Different phases of the moon determine its size

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"Why is the moon more like a fingernail sometimes and sometimes it's a circle moon?" asked a young patron at Cook Memorial Library in Libertyville.

The moon is an amazing object. Sometimes it bursts into the sky like a giant, glowing balloon - so large and so bright that it seems like nothing else could possibly fit in the sky. Other times it's just the tiniest sliver, balancing like a thin lemon peel in the dark night sky.

The amount of time it takes for the moon to rotate on its axis is exactly the same amount of time it takes to orbit the earth - 27 days and a little bit more. At the same time, the earth spins on its own axis making a 24-hour day.

The moon makes a slight arc as it rides across the sky through its monthly rotation, traveling lowest in the sky at the full phase and highest at the new moon phase. Each day, the moon takes on a different shape, depending on where it is in its orbit around the earth.

"Night after night, as we look up, the moon appears to change shape," said Geza Gyuk, vice president of astronomy, research and collections at Chicago's Adler Planetarium. "It starts as a full moon, a glowing circle in the sky."

As the days progress, the moon seems to become smaller, or wane, making the right side smaller and smaller until we only see a half circle on the left side - like a fingernail. Up to this point, the full-moon-to-half-circle cycle is called the waning crescent.

The new moon then appears and the waxing crescent phases begin. First, it seems like there's no moon, then the next night the right side reappears as a very thin crescent, and it grows larger as each day passes.

About a week later, the moon looks like a half circle. In another week, the moon is back to full or circle moon. The process takes about a month.

"The moon emits no light of its own, it only reflects light," Gyuk explained.

Even though it is always there, we can only see parts of the moon depending on its position as it orbits Earth, and also depending on the Earth's position as its moves through space and orbits the sun.

Gyuk describes the moon's phases: "When the moon is farther from the sun than the Earth is when we see the whole illuminated side, and it is a full moon. When it is between the sun and the Earth, then we see only the shadowed side of the moon, and it's a new moon. At the sides, then a half circle.

"This is best understood by standing in the dark at night with a bright light nearby, but not too close, and moving a baseball around your head. Look at the pattern of light and shadow on the baseball. It'll look just like the phases of the moon."