Quick: What causes the phases of the moon? Earth’s shadow?
Wrong. This is a common misconception – over half my Introductory Astronomy students at Appalachian State jump at that answer on the pre-test I give at the start of the semester.
Earth’s shadow has nothing to do with the crescent or quarter moons but has everything to do with lunar eclipses, and we have a fine, prime-time lunar eclipse near the end of this month.
As we approach that event you can check the waxing moon as it now grows from new noon to a first-quarter phase on Sept. 20. The changing angle between the sun, Earth and moon causes us to see a changing fraction of the moon’s night side. That’s the cause of the phases. After all, take a look at it just about sunset near first quarter and ask yourself how could Earth’s shadow from the sun setting in the west take a right turn to get to the moon that’s seen to the south?
A week after that quarter moon – one Sunday later, Sept. 27 – the moon will have moved to a full phase, when the sun, Earth and moon line up. This month’s alignment is pretty close to dead-on, so the moon will go through our shadow that evening and provide us with a total lunar eclipse. Most full moons occur with the moon a little too far north or south, due to the tilted lunar orbit, and the moon slides above or below our shadow. No eclipse!
But twice a year, about six months apart, we get the close alignment that provides chances for both lunar and solar eclipses. Right now, the eclipse seasons are in September and March, but they slide about 20 days earlier each year, due to changes in the moon’s orbit.
The eclipse starts about sunset with the more visible partial phases starting just after 9 p.m. It will be totally eclipsed by 10:11 p.m., and not start emerging until 11:23, for more than an hour of totality. A nice, prime-time event!
If Earth’s twilight zone is mostly clear, some sunlight will be bent through it, stripped of the blue light that makes our sky blue, and it will paint the moon a nice “blood” red. If it is largely cloudy around the band of the Earth’s twilight, not much light will get through, leaving us with a dark, gray moon.
This is a fine view for the unaided eye, binoculars or telescope. During the eclipse’s partial phases, notice that Earth’s shadow is curved – an early proof of a round Earth, as noted by Aristotle. Only a sphere always casts a round shadow. So, enjoy the eclipse and a demonstration of our round planet!
Daniel B. Caton is a physics and astronomy professor and director of observatories at Appalachian State. Email: firstname.lastname@example.org. More on this column is at: www.upintheair.info.