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Figure 1.4 Greenhouse heating ofVenus. Visible sunlight strikes the planet's surface and is converted into heat. Much of that heat is trapped by Venus's thick, carbon dioxide-rich atmosphere.

88 Earth days long; thus, a day on Mercury is twice as long as a year there. As a result of this incredibly slow rotation, the night side of Mercury has 88 Earth days to cool off. Since there is little atmosphere to capture any of the heat emitted by the planet, the temperature on the night side of Mercury plummets to -28o°F or -i75°C, making half of the closest planet to the Sun among the coldest bodies in the solar system.

Moonshine and Earthshine

As with Mercury, slow rotation, sweltering days, and frigid nights are also hallmarks of our Moon—as are a variety of incorrect beliefs. We saw in the preface that the Moon rotates as it orbits the Earth. The length of a day on the Moon is roughly 29 i2 Earth days. It lacks a significant atmosphere. Therefore, the temperature variations are also extreme, ranging from 2io°F or ioo°C at noon down to -250°F or -i50°C at night. Day and night have a bearing on one common area of confusion about the Moon, namely, the cause of its phases.

The most common incorrect explanation for the Moon's phases is that they are caused by Earth's shadow covering part of the Moon in the course of its orbit around Earth. A similar idea suggests that the Moon's phases are caused by shadows from other objects in space. Let's start by addressing the idea of the Earth's shadow as the cause.

Moon in the Shadows

I think everyone would accept the assertion that the Moon continually goes through a smooth cycle of phases. A quick glance at the photographs in an astronomy text or on an astronomy Web site, or a month of looking at the Moon, will convince you. The cycle goes like this: the new moon is the phase when the Moon appears either the smallest of crescents (when it passes slightly off the imaginary straight line between the Earth and the Sun) or completely dark (when it passes directly between the Earth and Sun—only on these new moons does it create a solar eclipse). As the crescent broadens, the Moon is in the waxing crescent phase; this continues for about a week until half of the side of the Moon facing us is bright. We call this the first quarter moon (because it is one quarter of the way through its cycle). The bright part of the Moon continues to widen for another week, the waxing gibbous phase (from a word that once meant "humped"), until we see a circular full moon. Then the cycle reverses: waning gibbous, third quarter, waning crescent, back to new moon.

If the phases are caused by the Earth's shadow covering different amounts of the Moon's surface on different days, then we can set up the geometry of the Sun, Moon, and Earth necessary to create the cycle. The Earth's shadow is cast when the Earth blocks sunlight. The only place the shadow exists is on the far side of the Earth from the Sun (see figure 1.5). You can see that the Sun and Moon must be on essentially opposite sides of the Earth on each day that we see less than a full moon.

This raises a problem. The Moon is only full on one day in each cycle of phases, meaning roughly one day a month. If the Earth's shadow causes the phases, then the Moon must be on the opposite side of the Earth from the Sun virtually every day of the year so that part of it can be in the Earth's shadow. But under such circumstances, the Moon would not orbit the Earth. If the Moon were positioned only on one side, the Earth's gravitational attraction would pull it straight in and onto our planet's surface. In reality, the Moon is continually falling Earthward, but because of its orbital motion it continually misses, which is a good thing. In reality, the Moon continually orbits the Earth and spends as much time on the Sun's side as it does on the side of the Earth far from the Sun. For example, take a look at a crescent moon sometime. You will always see it less than half a sky away from the Sun.

Even more challenging to the shadow-lunar phase relationship is the shape of the Earth's shadow on the Moon. For all intents and purposes, the Earth is a spherical body. When the Moon is passing through the shadow, the dark part of the Moon should always cut an arc "into" the Moon, as shown in figure 1.5. That is fine for the crescent moon, but the gibbous moon (inset) has the bright part bulging out on both sides, which is inconsistent with the idea that the shape of the Earth's shadow creates the dark part of the Moon.

Earth's shadow

Gibbous moon

Gibbous moon

Missing phase

Missing phase


Figure 1.5 The scenario if lunar phases were caused by the Moon in the Earth's shadow. This model can re-create the crescent moon (1) and new moon (2), but not the gibbous moon (inset). In reality, the shape at (3) is only seen during a lunar eclipse.

Another problem with the Earth's shadow as the reason for the phases is lunar eclipses. You might well argue (correctly) that the Earth's shadow causes these eclipses. But if our shadow causes the lunar phases, why isn't there an eclipse every month? Even more confounding is the fact that we observe solar eclipses when the Moon covers the Sun. That means, of course, that the Moon must be directly between the Earth and the Sun. A few hours before or after a solar eclipse, the Moon appears as the slimmest of crescents in the sky very close to the Sun. But if the Earth's shadow caused the phases, that nearly invisible Moon would have to be almost exactly on the opposite side of the Earth from the Sun. These two assertions are inconsistent, so the belief that the Earth's shadow causes the phases must be wrong.

True Lunar Phases

The actual cause of the phases of the Moon is related to the idea behind the title of a record album by Pink Floyd, Dark Side of the Moon. Many people believe that the Moon has a dark side, defined to be the side we never see.6 The Moon does have a side we never see from Earth, called the "far side." But is it always dark?

Put yourself on the Moon for a moment. Choose a nice spot with the Earth high in the sky. Let's make it sunrise. Keep in mind that even with the Sun up, the lunar sky looks black because there is virtually no air to scatter sunlight and obscure the stars. Set up camp and watch the Earth and Sun. Because the Moon is rotating at exactly the same rate that it orbits the Earth, the Earth won't budge, no matter how many days, weeks, months, years, or decades you watch it. However, the Sun does move across the sky.

It will take the Sun just over two weeks to cross the sky. When it goes down, you will be plunged into a cold, dark night for an equal length of time. In other words, it takes the Sun one day to go from sunrise to sunrise on the Earth and nearly thirty Earth days to go from sunrise to sunrise on the Moon. But where is the Sun during the time that it is "down" in your sky on the Moon? It is illuminating the other half of the Moon under your feet, just as the Sun sheds light all around the Earth at some time each day. While half the Moon is in darkness at any time, virtually all places on it (except in some craters at its poles) receive sunlight at some time throughout the month. The dark side of the Moon is continually changing.

Even when you were in darkness on the Moon watching us, we could watch you. You stayed in the same spot, but that spot went from

6 At the end of the album is a disclaimer stating very quietly that the Moon does not have a dark side, as just defined.

being in the bright part of Moon to being in the dark region. We on Earth see at least part of the dark side of the Moon whenever we see less than a full moon. The phases occur because we see different amounts of that dark side as the Moon orbits the Earth.

Let's see how this works, starting again at the new moon, which occurs when the Moon is between the Earth and the Sun. As the sunlit side of the Moon faces away from us, we see virtually all of the Moon's "dark side." A crescent of light appears as the sunlit side comes into view. The crescent's two points aim away from the Sun, and in this phase the Moon rises shortly after the Sun. Over roughly the next week, as the Moon orbits the Earth and moves farther away from the Sun in the sky, we see more of the sunlit side until we see half of it—the first quarter moon. From first quarter to full and then to third quarter, the Moon is more than halfway across the sky from the Sun. Indeed, when the Moon is full, it is on the opposite side of the Earth from the Sun, so that we see the entire lit side. This directly contradicts the model with the phases caused by the Earth's shadow on the Moon. There, if the full moon were opposite the Sun, it would be in our shadow. The cycle continues as shown in figure 1.6.


If the Moon is on the opposite side of the Earth from the Sun every month, why isn't there an eclipse every month? There would be if the Moon orbited the Earth in exactly the same plane as the Earth orbits the Sun (the ecliptic). In that case, every time the Moon was about to be full, it would slide into the Earth's shadow, creating a lunar eclipse. Similarly, every time the Moon was about to be new, it would pass exactly between the Earth and the Sun, blocking some sunlight and thereby creating a solar eclipse.

In reality, the Moon's orbit is tilted by about five degrees to the ecliptic, so that usually it is slightly above or below the Earth's shadow when it is full and the Earth is slightly above or below its shadow when it is new. Eclipses only occur when the Moon is crossing the ecliptic at those two phases.

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