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Stargazing: Watching rising moon creates wonders

Twilight stretched across the Mid-Columbia as I looked eastward toward the Blue Mountains.

That evening, I knew where and when to catch the first glint of the full moon.

The moon rise began as a whisper of light that grew into a backdrop of thick orange. The moon took nearly two minutes to loosen itself from Earth's edge.

During summer, the full moon slides lazily southward, playing hide-and-seek behind trees and rooftops. People notice summer moons because of clear skies and evening activities inviting them to remain outside after sunset.

Scanning the moon with binoculars reminds me that it's an ancient landscape still bearing the cratered memories of our solar system's past.

When the moon rises, people notice four of its features: the moon is round, slightly orange, somewhat squashed and covered with dark splotches.

Why is the moon, or for that matter all planets, spherical? Why not cubic or perhaps octahedral like a raw diamond?

The moon is round because the weakest force in nature, gravity, acts as if it originates from the moon's center, pulling everything inward. The only way to efficiently tuck all the moon's mass as close as possible to the moon's center is to form a sphere.

Why does the rising or setting moon look so large? Well, that's an illusion. Photographs of the moon, regardless of its height above the horizon, show the moon's size unchanged.

How can this be? The answer rests in how our brain perceives distance. We sense the horizon lies closer than the sky overhead. Therefore, when the same-size moon rises or sets, we think it's also closer, and therefore larger, compared to when the moon shines overhead against a "more distant" sky.

What makes the rising moon orange? Is that an illusion too? No, the orange is real.

The full rainbow of colors combines to make white light. And because the moon shines from borrowed (reflected) sunlight, moonlight also is white.

However, when the moon lounges low on the horizon, its light passes through a few hundred miles of atmosphere. Tiny particles and gases in the atmosphere scatter the short wavelength (blue and green) colors of moonlight leaving mostly yellow, orange and red behind.

The rising or setting moon also looks slightly squashed. That, too, is real, though only the moon's image is flatter than usual, not the moon itself.

Moonlight is bent (refracted) when passing through the atmosphere. As the moon rests on the horizon, this bending appears to lift the lower limb of the moon more than the upper limb. The result is a squashed-looking moon.

In fact, this light bending is so pronounced that when the moon appears to be setting, the real moon actually lies below the horizon. This is easily demonstrated by laying a clear glass jar flat on the edge of a table and then having someone hold a dark marble just below the table's edge next to the jar. Now look at the jar from the other side of the table. You'll actually see the marble though it's physically below your line of sight. The jar acts like Earth's atmosphere, bending the marble's image-upward.

Those large circular gray patches visible on the moon mark locations where asteroid-size rocks gouged huge basins into the lunar surface 3.9 billion to 4.3 billion years ago. This denotes the time when rocky debris, remaining from the solar system's formation, pelted the young planets.

Molten dark-colored basalt rock flowed like hot maple syrup through cracks in the lunar surface, filling these basins. Basalts are the moon's youngest rocks -- a mere 3 billion to 4 billion years old.

The brightest portions of the moon identify the oldest and most heavily cratered landscapes. These highlands are covered by light-colored rocks called anorthosites and fused rock fragments (breccias). The Apollo astronauts retrieved their oldest lunar rocks, dating back 4.5 billion years, from these areas.

If you're looking at the moon using binoculars, notice the bright crater near the bottom of the moon. This is 50-mile-wide crater, named Tycho, formed when a large meteorite struck the moon 100 million years ago. Tycho is unmistakable because the moon's brightest ray pattern of ejected rock radiates 1,000 miles outward from the crater.

So, grab your lawn chair and watch the full moon rise. The next dates are Saturday, Aug. 16 and Sept. 15.

* Roy Gephart is an earth scientist with the Pacific Northwest National Laboratory and an avid amateur astronomer. E-mail: roy.gephart@pnl.gov.

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