Answer 6:

Like any body in our solar system, the Moon has two sides: one illuminated by the Sun (the day side), and one on the other side of the Sun (the night side). When we look at the Moon, we normally see only the day side, since the night side is dark like the space behind it.

If the Sun is behind us when we look at the Moon, the Moon will appear to be full, since its day side is facing us (and the Sun behind us). If the Sun is to our side when we look at the Moon, we see only half of the day side. If the Sun and the Moon are together, we will see no Moon at all (New Moon) because the light side of the Moon is facing away from us (toward the Sun).

The Moon orbits the Earth once every 28 days (the reason why it rises and sets each night or day is because the Earth is spinning underneath it). This means that every 14 days, it is on the opposite side of the Earth, causing the New and Full Moons to alternate.

You can do the geometry with a sheet of paper or with a light bulb (the Sun) a tennis ball (the Moon) and a basketball (the Earth), as the Moon orbits the Earth.


Click Here to return to the search form.

University of California, Santa Barbara Materials Research Laboratory National Science Foundation
This program is co-sponsored by the National Science Foundation and UCSB School-University Partnerships