Follow the Moon


Last updated 2/15/2024 at 11:16am

Use your hand to predict the path of the moon through the sky.

The moon returns to our evening sky again this week, and you can have some fun watching it cross our sky from west to east.

Our nearest cosmic neighbor reached its "new" phase on Dec. 12, when it was located between the Earth and the sun. Of course, it wasn't directly in line; if it had been, it would have blocked the sun from view and caused a Solar eclipse. Most of the time it passes slightly above or below the sun's disk.

From that day on, the moon begins appearing in our evening sky again as it continues orbiting the Earth. First, it appears as a crescent moon at dusk, when it still lies generally in the direction of the sun. Its spherical body is lit mostly from the back side and we see a bright crescent because sunlight spills over onto the lunar side facing Earth.

About a week after its new phase, the moon always appears in its "first quarter" position. This means that, from our vantage point, a quarter of the lunar sphere is lit by sunlight. If you can recall where the sun is on that night (below the western horizon where it had set earlier), you'll understand how the moon is illuminated in this way.

After another week passes, the moon will have drifted into the eastern sky; now it lies opposite the sun, and from our viewpoint, its disk appears to be fully illuminated (hence the term "full" moon). And since the full moon always lies directly opposite the sun, it does everything opposite the sun. It rises as the sun sets and sets as the sun rises. In addition, during the winter months when the sun travels quite low in our daytime sky, the full moon appears high in our sky at night.

Every month, the moon makes one complete circuit eastward around our planet. In other words, it travels 360 degrees around the heavens every month (let's say 30 days, just to keep the arithmetic simple). That means we should see its position change against the starry background by 12 degrees every night (360 degrees divided by 30 days). And, indeed, we can.

This week, we've got a couple of bright celestial "markers" in the sky to help us measure this motion: Jupiter and Saturn. Keep watch and you'll notice that the moon passes by the planet Saturn on the night of Dec. 17 and by Jupiter on the nights of Dec. 21 and 22.

So how can you measure the moon's motion? You have all the tools you need right at the end of your arm! If you hold your fist at arm's length, it will appear about 10 degrees from thumb to little finger. Now hold out just your thumb at arm's length; this is approximately two degrees across. And your little finger, also at arm's length, appears about 1 degree across.

By holding up your fist, thumb or little finger to measure 12 degrees east of the moon's current position, you can predict where it will appear the following night.

Then the next night, go out and see if your guess was correct.

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