Astro Bob: Lunar observing by daylight — what could be easier?
For the next week, you can follow the moon and its phases in the daytime sky.
Want to know what's going on in the sky? Make a habit of looking up. We humans mostly look straight ahead or from side to side but rarely skyward. I'm sure there are good reasons for this. In the industrialized world we don't usually anticipate predators striking from above. Or maybe it has to do with the hazards of walking a straight line with your head stuck in the heavens. That one I can vouch for.
On Sunday morning (Nov. 13) I caught a glimpse of the waning gibbous moon out the window before clouds moved in for the kill. Lucky for us, we have at least a week of morning moon-watching left before the crescent disappears in the solar glare and returns to the evening sky. Sometimes it's hard to get out at night especially as the weather turns cold in November. Mornings can be easier.
I'm usually out between 8 and 9 a.m. local time, ideal for catching the moon at a nice elevation in the west or southwestern sky. At that hour, the lunar globe looks chalky white against the blue blackboard of the sky. You can also clearly distinguish the bright, white heavily cratered regions dubbed the Southern Highlands that contrast so well with the darker lunar seas, known as maria (MAH-ree-uh).
In binoculars the maria and highlands are more distinct, with the largest craters visible along the lunar terminator, the boundary between day and night on the moon. During the moon's waning phases, the terminator marks the line of advancing lunar sunset.
I could use words to describe why the moon's phase changes from crescent to half to gibbous and full (and back again) every time the moon circles the Earth. But I'll let Emily Morgan and her wonderful video on YouTube do that. It's one of the best demonstrations I've seen.
On Earth a day lasts 24 hours from one sunup to the next. The moon rotates about 28 times slower than the Earth, so day and night each last about 14 Earth days for an astronaut on the moon. Four weeks is also about the same time it takes the moon to revolve around the Earth. That's why we see only one face — Earth's gravitational tug has slowed the moon's spin so both its rotation and revolution are in lockstep with each other. The moon spins at the same rate it orbits the planet.
As you keep an eye on the morning moon not only will you notice its changing phase, but you'll see it gradually moving upward and left (to the east) from the western into the southern sky. What's up? You're witnessing the moon's daily motion in its orbit. It advances about 12° each day or a little more than a fist held at arm's length to the sky, while scooting along at around 2,300 miles an hour (3,700 km/hour).
Although you generally can't see stars by daylight, the moon is all the while passing through the 12 zodiac constellations. For example, on Monday, Nov. 14, it will be in Cancer the Crab. On Nov. 16, it's right next to Leo the Lion's scary fangs.
In the diagrams I've added in the ecliptic — the path the sun, planets and moon follow as they truck across the sky, each according to its own orbital cycle. They all follow this great circle because every one of them, Earth included, orbit the sun in approximately the same plane. Since we're more special, astronomers define the ecliptic as the plane of Earth's orbit projected onto the sky. You can imagine that plane as a perfectly flat football field extending from where you stand all the way to the stars. You'll notice the moon doesn't exactly stick to the ecliptic — it weaves a bit — because it's orbit around Earth is tilted 5 degrees.
Wishing you lots of clear mornings to get better acquainted with our amazing moon.