Explain the cause of the lunar phasesUnderstand just how the Moon rotates and revolves approximately Earth

After the Sun, the Moon is the brightest and also most apparent object in the sky. Unequal the Sun, the does no shine under its own power, but merely glows v reflected sunlight. If you to be to follow its progression in the skies for a month, you would observe a bike of phases (different appearances), v the Moon starting dark and getting much more and more illuminated by sunlight over the food of around two weeks. ~ the Moon’s disk becomes totally bright, it starts to fade, return to dark around two mainly later.

You are watching: The phases of the moon depend on how much of the lighted side of the moon can be seen from earth.

These changes fascinated and also mystified plenty of early cultures, which came up v marvelous stories and also legends to explain the bicycle of the Moon. Even in the modern world, many people don’t know what causes the phases, reasoning that they are somehow regarded the shadow of Earth. Let us see how the phases have the right to be described by the activity of the Moon relative to the bright light source in the solar system, the Sun.

Lunar Phases

Although we recognize that the sun moves 1/12 that its path roughly the skies each month, for purposes of explaining the phases, we have the right to assume the the Sun’s light comes from about the same direction during the food of a four-week lunar cycle. The Moon, ~ above the other hand, moves totally around planet in the time. As we watch the Moon from our vantage point on Earth, how much of its face we check out illuminated by sunlight counts on the angle the Sun provides with the Moon.

Here is a simple experiment to present you what us mean: stand around 6 feet in front of a bright electric light in a fully dark room (or outdoors at night) and hold in your hand a tiny round object such together a tennis ball or an orange. Her head deserve to then represent Earth, the light represents the Sun, and also the sphere the Moon. Move the ball about your head (making sure you don’t cause an eclipse by prevent the light with your head). Girlfriend will see phases as with those that the Moon on the ball. (Another an excellent way to acquire acquainted through the phases and also motions of the Moon is to follow our satellite in the skies for a month or two, recording its shape, its direction from the Sun, and when the rises and also sets.)

Let’s research the Moon’s bicycle of phases using figure 1, i m sorry depicts the Moon’s actions for the whole month. The trick to this number is that you must imagine yourself standing ~ above Earth, encountering the Moon in each of that phases. So, because that the place labeled “New,” you space on the best side of Earth and also it’s the center of the day; because that the place “Full,” you room on the left side of planet in the center of the night. Note that in every position on number 1, the Moon is half illuminated and fifty percent dark (as a sphere in sunlight need to be). The difference at each position needs to do with what component of the Moon deals with Earth.


Figure 1: Phases that the Moon. The illustration of the Moon alters over the food of a complete monthly cycle. The images of the Moon top top the white circle show the view from space, v the sun off come the ideal in a fixed position. The outer images present how the Moon appears to girlfriend in the sky from each allude in the orbit. Imagine yourself standing top top Earth, facing the Moon at each stage. In the place “New,” for example, you are encountering the Moon native the ideal side of earth in the middle of the day. (Note the the street of the Moon from planet is not to range in this diagram: the Moon is approximately 30 Earth-diameters away from us.) (credit: modification of work-related by NASA)


The Moon is stated to be new when it is in the same basic direction in the skies as the sun (position A). Here, the illuminated (bright) side is turned far from us and also its dark next is turned towards us. You might say that the sunlight is shining on the “wrong ” side of the Moon from ours perspective. In this step the Moon is invisible come us; its dark, rocky surface does not give off any type of light that its own. Because the new moon is in the same part of the skies as the Sun, that rises at sunrise and sets in ~ sunset.

But the Moon does not stay in this phase long because it moves eastward every day in its monthly path roughly us. Due to the fact that it takes around 30 days to orbit Earth and also there room 360° in a circle, the Moon will certainly move about 12° in the skies each work (or around 24 time its own diameter). A day or 2 after the brand-new phase, the thin crescent first appears, as we begin to view a small component of the Moon’s illuminated hemisphere. It has actually moved right into a place where it currently reflects a little sunlight towards us along one side. The shining crescent boosts in size on successive days as the Moon moves farther and also farther about the sky away native the direction that the sun (position B). Because the Moon is moving eastward away from the Sun, that rises later and also later each day (like a student throughout summer vacation).

After about one week, the Moon is one-quarter the the method around that is orbit (position C) and also so us say it is at the first quarter phase. Half of the Moon’s illuminated next is visible to planet observers. Because of that is eastward motion, the Moon now lags about one-quarter that the work behind the Sun, rising about noon and setting around midnight.

During the main after the very first quarter phase, us see an ext and much more of the Moon’s illuminated hemisphere (position D), a phase the is called waxing (or growing) gibbous (from the Latin gibbus, meaning hump). Eventually, the Moon come at place E in our figure, whereby it and also the Sun space opposite each various other in the sky. The next of the Moon turned towards the sunlight is additionally turned toward Earth, and also we have the full phase.

When the Moon is full, that is the contrary the sun in the sky. The Moon does the opposite of what the sunlight does, climbing at sunset and setup at sunrise. Keep in mind what that method in practice: the totally illuminated (and thus really noticeable) Moon rises just as it it s okay dark, remains in the skies all night long, and also sets as the Sun’s first rays space seen at dawn. That illumination throughout the night helps lover on a romantic stroll and also students detect their means back to your dorms ~ a long night in the library or one off-campus party.

And as soon as is the full moon greatest in the sky and also most noticeable? in ~ midnight, a time made renowned in generations of horror novels and films. (Note how the habits of a vampire choose Dracula parallels the behavior of the full Moon: Dracula rises at sunset, walk his worst mischief in ~ midnight, and also must be earlier down in his coffin through sunrise. The old legends were a means of personifying the behavior of the Moon, which to be a much much more dramatic part of people’s stays in the days before electric lights and also television.)

Folklore has it that an ext crazy behavior is seen during the time of the complete moon (the Moon also gives a surname to crazy behavior—”lunacy”). But, in fact, statistical tests of this “hypothesis” involving thousands of documents from hospital emergency rooms and also police documents do no reveal any kind of correlation of human actions with the phases the the Moon. For example, homicides take place at the exact same rate during the brand-new moon or the crescent moon as throughout the complete moon. Many investigators think that the actual story is no that an ext crazy behavior happens top top nights with a full moon, yet rather that us are more likely to notification or remember such habits with the aid of a glowing celestial light that is up every night long.

During the 2 weeks adhering to the full moon, the Moon goes v the very same phases again in reverse order (points F, G, and H in number 1, returning to new phase after around 29.5 days. About a mainly after the complete moon, because that example, the Moon is in ~ third quarter, an interpretation that the is three-quarters of the way around (not the it is three-quarters illuminated—in fact, fifty percent of the visible next of the Moon is again dark). In ~ this phase, the Moon is now rising around midnight and setting around noon.

Note the there is one thing fairly misleading about Figure 1. If girlfriend look in ~ the Moon in position E, although that is full in theory, it shows up as if the illumination would in truth be blocked by a big fat Earth, and hence we would certainly not check out anything ~ above the Moon other than Earth’s shadow. In reality, the Moon is nowhere near as close to planet (nor is its path so similar with the Sun’s in the sky) together this diagram (and the diagrams in most textbooks) could lead you come believe.

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The Moon is in reality 30 Earth-diameters away from us; Science and the Universe: A brief Tour includes a diagram that shows the 2 objects to scale. And, since the Moon’s orbit is tilted family member to the path of the sun in the sky, earth’s shadow misses the Moon most months. That’s why we regularly obtain treated to a full moon. The times as soon as Earth’s shadow does fall on the Moon are referred to as lunar eclipses and are discussed in Eclipses of the Sun and also Moon.