Night Moves

By Suzanne Wilson
Photograph by Vince Heptig
Digital imaging by Marci Rodriguez

When your budding young astronomers ask questions about that shining orb in a black-velvet sky, be prepared to explain the mysteries of the moon.

Seeing is deceiving: Your eyes sometimes play tricks on you as the moon goes through its phases.

Who doesn’t get a thrill looking up at the moon?

Whether you see it as a giant, glowing orb rising from the eastern horizon at dusk or as a slender, silvery crescent hanging on the black-velvet backdrop of a pre-dawn eastern sky, Earth’s tag-along partner always has inspired mystery and awe.

How many people, though, really know what they’re seeing — or not seeing in the case of the long misunderstood “far side” of the moon (or the “dark side” as classic rockers Pink Floyd called it).
Is the moon waxing or waning? What’s the difference between a new moon and a full moon? And what the heck is a waning gibbous moon, anyway?

Your Cub and Boy Scouts might ask those same questions, and more, as they ponder the ever-changing appearance of Earth’s moon. How can you teach them valuable lessons about the phases they see?

First, some basics.

A celestial dance

Scientists theorize the moon formed about four-and-a-half billion years ago and gradually settled into an orbit around the Earth as tidal forces between the two bodies slowed the moon’s spin.

What you probably call moonlight isn’t. The moon gives off no light of its own. Moonlight is merely reflected sunlight.

Sometimes you see the moon as a curved sliver (crescent). Sometimes you see it in half-light (first- and third-quarter phases). Sometimes you see an entire side of the moon illuminated by sunlight (full moon). And sometimes the moon appears in “gibbous,” or protruding, phase (more than half, but less than fully illuminated).

Living in the Northern Hemisphere, you can identify a waxing crescent by noting that if the illuminated area appears on the right, it’s increasing toward a full moon. If the illuminated area appears on the left, it’s called a waning crescent and is decreasing toward a new moon.

Don’t bother scanning the sky for a new moon, which occurs when the orb moves between Earth and the sun. You can’t see it then, which is why astronomers often call it a “dark moon.”

Think of the phases of the moon as the result of movements in a complex celestial dance. They don’t occur, as some people believe, because the sun is casting Earth’s shadow on the moon. That’s a lunar eclipse. The explanation for phases of the moon lies in the rotational relationships of the Earth to the sun and the moon to the Earth.

The Earth spins as it travels around the sun, and the moon spins as it travels around the Earth—though more slowly. To complete its elliptical (oval-shaped) orbit of the sun, Earth takes a bit longer than a calendar year (365 ¼ days). The planet also spins on its axis about once every 24 hours.

Similarly, the moon travels around the Earth in an elliptical journey that takes about 29 ½ Earth days as it completes the phase cycle. Significantly, though, during this period the moon spins only once on its axis. And because the moon spins on its axis in the same length of time that it takes to orbit the Earth, the moon always presents the same side, or “face,” to viewers.

The moon turns on its axis, and sunlight always illuminates the side turned toward the sun. We see varying amounts of the lighted side as the moon orbits Earth.

Because of slight variations in our view of the moon, we see 59 percent of its surface over a period of time—a bit more to its north and south poles, or either side.

Use your illusion

When you watch the moon, it appears to move across the sky from east to west. In truth, it moves west to east in its orbit around the Earth. Why?

“The fun thing about looking at the moon is that it shows the sky does change,” says Dr. David Kuehn, professor of physics at Pittsburg State University, Pittsburg, Kan. “You can see a phase change over a couple of days,” he says.

Kuehn, who presents astronomy programs for area Boy Scouts and school children at the Greenbush Astrophysical Observatory, notes that in just one night you can witness the moon’s movement from west to east in its orbit.

After sundown, note the position of stars that appear near the moon. Three hours later, take another look. “The moon will have moved east relative to those background stars, even though the whole sky appears to be going east to west because of the Earth’s spin.”

In other words, the Earth’s rotation is speeding you west to east.

Scientists call another visual trick the “moon illusion.” It occurs when you see the full moon rising above the eastern horizon. You’re probably amazed at how close and how large it seems.

How can it be larger at moonrise than when it’s high in the sky? It isn’t. And you can prove it.

At moonrise, hold your hand out at arm’s length so that your little finger covers the moon. Later that night, when the moon is higher in the sky, repeat the action. Nothing’s changed. The size of the moon remains the same.

You also can test that illusion with a piece of clear plastic and a black marker. Use the marker to trace the size of the moon as it rises. Later, hold the drawing at the same distance and compare it to the moon overhead.

Scientists disagree on the reason our eyes and brain play tricks on us this way. But even knowing that you’re seeing an optical illusion, you can still enjoy the beauty of a rising moon, especially when it’s an autumnal “Harvest moon,” often rendered a gorgeous orange hue by the haze and humidity in the atmosphere along the horizon.

For optimum viewing of the entire lunar cycle:

  • Check dates of moon phases for each day of the month. Note moonrise and moonset times for each day.
  • Begin your watch with the waxing crescent before it sets in the evening. The next night, you’ll find the crescent higher above the western horizon because moonset is later.
  • If you go out at roughly the same time after dark each evening, you’ll find the moon at a point farther east and with a growing shape. But moonrise is always getting later, so check the time and adjust your watching for the best view.
  • Be ready to watch the full moon rise as the sun sets. See if you experience the “moon illusion,” with the moon appearing large on the horizon.
  • As the waning gibbous moon rises later and later each night, the best viewing time creeps past bedtime for most of us. Nighttime moon watching is over until the next appearance of the waxing crescent.
  • Check rise and set times to view the moon in daytime. The last quarter will be in the western sky in the morning because it sets around noon.

Get out the binoculars

Want to become a moon watcher? It’s easy. Just look up. But knowing where and when to look and paying attention to the entire night sky are crucial to getting the most out of your experience.

A full moon offers too much light to view its craters, dark maria (seas, in Latin) of old lava flow, and mountains.

So view the moon when it’s just partially illuminated. That way you can discern the features along what’s called the “terminator,” the line of contrasts between illuminated and non-illuminated areas. Long shadows provide geographic relief and a feeling of three dimensions.

All the gear you and your Scouts need to enjoy gazing at the moon is a pair of binoculars or a small telescope. Both work well at spotting detail in the moon’s surface.

You might also recruit an amateur stargazer that owns a large telescope to introduce kids to the joys of astronomy. Or you can arrange a trip to the local observatory, planetarium, or university for an even more professional view.

And when you’ve mastered the phases of the moon, you’ll be ready for your next nighttime outing.

“Look,” you can instruct your Scouts with confidence, “that’s a waxing gibbous moon.”

They might think you’re speaking a different language. But consider that high praise.

Writer Suzanne Wilson enjoys stargazing near her home in Joplin, Mo.

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November - December 2008 Table of Contents