Why Does the Moon Shine in the Daytime?

A poplar tree in full fall regalia frames the waning gibbous moon this morning. Bob King

I've written about seeing the moon in the daytime sky before. I hope you don't mind if I take another swing at the topic. Did you see the moon this morning? Just amazing. It caught my eye around 8 a.m. high in the southwestern sky couched in deep blue and framed by autumnal colors.

Portrait of the full moon if the sun's light were suddenly extinguished. Can you find it? Stellarium with additions by the author

For the next two hours as I went about errands it hung in clear view opposite the sun, illuminated by the light kindled by fusing atoms in its core. At a glance you could see one of the most important differences between a planet (or moon) and a star. A star makes its own energy and beams it across space in all directions. Moons and planets reflect a small portion of that light back, which renders them visible. Otherwise they'd appear as black silhouettes against the stars. 

This morning (Oct. 7, 2020), the waning gibbous moon occupied the same place in the sky the sun did back in late spring and summer, the reason it appeared so high in the sky. It will remain easy to see the next few mornings. Stellarium

Every fall, several factors conspire to make the waning gibbous moon easily visible in the daytime. Key is the moon's location on the ecliptic, the path also followed by the sun and the planets through the 12 constellations of the zodiac. 

By October, the sun has slid southward from the lofty perch in the constellation Taurus it occupied back in June. Days are shorter and the solar arc across the sky is noticeably lower. With Taurus now "vacant," the waning gibbous moon has moved into the sun's former location. As you'd expect, it follows the same arc the sun did in summer, so it's high in the morning daytime sky. High altitude also means we see it through less air and dust, which increases its contrast and brilliance even more.

I took this photo through my scope this morning with a mobile phone. Lots of craters were visible! Bob King

Late sunrises also help. From many locations sunup occurs after 7 a.m. By the time many of us are up and about the sun is still low and the moon relatively high. As well, a low sun in the east means the western sky is less affected by solar glare, making it a deeper shade of blue.

If you have a telescope and don't like staying up late to see the moon's waning phases — and who can blame you? — you can wait till daylight to do so. If you're out around 8 a.m. you'll be surprised at how much there is to see and photograph. Over the next few mornings, look up in the southwestern sky and give a nod to the moon.


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