➤ What's Really on the Far Side of the Moon

 

We've all heard the term "dark side of the Moon" as a description for the far side of our planet's satellite. It's actually quite a mistaken idea based on a misconception that if we can't see the other side of the Moon, it must be dark. It doesn't help that the idea crops up in popular music (the Dark Side of the Moon by Pink Floyd is one good example) and in poetry.

 The far side of the Moon as seen and photographed by the Apollo 16 astronauts. NASA 

In ancient times, people really did believe that one side of the Moon was always dark. Of course, we now know that the Moon orbits Earth, and they both orbit the Sun. The "dark" side is merely a trick of perspective. The Apollo astronauts who went to the Moon saw the other side and actually basked in the sunlight there. As it turns out, different parts of the Moon are sunlit during different parts of each month, and not just one side.

 This image shows the phases of the Moon and why they happen. The center ring shows the Moon as it orbits around the Earth, as seen from above the north pole. Sunlight illuminates half the Earth and half the moon at all times. But as the Moon orbits around the Earth, at some points in its orbit the sunlit part of the Moon can be seen from the Earth. At other points, we can only see the parts of the Moon that are in shadow. The outer ring shows what we see on the Earth during each corresponding part of the moon's orbit. NASA

Its shape seems to change, which is what we call the phases of the Moon. Interestingly, "New Moon," which is the time when the Sun and Moon are on the same side of Earth, is when the face we see from Earth actually IS dark and the far side is brightly lit by the Sun. So, calling the part that faces away from us as the "dark side" really IS a mistake. 

Call It What It Is: The Far Side

So, what do we call that part of the Moon we don't see each month? The better term to use is the "far side." It makes perfect sense since it is the side farthest away from us.

To understand, let's look more closely at its relationship to Earth. The Moon orbits in such a way that one rotation takes just about the same length of time as it takes for it to orbit around Earth. That is, the Moon spins on its own axis once during its orbit around our planet. That leaves one side is facing us during its orbit. The technical name for this spin-orbit lock is "tidal locking."

 Earth and the Moon as seen from a passing spacecraft. NASA

Of course, there is literally a dark side of the Moon, but it's not always the same side. What is darkened depends on which phase of the Moon we see. During a new moon, the Moon lies between Earth and the Sun. So, the side we normally see from here on Earth that's normally lit by the Sun is in its shadow. Only when the Moon is opposite from the Sun do we see that part of the surface lit up. At that point, the far side is shadowed and is truly dark. 

Exploring the Mysterious Far Side 

The far side of the Moon was once mysterious and hidden. But that all changed when the first images of its cratered surface were sent back by the USSR's Luna 3 mission in 1959. 

Now that the Moon (including its far side) has been explored by humans and spacecraft from several countries since the mid-1960s, we know much more about it. We know, for example, that the lunar far side is cratered, and has a few large basins (called maria), as well as mountains. One of the largest known craters in the solar system sits at its south pole, called the South Pole-Aitken Basin. That area is also known to have water ice hidden away on permanently shadowed crater walls and in regions just below the surface.

 A Clementine view of the south pole/Aitkin Basin region. This is where the Chang'e 4 lander from China landed.  NASA

It turns out that a small sliver of the far side can be seen on Earth due to a phenomenon called libration in which the moon oscillates each month, revealing a tiny bit of the Moon we'd otherwise not see. Think of libration as a little side-to-side shake that the Moon experiences. It's not a lot, but enough to reveal a bit more of the lunar surface than we normally see from Earth.

The most recent exploration of the far side has been undertaken by the Chinese space agency and its Chang'e 4 spacecraft. It's a robotic mission with a rover to study the lunar surface. Ultimately, China is interested in sending humans to study the moon personally.

The Far Side and Astronomy

Because the far side is shielded from radio frequency interference from Earth, it's a perfect place to put radio telescopes and astronomers have long discussed the option of placing observatories there. Other countries (including China) are talking about locating permanent colonies and bases there. In addition, space tourists could find themselves exploring all over the Moon, both near and far side. Who knows? As we learn to live and work on all sides of the moon, maybe one day we'll find human colonies on the far side of the moon. 

Fast Facts

• The term "dark side of the Moon" is really a misnomer for the "far side".
• Each side of the Moon is dark for 14 earth days each month.
• The far side of the Moon has been explored by the United States, Russia, and China.

➤ The Once-Mysterious Phases of the Moon

 

The Moon's shape changes for the following reasons:

• The Moon orbits Earth.
• Both Earth and the Moon orbit the Sun.
• The Moon's orbit is the same length as the time it spins on its axis (about 28 Earth days), which means that we see the same part of the lunar surface all month.
• The Sun illuminates both Earth and the Moon.

Get to Know the Lunar Phases

There are eight phases of the Moon to track each month.

New Moon: During New Moon, the side of the Moon facing us is not illuminated by the Sun. At this time, the Moon is not up at night, but it is up during the day. We just can't see it. Solar eclipses can occur during the New Moon, depending on how the Sun, Earth, and Moon line up in their orbits.

Waxing Crescent: As the Moon waxes (grows) into its crescent phase, it begins to show up low in the sky right after sunset. Look for a silvery-looking crescent. The side facing the sunset direction will be lit up.

First Quarter: Seven days after New Moon, the Moon is in first quarter. Only half of it is visible for the first half of the evening, and then it sets. 

Waxing Gibbous: After First Quarter, the Moon appears to grow into a gibbous shape. Most of it is visible, except for a dark sliver that shrinks over the next seven nights. Look for the Moon at this time during the afternoon, too. 

Full Moon: During the Full Moon, the Sun lights up the entire surface of the Moon that faces Earth. It rises just as the Sun sets and disappears beneath the western horizon when the Sun rises the next morning. This is the brightest phase of the Moon and it washes out the nearby part of the sky, making it difficult to see stars and faint objects such as nebulae. 

Ever hear of a Super Moon? That's a Full Moon that happens when the Moon is closest in its orbit to Earth. The press likes to make a big deal about this, but it's really a very natural thing: On occasion, the Moon's orbit brings it closer to Earth. Not every month has a Super Moon. Despite the hype about Super Moons in the media, it's difficult for the average observer to notice one, because the Moon might appear only slightly larger in the sky than normal. In fact, the well-known astronomer Neil deGrasse Tyson pointed out that the difference between a regular Full Moon and a Super Moon is analogous to the difference between a 16-inch pizza and a 16.1-inch pizza. 

Lunar eclipses occur only at Full Moons because the Moon is passing directly between Earth and the Sun in its orbit. Due to other perturbations in its orbit, not every Full Moon results in an eclipse. 

The other Full Moon variation that often grabs media attention is a "Blue Moon." That's the name given to the second Full Moon that occurs in the same month. These don't happen all the time, and the Moon certainly doesn't appear blue. Full Moons also have colloquial names based on folklore. It's worth reading about some of these names; they tell fascinating stories about early cultures.

Waning Gibbous: After the glorious appearance of Full Moon, the lunar shape starts to wane, meaning it gets smaller. It's visible later at night and into the early morning, and we see a steadily shrinking shape of the lunar surface that's being lit up. The side that is lit up is facing toward the Sun, in this case, the sunrise direction. During this phase, look for the Moon during the day—it should be in the sky in the morning. 

Last Quarter: At Last Quarter, we see exactly half the sunlit surface of the Moon. It can be seen in the early morning and daytime sky. 

Waning Crescent: The last phase of the Moon before returning to New Moon is called Waning Crescent, and it is exactly what it says: a steadily-shrinking crescent phase. We can see only a small sliver from Earth. It's visible in the early morning, and by the end of the 28-day lunar cycle, it has vanished almost entirely. That brings us back to New Moon to start the new cycle.

Making Lunar Phases at Home

Creating lunar phases is a great classroom or home science activity. First, set up a light in the middle of a darkened room. One person holds a white ball and stands a short distance from the light. He or she turns in a circle, just like the Moon does as it turns on its axis. The ball is illuminated by the light in ways that almost exactly match lunar phases.  

Observing the Moon over the course of a month is a great school project, as well as something anyone can do on their own or with family and friends. Check it out this month! 

➤ Exosphere Definition and Facts

 

The exosphere is the outermost layer of the Earth's atmosphere, located above the thermosphere. It extends from about 600 km until it thins out to merge with interplanetary space. This makes the exosphere about 10,000 km or 6,200 miles thick or about as wide as the Earth. The top boundary of Earth's exosphere extends about halfway to the Moon.

For other planets with substantial atmospheres, the exosphere is the layer above the denser atmospheric layers, but for planets or satellites without dense atmospheres, the exosphere is the region between the surface and interplanetary space. This is called the surface boundary exosphere. It has been observed for the Earth's Moon, Mercury, and the Galilean moons of Jupiter.

The word "exosphere" comes from the Ancient Greek words exo, meaning outside or beyond, and sphaira, which means sphere.

Exosphere Characteristics

The particles in the exosphere are extremely far apart. They don't quite fit the definition of a "gas" because the density is too low for collisions and interactions to occur. Nor are they necessarily plasma, because the atoms and molecules aren't all electrically charged. Particles in the exosphere can travel hundreds of kilometers along a ballistic trajectory before bumping into other particles.

The Earth's Exosphere

The lower boundary of the exosphere, where it meets the thermosphere, is called the thermopause. Its height above sea level ranges from 250-500 km up to 1000 km (310 to 620 miles), depending on solar activity. The thermopause is called the exobase, exopause, or critical altitude. Above this point, barometric conditions do not apply. The temperature of the exosphere is nearly constant and very cold. At the upper boundary of the exosphere, the solar radiation pressure on hydrogen exceeds the gravitational pull back toward Earth. The fluctuation of the exobase due to solar weather is important because it affects atmospheric drag on space stations and satellites. Particles that reach the boundary are lost from the Earth's atmosphere to space.

The composition of the exosphere is different from that of the layers beneath it. Only the lightest gases occur, barely held to the planet by gravity. The Earth's exosphere consists mainly of hydrogen, helium, carbon dioxide, and atomic oxygen. The exosphere is visible from space as a fuzzy region called the geocorona.

The Lunar Atmosphere

On Earth, there are about 1019 molecules per cubic centimeter of air at sea level. In contrast, there are fewer than a million (106) molecules in the same volume in the exosphere. The Moon does not have a true atmosphere because its particles don't circulate, don't absorb much radiation, and have to be replenished. Yet, it's not quite a vacuum, either. The lunar surface boundary layer has a pressure of about 3 x 10-15 atm (0.3 nano Pascals). The pressure varies depending on whether it's day or night, but the entire mass weighs less than 10 metric tonnes. The exosphere is produced by outgassing of radon and helium from radioactive decay. The solar wind, micrometeor bombardment, and the solar wind also contribute particles. Unusual gases found in the Moon's exosphere, but not in the atmosphere's of Earth, Venus, or Mars include sodium and potassium. Other elements and compounds found in the Moon's exosphere include argon-40, neon, helium-4, oxygen, methane, nitrogen, carbon monoxide, and carbon dioxide. 

A trace amount of hydrogen is present. Very minute quantities of water vapor may also exist.

In addition to its exosphere, the Moon may have an "atmosphere" of dust that hovers above the surface due to electrostatic levitation.

Exosphere Fun Fact

While the exosphere of the Moon is nearly a vacuum it's larger than the exosphere of Mercury. One explanation for this is that Mercury is much closer to the Sun, so the solar wind can sweep away particles more easily.

➤ The 5 Layers of the Atmosphere

The envelope of gas surrounding our planet Earth, known as the atmosphere, is organized into five distinct layers. These layers start at ground level, measured at sea level, and rise into what we call outer space. From the ground up they are:

• the troposphere,
• the stratosphere,
• the mesosphere,
• the thermosphere, and
• the exosphere.

In-between each of these major five layers are transition zones called "pauses" where temperature changes, air composition, and air density occur. Pauses included, the atmosphere is a total of 9 layers thick!

The Troposphere: Where Weather Happens

Of all the atmosphere's layer's, the troposphere is the one we're most familiar with (whether you realize it or not) since we live at its bottom -- the Earth's surface. It hugs the Earth's surface and extends upward to about high. Troposphere means, ‘where the air turns over’. A very appropriate name, since it is the layer where our day-to-day weather takes place.

Starting at sea level, the troposphere goes up 4 to 12 miles (6 to 20 km) high. The bottom one third, that which is closest to us, contains 50% of all atmospheric gasses. This is the only part of the whole makeup of the atmosphere that is breathable. Thanks to its air being heated from below by the earth's surface which absorbs the sun's heat energy, tropospheric temperatures decrease as you travel up into the layer.

At its top is a thin layer called the tropopause, which is just a buffer between the troposphere and the stratosphere.

The Stratosphere: Ozone's Home

The stratosphere is the next layer of the atmosphere. It extends anywhere from 4 to 12 miles (6 to 20 km) above Earth's surface up to 31 miles (50 km). This is the layer where most commercial airliners fly and weather balloons travel to.

Here the air doesn’t flow up and down but flows parallel to the earth in very fast moving air streams. It's temperature also increases as you go up, thanks to the abundance of natural ozone (O3) -- the byproduct of solar radiation and oxygen which has a knack for absorbing the sun's harmful UV rays. (Anytime temperatures increase with elevation in meteorology, it's known as an "inversion.")

Since the stratosphere has warmer temperatures at its bottom and cooler air at its top, convection (thunderstorms) is rare in this part of the atmosphere. In fact, you can visibly spot its bottom layer in stormy weather by where the anvil-shaped tops of cumulonimbus clouds are. How so? Since the layer acts as a "cap" to convection, the tops of storm clouds have nowhere to go but spread outward.

After the stratosphere, there is again a buffer layer, this time called the stratopause.

The Mesosphere: The "Middle Atmosphere"

Starting roughly 31 miles (50 km) above Earth's surface and extending up to 53 miles (85 km) is the mesosphere. The mesosphere's top region is the coldest naturally occurring place on Earth. Its temperatures can dip below -220 °F (-143 °C, -130 K)!

The Thermosphere: The "Upper Atmosphere"

After the mesosphere and mesopause come the thermosphere. Measured between 53 miles (85 km) and 375 miles (600 km) above the earth, it contains less than 0.01% of all air within the atmospheric envelope. Temperatures here reach upward to 3,600 °F (2,000 °C), but because the air is so thin and there are so few gas molecules to transfer the heat, these high temperatures would amazingly feel very cold to our skin.

The Exosphere: Where Atmosphere and Outer Space Meet

Some 6,200 miles (10,000 km) above the earth is the exosphere -- the atmosphere's outer edge. It is where weather satellites orbit the earth.

What About the Ionosphere?

The ionosphere isn't its own separate layer but is actually the name given to the atmosphere from about 37 miles (60 km) to 620 miles (1,000 km) high. (It includes the top-most parts of the mesosphere and all of the thermosphere and exosphere.) Gas atoms drift into space from here. It is called ionosphere because in this part of the atmosphere the sun’s radiation is ionized, or pulled apart as it travels earth’s magnetic fields to the north and south poles. This pulling apart is seen from earth as auroras.