The Moon

While the beauty of the stars cannot fail to captivate the imagination, it must have been the Moon that first gave ancient astronomers the concept of other worlds in the heavens upon which humans may someday set foot. Even the naked eye can see the larger mountains, craters, and dark basalt lowlands which early astronomers named maria meaning seas.

The Moon also gave ancient people a regular calendar, progressing through phases from New Moon to Full Moon and back again in a period of just over 29 days. We call this period of time a month which is from the Old English word for moon.

The Moon actually takes only 27.3 days to move completely around the celestial sphere, as observed from Earth. This period of time is called a sidereal month reflecting the Moon's true orbital period around the Earth. The Moon takes 29.5 days, however, to return to the same point on the celestial sphere as referenced by the Sun due to the Earth's motion around the Sun. This period of time is called a synodic month and the lunar phases are correlated with this time period meaning there is one synodic month from one New Moon to the next.

Since the Moon must move eastward enough to go completely around the sky in 27.3 days, it moves eastward by 13.2 degrees each day. Thus, with respect to the stars in the background, the Moon will be 13.2 degrees further east each night. Since the celestial sphere appears to turn 1 degree about every 4 minutes, the Moon crosses our celestial meridian, an imaginary line between the zenith and the celestial equator, about 13.2 x 4 = 52.8 minutes later each night. This eastward movement also means that the Moon will rise later and later on successive days. Moonrise and moonset times for your location can be calculated by the U.S. Naval Observatory site.

Those of us in the northern hemisphere are familiar with the Man in the Moon, where the shading of the surface takes on the likeness of a face. In the southern hemisphere, however, the Moon appears to be "upside-down" in the sky, so there is no Man in the Moon, which can be a shock to northerners when traveling south of the equator.

The Moon as it Appears in the Southern Hemisphere
Notice the bright rayed Tycho Crater is seen at the "top" of the Moon.


Lunar Phases

The Moon is tidally locked with the Earth, meaning that the same side of the Moon is always facing the Earth (the near side), and the other side always remains hidden from us (the far side). The Moon is only visible to us when sunlight reflects off its surface. The Sun always illuminates one side of the Moon, like Earth, and the phase of the Moon is caused by how much illumination is on the side of the Moon that faces Earth. This is determined by the Moon's orbit around the Earth. The Moon moves counterclockwise around the Earth, and as the Sun's illumination extends further onto the near side, more of the surface becomes visible and we say that the Moon is waxing. After Full Moon, when the whole of the near side is illuminated by the Sun, the Sun's illumination begins to move again toward the far side, and we say the Moon is waning.

Elliptical Orbit

Like most bodies in the solar system, the Moon is also in a slightly elliptical orbit around the Earth. The point in its orbit in which it is nearest to the Earth is called the perigee. The point where it is farthest from Earth is the apogee. These terms are used for any satellite having an elliptical orbit.

Far Side

Far Side of the Moon
Photograph taken by Apollo 16

Here is a picture of the heavily cratered far side of the Moon. Compare this with the photograph of the familiar near side at top of the page. Since the moon is tidally locked with the Earth, the far side cannot be seen from Earth. The first men to ever see the far side of the Moon were the crew of the Apollo 8 mission in 1968. (The far side was first photographed in 1959 by the Soviet lunar orbiter Lunik 3.) Before the space age, astronomers could only guess as to what the far side might look like.

The Apollo 8 astronauts also took the famous picture of the Earth rising from the Moon's limb. While the curve of the Earth's horizon can be seen from low Earth orbit, the Apollo 8 astronauts were the first people to ever see the Earth from deep space, as a beautiful blue globe against the blackness.

Lunar Eclipse

Earth's Shadow

A lunar eclipse occurs when the Moon enters the Earth's shadow. The Moon is only visible to us because sunlight reflects off its surface. When the Earth comes between the Moon and the Sun, the Earth's shadow blocks most of the light that illuminates the moon, turning it dim shades of coppery red.

Any shadow consists of two parts: an inner region of total shadow called the umbra and an outer region of partial shadow called the penumbra. When the Moon only passes through the penumbra, it is only dimmed slightly and this is called a partial eclipse. When the Moon passes through the umbra, however, it is called a total eclipse. When the Moon is in the umbra, some sunlight still reaches its surface because it is refracted as it passes through the Earth's atmosphere. Dust particles in the Earth's atmosphere scatter most of the shades of blue (this is why the sky is blue), allowing only shades of red to pass and make it to Moon. So, the same effect that creates spectacular shades of red during a sunset, also paints the Moon a coppery red color during a total lunar eclipse.

Total Lunar Eclipse in Three Exposures

The picture above shows a total lunar eclipse at three different times during the eclipse. The image on the left shows the Moon as it is entering the Earth's umbra, the middle when it was fully in the umbra, and the right image shows the Moon as it exits the umbra.

Solar Eclipse

Total Solar Eclipse

The Moon's shadow also has an umbra and penumbra, though it is much smaller than the Earth's shadow. A total solar eclipse occurs when the Moon's umbra hits the Earth, and a partial solar eclipse occurs when the Moon's penumbra does so. The image above is of a total solar eclipse, when the bright disc of the Sun is totally obscured by the Moon. A circular shadow is projected in a cone shape, and only the very tip of the Moon's shadow hits the Earth and covers a diameter of at most 270 kilometers. This is why a particular solar eclipse can only be seen on certain places on Earth. The shadow also moves along the Earth's surface at over 1,600 kilometers per hour, so a total solar eclipse will only last 7.5 minutes at the most.

As the Moon orbits its distance from Earth varies. When it is at a greater distance, sometimes only the the Moon's penumbra reaches the Earth, even when the Moon is on the ecliptic and exactly in New Moon phase. In this case, a bright ring will be visible around the Moon when it is lined up with the Sun and this is called an annular eclipse because of the annulus or ring of light around the moon. If the Moon were closer to the Earth during an annular eclipse, a total solar eclipse would occur instead.

It seems as if there should be both a lunar and solar eclipse every month as the Moon makes a complete orbit. This does not happen, however, because the Moon's orbit is tilted 5 degrees from the Earth's orbit (the ecliptic) which usually causes the shadows to miss, as shown in the above image. An eclipse can only occur when the Moon crosses the ecliptic while it is in New Moon phase, creating a solar eclipse, or Full Moon phase, creating a lunar eclipse. This only happens about twice a year.

The Tides

The tides are the periodic rising and falling of large bodies of water. They are chiefly caused by the gravitational interaction between the Earth and the Moon. The Moon's gravity pulls at the Earth (and vice versa), causing the oceans to bulge out in the direction of the Moon. The oceans at the opposite side of the Earth also bulge out due to centrifugal effects, and therefore the Earth's rotation causes two tides per day. Isaac Newton was the first to explain the tides scientifically in his second volume of the Principia.

The Sun's gravity also has an effect on the tides, though it is less significant than the Moon. Nevertheless, on certain times of the month the tides are either exceptionally strong or weak, and this is because of the combined effects of both the Sun's gravity and the Moon's gravity on Earth.

Especially strong tides called spring tides occur when there is a New Moon or a Full Moon, as the Sun, Moon, and Earth are in a line, and therefore the effects of the Sun's gravity and the Moon's gravity are combined. (Spring tides have nothing to do with the spring season.)

A neap tide is an especially weak tide, and occurs when the Sun's gravitational forces are perpendicular to the Moon's forces with respect to Earth. Neap tides occur during Quarter Moons.

A rare exceptionally high tide called a proxigean spring tide occurs at most every one and a half years. Remember, the Moon is in a slightly elliptical orbit around the Earth, and when it is nearest to the Earth it is at a point called perigee (or proxigee). If the Moon is at perigee when it is in New Moon phase, the combined effect of the Moon's gravity and the Sun's gravity is the greatest, producing these very high proxigean spring tides.

As impressive as these tidal forces can be on Earth, they pale in comparison to the tidal forces wreaked by Jupiter upon its moons, especially the four large Gallilean Moons. The most horrific of these tidal effects occurs on Io. Two other large moons Europa and Ganymede perturb Io's orbit into an irregular elliptical orbit, which makes Io the victim of tremendous tidal forces. These forces cause Io's surface of solid rock to bulge in or out as much as 100 meters. (On Earth the liquid oceans only change by as much as 18 meters in height between high and low tides.) Io is the most, volcanically active body in the solar system because the relentless tidal forces generate a lot of heat tending to keep the subsurface crust in liquid form. Therefore impact craters on Io soon become molten lava lakes, and the entire surface is constantly being renewed by lava flows. Io's orbit also cuts across Jupiter's intense magnetic field lines, turning Io into a powerful electric generator. Io can generate 400,000 volts across itself and create a current of 3 million amperes, which often arcs across space toward Jupiter's surface (following the magnetic field lines, the path of least resistance) creating spectacular lightning displays in Jupiter's upper atmosphere. As Jupiter rotates and sweeps its magnetic field across Io, it strips off 1,000 kilograms (1 ton) of Io's surface material every second! Io is a moon tortured by Jupiter.

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Please contact Adam if you have questions or comments about this page. Research and image sources are provided when possible.


Images are shown here for noncommercial educational purposes.
The image of the Full Moon a the top of the page is by astroimager Rob Gendler taken from NASA's Astronomy Picture of the Day site.
The lunar phases graphic is from a textbook scan for an online astronomy course Birth and Death of Stars presented by Dr. James Schombert at University of Oregon.
The image of the far side of the Moon is a NASA image from Apollo 16.
The image of the Earth's shadow is from NASA's Lunar Prospector site.
The three exposures of the lunar eclipse were taken by Stephen Barnes and are from NASA's Astronomy Picture of the Day site.
The image of the solar eclipse is by Fred Espenak and is also from NASA's Astronomy Picture of the Day site.
The greyscale line drawings of the Moon's orbit are from Nick Strobel's Astronomy Without a Telescope site.