If you follow me on twitter, you know I love to muse about the Moon. The Moon can be metaphor or quiet companion. There is comfort in watching the Moon’s monthly cycle slowly unfold. All of my moon-gazing brought questions along with it. I suppose I could have consulted NASA for the answers, but what would be the fun in that? Instead, I looked skyward, right to the source. I call it my Moon Experiment, but don’t worry. The experiment is observational only.
First, a little background, for context:
- The Moon is represented as nocturnal, like all night’s creatures. What would owls and coyotes be without a Moon as a backdrop?
- We forget that the Moon appears in the daytime too. Even as its brightness is lost in the sunlight, the Daytime Moon can be inspiring. Each time I see it, I am reminded not to be limited by expectations.
- Each month, the Moon seems to disappear for about a week or two. More nights are moon-less than can be explained by the New Moon. Where is the Moon during these dark nights?
- The Full Moon is on the opposite side of the Earth as the Sun
- The New Moon is on the same side of the Earth as the Sun. We cannot see the New Moon because the dark side of the Moon is facing us. Even so, we would have difficulty looking at the New Moon because it would require looking toward the Sun! A way to think about the New Moon is that it rises and sets with the Sun, and is directly above us at noon!
In my experiment, I sought the answer to two questions.
Question 1: The Moon has incredible periodicity. How can I predict the Moon’s position each night?
To answer this, my thought experiment went as follows:
- A day has 24 hours
- The circumference of the Earth is about 25,000 miles around at the equator
- Therefore, the Earth spins about 1000 miles per hour at the equator
Adding the Moon to our equations:
- The Moon travels around the Earth about once every 27 days (this is the basis of a month, or a moonth!)
- Using the above numbers, we can estimate that the Moon must change its position by about 1000 equator-miles per day in order to make it all the way around the Earth in one lunar month (~25000 equator-miles in 27 days ≈ 1000 equator-miles per day)
- A daily off-set of ~1000 equator-miles is equivalent to a change in the Moon’s peak by about an hour each day
More simply stated, the Moon rises and sets about an hour later each day. To see this for yourself, look up at the evening sky at the same time each day starting at the Half-Moon phase, and notice that the Moon rises later. Look up again an hour later, and the Moon will be in the same place it was the night before.
I empirically tested this hypothesis over the course of the last month, and my thought experiment held up to observation. After I was convinced I was right, I googled and confirmed this is true. Multiple sources state the Moon rises and sets 50 minutes later each day. Now I can look at the sky and know the position of the Moon based on the previous night’s position!
So, we understand the Moon’s patterns a bit more. What about those extra moon-less nights? What about the Daytime Moon?
Question 2: When does the Daytime Moon appear, and why does the Moon seem to disappear for a week or two each month when the New Moon period is much shorter?
My thought experiment for this question went as follows:
- My experience is the smallest Crescent Moon one can see with the unaided eye is about a 9% illuminated Crescent Moon. I think smaller Crescent Moons cannot be easily seen in part because they appear at dawn and dusk. That is, there is too much light in the sky for easy viewing. Further, the Moon must be positioned far enough to the side of Earth that we can view the Moon’s illuminated face askance (hence the crescent shape).
- Therefore, there are more moon-less nights than one would expect from the Night of New Moon and a night or two before and after. The Moon must be positioned and illuminated enough for unaided viewing.
So where is the Moon during “moon-less” nights? Consider:
- The Moon travels completely around the Earth
- The Moon moves in one direction (East to West) around the Earth, and rises/sets about an hour later each day
- Therefore, at some point during the month, the Moon is directly overhead at each point in the day/night cycle.
In other words, the Moon’s arc peaks at midnight one night, then peaks at ~1am the next night, then peaks at ~2am the night after that, and so on. Some of the nights that seem moon-less are just nights when the Moon’s cycle and my waking cycle do not coincide. If I were to forgo sleep for a month and stare at the skies, I could see the Moon for more nights of the month than I currently do.
Okay, then what is the Daytime Moon?
- The unidirectional movement of the Moon around the Earth means that sometimes the Moon is overhead as we are spinning into nighttime (i.e., dusk), and sometimes the Moon is overhead while we are spinning into daytime (i.e., dawn).
- Given the direction of all the spinning, we know the waxing Moon is visible at dusk and early evening, and the waning Moon is visible in dawn!
In other words, the Daytime Moon, which we see in the morning, is waning!
Some parting thoughts:
I leaned so much about the Moon through observation. I have a new appreciation for early astronomers who uncovered the dance of our Solar System long ago, and fully believe this was possible with a dedicated and observant eye.
I leave you with this wonderful movie of the Moon’s movement around the Earth (source: NASA): Orbit of the Moon in 2013.ogv