• If you look in a clear western sky after sunset on Monday, December 17, you should spot the waxing crescent moon. The planet Neptune is near the waxing moon tonight, but you won't see

Neptune with the unaided eye.
• More about Neptune below. For now, let's linger on that little crescent

moon. You know that the moon passes through a series of phases. But did you know that you can tell where the sun is - even when the sun is below the horizon - just by observing the moon phase? It's especially easy with a crescent moon up to first quarter phase. And, once you learn to use the moon in this way, you'll gain a lot understanding about what causes the moon to change its phase in the first place.
• The crescent has two ends or "horns." Imagine that you draw a straight line from one horn to the other. Then from the middle of that line, between the horns, imagine another straight line perpendicular to the first (that is, at a "right" angle). Imagine that line drawn down through the illuminated part of the crescent. That line points to the sun. You can extend the line down to the point on the horizon roughly where the sun went down.
• By the time the moon waxes to the first quarter phase, of course, the first line above is just the "flat" edge of the moon. After that phase, it becomes a bit harder to use the moon to find the sun's position (when the sun is below the horizon), but if you think about it, you can always do it - and it'll add to your perception of yourself as a person standing on a world in space.

• Why does this work? Well, it's the sun that causes the moon's phases. The moon is a globe in space, just as Earth is a globe. And like the Earth, the moon has a day side and a night side. The varying moon phases we see are just varying fractions of the moon's day side. Want more about this? Try our article on Understanding moon phases.

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