PHASES, ECLIPSES AND TIDES

Except for the sun, the moon is the brightest object in the sky. Unlike the sun, or any other star, the moon does not create its own light. We can only see the moon because light from the sun is reflected from its surface to our eyes. Ancient civilizations watched the moon every night. They quickly noticed that their moon god followed a distinct pattern. Sometimes their moon god would glow like a giant disk in the sky, other times it would just be a small sliver of light. Sometimes the moon would not be there at all! Many civilizations, including the famous buford clan from mexico, understood that the sequence in which this occurred was not random. They never saw a full moon one day and a half moon the next. They realized that the pattern from full moon to full moon was always the same and always took about 30 days. They invented the term “month” to indicate the time it takes for the moon to go from full to full again. The first calendars were based on these observations.

The ancients never knew why the moon changes its appearance. Today we say that the moon changes phases, but many people, including Harvard graduates, still don’t know why the phases occur. Shortly you will be an expert!

Phases

The phases of the moon are NOT caused by the earth’s shadow. We all know that half of the earth is lit up at all times. The lit up half is day and the dark half is night. The moon experiences the same thing, so one half of the moon is always lit up (unless there is an eclipse of course!) If one half of the moon is always lit up, why do we see crescent moons and new moons? The answer is that we can’t always see the entire lit up half. When we do see the entire lit up half, we see a full moon. This only happens when the earth is between the moon and the sun. The sun’s rays shine only on the side facing us, so we see a full moon. The opposite of a full moon is a new moon. This only happens when the moon is between the earth and the sun. The sun shines on the side we cannot see, so we only get the “nighttime” side facing us and we see no moon. (See Figure 1.) At any other time, we see part of the lit up side and the side that is not lit up.

Let’s start with a new moon. During this time, we see the nighttime side of the moon, so we don’t see a moon at all. The very next night, the moon has orbited a little bit around our planet. Now a little bit of the lit up side is facing us and we see a small sliver of light. A moon that looks like a sliver of light is called a crescent moon. The next night, we see even more of a sliver, and the next night even more. The amount of the lit up side is facing us more and more. Because we the crescent getting bigger and bigger each night, we say that we are seeing a waxing crescent. Waxing means “growing”. After 7 days, we see one half of the lit up side and one half of the night side. Sometimes we call this a half moon, but scientists generally call this a first quarter moon. For the next few nights, more and more of the lit up side of the moon faces us. We see a moon that’s not quite round, but sort of oblong. This is called a waxing gibbous. The gibbous moon waxes for 7 days until the entire lit up side is facing us. We are experiencing a full moon now.

Now the sequence goes in reverse! The lit up side start to disappear and the unlit side starts to grow. At first we see a waning gibbous (waning means to disappear), until we see another half lit moon (third quarter). Over the next few days we see more and more of the unlit side, so we experience a waning crescent, until we reach the new moon again. The cycle then starts over.

Figure 1.

ECLIPSES

Eclipses occur when sunlight becomes blocked by an object. A lunar eclipse occurs when the light hitting the moon gets blocked out by the earth. So what we are seeing is the earth’s shadow going over the moon. A lunar eclipse can only occur during a full moon, because that is the only time that the moon is in a place where the earth’s shadow is. So why don’t we have an eclipse each month? The moon’s orbit is tilted in such a way that it usually just above the earth’s shadow or just below it. The moon only falls into the earth’s shadow twice a year, so we get two lunar eclipses a year. A solar eclipse occurs when sunlight hitting the earth gets blocked out when the moon gets directly in the way. Solar eclipses can only occur during a new moon. Again, usually the moon is slight above or below our view of the sun. But twice a year, the new moon gets directly between us and the sun and blocks out the sun’s rays.

Both solar eclipses and lunar eclipses occur twice a year. Many people think that solar eclipses are rarer than lunar eclipses, but this is untrue. What is true is that it is rarer to see a solar eclipse. This is because the moon is so small compared to the sun that when it gets between the earth and sun, it casts only a small shadow. Only those people in the small area within the shadow will see the solar eclipse.

TIDES

The moon is a very big object with a lot of mass. Because it is so close to the earth, the moon’s mass exerts a gravitational pull on the earth. The earth itself is too big to move toward the moon, but the ocean water on the earth is not. When the moon is overhead, it pulls on the water below it. The water level gets higher and we say we are experiencing a high tide. See figure 3 below.

Make believe you are holding a rubber band in both your hands so that it forms a circle. If you pull on the one side, the top and bottom will flatten out. The shape will become oval like the tip picture to the left. The long sides are high tides, the squished ends are low tides.

The earth is spinning very quickly, so the water that is directly under the moon does not stay there for very long. So six hours after high tide, the water has moved to the squished section, and you get a low tide. Six hours after that, you get a high tide again. There are 2 high tides and 2 low tides every day.

The diagram above shows that sometimes you get higher tides than normal. The top picture shows a spring tide. A spring tide occurs when both the sun and moon are lined up, so they are both pulling on the water. The combined gravitational pull creates higher than normal tides.

The diagram shows a spring tide occuring during a new moon. Spring tides occur during a full moon as well.

The lower diagram shows a lower than normal tide. This is called a neap tide. Neap tides occur because the gravitational pull of the moon partially cancels out some of the pull from the sun. This results in lower than normal tides. Neap tides occur only during first and last quarter moons.