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Shape and Orientation of the Third-Quarter Moon

-- Brittany | December 12, 2019

Question:

My husband recently saw the waning moon about at 3rd quarter, coming up really orange, lit on the bottom around the evening of Nov. 17th(2019). He called freaking out because he had never seen this. I’ve only seen this a few times maybe myself, but never focused on a thought about it. I never remember seeing this as a kid and no one around me remembers ever seeing the terminator laying horizontal either. My husband used to drive at night for work and said has never seen the moon look like this except during an eclipse. I know with the recent big earthquakes that our axis gets knocked around. Is this a result of that and it shifted our view or has this really been happening all of our lives and somehow so many of us have never noticed it before? I cannot find historical writings, images, or anything mentioned about this horizontal looking phase anywhere to see that is has been happening all along, and not just a more recent thing. All I find are forums of people freaking out and asking the same thing because they’ve never seen it until now. Are there historical records or anything that I can read and show him, and even get an education myself on this?

Thank you,

Brittany

-- Brittany

Answer:

The orientation of the illuminated portion of the Moon when it is at third quarter depends upon where on the Earth you are viewing the Moon. From the northern hemisphere the left-half of the Moon is illuminated, while from the southern hemisphere it is the right-half of the Moon that is illuminated. From equatorial regions, the lower-half of the Moon is illuminated at moonrise. You can find a nice description of the third quarter moon’s properties on timeanddate.com.

Earthquakes have no affect on the Earth’s rotational axis tilt. What you are seeing is just how the orientation of the Moon’s phases appear from different places on the Earth.

-- Jeff Mangum

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