Astronomy EssentialsMoon Phases

Do we all see the same moon phase?

3 gibbous moons tilted a little differently each labeled with time of night.
View at EarthSky Community Photos. | This is a crop of an image taken by Andy Bentley in Mountainview, California, showing the moon on a single night, February 21, 2021. See the full image here. We all see the same moon phase. But, as the image shows, the moon’s orientation to your horizon changes throughout the night, or as you move northward or southward on Earth’s globe. Thanks, Andy!

Moon phase is a whole-Earth phenomenon

Everyone looking up at the moon from any spot on the globe sees the same moon, in more or less the same phase. So why might photos of the moon taken on a single night – but from different parts of Earth – look different?

And here’s a related question. Why does the moon look different – even from the same spot – over the course of a single night?

Let’s take the second question first. The moon’s orientation with respect to your horizon shifts throughout the night because we live under a curved dome of sky. Think of the curved arc traveled by the sun during the day. The moon and the stars follow similarly curved paths.

The illustration below – by Courtney Seligman – shows the curved line traveled by the stars and the moon, as they traverse Earth’s dome of night (or day). As an aside, Professor Seligman was also trying to illustrate in this drawing that, although Earth’s rotation under the sky causes the stars and moon to move westward throughout the night, the moon’s motion in orbit is eastward.

Diagram showing the arcing line traveled by the stars and moon.
The stars and moon travel on curved lines across Earth’s sky. That’s why the moon’s orientation with respect to your horizon changes throughout the night, as the photos above and below show. Illustration by C. Seligman.
3 images: Half moon angled up, half moon vertical, half moon angled down.
Bridget Borchert in Bloomington, Minnesota, captured this image in 2018. She wrote: “The tilt of the moon through the evening fascinates me.”

Same moon phase, so why the differences?

People in the Northern and Southern Hemispheres see the moon oriented differently from one another. It’s not a change in phase. It’s a change in the orientation of the moon with respect to your horizon. The differences can be hard to comprehend!

For one thing, observers in the Northern and Southern Hemispheres see the moon apparently upside-down with respect to each other. You can see that, if you scrutinize the moon’s features in the images below. Notice that from the Northern Hemisphere, a waxing moon (from new moon to full moon) increases its phase from right to left.

From the Southern Hemisphere, meanwhile, a waxing moon (from new moon to full moon) increases its phase from left to right.

Why so different? Think of the moon’s path across your sky. Just picture yourself standing there, looking at it. From the Northern Hemisphere, we look generally southward to see the moon (or sun) crossing our sky. From the Southern Hemisphere, people look generally northward to see the moon (or sun) crossing the sky.

Can you see why that shift in your orientation on the globe would cause you to see the moon differently?

Row of moon phase images showing crescent and quarter moons pointing toward full moon.
Northern Hemisphere viewers see the moon like this. Image via primaryhomeworkhelp.co.uk.
Row of moon phase images showing crescent and quarter moons pointing away from full moon.
Southern Hemisphere viewers see the moon like this. Image via primaryhomeworkhelp.co.uk.

A final thought

What we on Earth call moon phases are really about sunrise and sunset on the moon. Astronomers call the line between light and dark on the moon the terminator line. That’s the line of sunrise or sunset on the moon, and it shifts, just as the line of sunrise and sunset on Earth is constantly shifting. Earth spins relatively fast, approximately once every 24 hours. The moon spins on its axis only once each earthly month, and its line of sunrise/set moves slowly. It’s wonderful fun to beg or borrow a telescope for a night when the moon is up … and watch for yourself over several hours as the shadows slowly shift on the moon, as the lunar sunrise or sunset slowly creeps across the moon’s face.

A great way to clear the mind!

Four vertical sections of moon showing large crater appearing farther to the right in each.
View larger. | Mark Seibold caught the progression of shadows and light on crater rims and floors – across about 1 1/2 hours – at lunar sunrise (along the terminator line, or line between light and dark on the moon) on a 7-day-old moon on April 22, 2018. The large crater here is 95-mile-wide (150-km-wide) Ptolemaeus. Celestron Nexstar 5i, with Sony NEX5R directly through telescope, and 12.4mm Meade eyepiece projection.
Labeled lines pointing to features on almost straight edge between dark and light sides of moon.
Flickr user Mike O’Hara kindly labeled the wealth of named features along the moon’s terminator line – line of sunrise – on a waxing moon. If you had a telescope, as the line of sunrise slowly shifts, you could watch over several hours to see the shadows playing along the rims of these and many other lunar mountains and craters.

Bottom line: The moon shows one phase to the Earth at the same time, but our different perspectives due to where we are on the globe can make the moon appear differently in our sky.

Want more? Visit this page at primaryhomeworkhelp.co.uk

Posted 
February 24, 2021
 in 
Astronomy Essentials

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