Moon Phases

Moon Phases 

Besides the Sun, the Moon is the most obvious object in the sky. Bright silvery with tantalizing features on its face, it's been the target of our imagination, poetry, science and even the occasional rocket. 

If we pay most the most cursory attention to it, we'll see that it changes everyday; sometimes it's up in the day, sometimes at night , and even its shape is always changing. 

What's causes this Behavior? 

The moon is basically a giant ball of rock 3500 km across in space. It's surface is pretty dark, with about the same reflectively as a chalkboard or asphalt.

However, it looks bright to us because it's sitting in full sunlight; the sun illuminates it, and it reflects that light down to us here on Earth. And because it's a sphere, and orbiting the Earth, the way we see it lit by the Sun changes with time. That's what's causes its phases : Geometry. 

The important things to remember through all is; the moon is a ball and in space half of it is always illuminate the Sun!

This is true for Earth too! and every spherical object in space; half faces the sun and half faces away, the side which faces the sun we calls it "the Bright side" and the half facing away is known as "The Dark side". 

The phase of the moon refers to what shape of the moon appears to us; how much of it we see illuminated from the Earth. The key to all of this is line, dividing the lit side from the unlit side which is called the Terminator.

If we're facing the moon with the sun behind us, we're seeing the half of the moon which is fully illuminated by sunlight and it looks like. 

If we're off to the side we see half of the lit side and half of the dark side and we say the moon is half full.

If the sun is on the other side of the moon, we're looking at the unlit half, and it looks dark.

The phase of the moon we see depends on from what direction the sunlight's hitting it, and the angle we see that from Earth.

The Moon orbits the Earth once per month, In fact that's where the word 'Month' comes from 'Moon'. The length of time it takes the moon to go through all its phases is 29.5 days.

Phases of Moon.

New Moon - happens when the sun, moon and earth are more or less in a line. The moon orbit's is actually tipped a bit to the Earth's. So, sometimes new moon happens when the moon is below the sun, or above it. 

But at some point in its orbit, at some points in the month it appears to be as close to the Sun as it can.

Whats does it look like from Earth ? 

The Moon is between the Earth and Sun, so from our prospective we only see the dark half, the un illuminated half of the moon, 

The other side, the far side, of the moon is lit, but we can't see it. It makes sense ten to call this the beginning of the Moon's cycle hence the term "New Moon".

At New Moon, the Moon stays near the sun, so it rises at sunrise, and sets at sunset. This makes it extremely difficult to see;  but not for long because the moon is orbiting the Earth, after a couple of days it's moved a bit to the East.

Now we are seeing it along a slight angle, and we can see a little bit of the illuminated half of the moon on its side towards the sun.

The Terminator, the day/night line, appears curved around the Moon, so what we see is a thin illuminated crescent moon. At this point the crescent is still very thin, with the horns of the crescent pointing away from the  sun.

Now that the moon is still pretty close to the sun in the sky, just a bit to the east, rising may be on hour or two after sunrise, But this means it's up all day, and then sets after the sun does. This is the best time to see the crescent moon, when the sun has already set and the sky starts to get dark.

The moon will be low over the western horizon and it will set soon the after the sun does.

After a couple of days - Now the sun has moved a bit more in its orbit around the Earth, and is farther from the sun in the sky.
We see a little more of the illuminated part, and the crescent is wider.  And this wider crescent is known as "Waxing crescent" Moon, waxing means growing. It's also well away from the sun now, so it's easier to spot, even during the day before sunset.

Seven or so days after New Moon we get to our first milestone; the moon is now one quater of the way around its orbit. It's 90 degree away from the sun in the sky, which means we're looking straight down on the Terminator. 

Next big steps comes two weeks after new moon when its moves halfway through its orbit. It is now opposite the sun in the sky 180 around.

The Earth is between he moon and the sun, so we're looking at the fully illuminated half of the moon. This is the full moon because it's opposite the sun its rises at sunset and sets at sunrise. It's up all the night shining down on the Earth.....