My writing buddy read a recent story that I’m working on set in Aemon-Tor, my constructed world. I wrote that a character simultaneously saw a waxing gibbous and a waning crescent in the sky. He told me this was impossible.
Wait a second… is it? Now I’m inclined to believe his assertion, but it’d be just plain lazy to take his word for it! So I went and did research of my own. My thanks to Moon Connection for the chart below.
Okay. So looking at this I got very confused for a second until my mind hopped in its time machine (thanks Stephen Hawking!) and journeyed back to those correspondence courses in Astronomy that I took five or six years ago.
Okay, I’m back in the mindset. So I took a look at the above chart again and picked a range of spots on the planet (the bottom left quadrant if you’re curious) to represent the time of night in the story, some time after midnight. This viewpoint blocks any position in the top right quadrant for sure. So never will you see a waxing crescent at that time of night. But this doesn’t answer the question, broken down into oh so many science-y parts:
1) Can a waning crescent be seen after midnight?
2) Can a waxing gibbous be seen at the same time as a waning crescent, provided this is a two-moon system?
3) Can two moons occupy these positions in the sky simultaneously?
1) A waning crescent can be seen in the eastern sky as sunrise approaches.
2) A waxing gibbous, depending on the precise location on the planet, may be seen in the western sky during the same time period, depending on the specific viewing angle of both objects.
3) A prerequisite for moons to have different phases simultaneously is that they must have different orbital periods. If both moons had the same orbital period as our moon: 29.53 synodic period, and orbited the planet so that they occupied the same area of the night sky at the same time, both moons would phase at the exact same time.
3a) Do moons with different orbital periods or distances exist in real life?
3a) Yes, moons very rarely orbit a planet at the same distance, or at the same rate. There are examples, especially in Jupiter, of moons which occupy approximately the same orbital path, but there are very specific positions at which such an orbital path is stable. Different orbital periods are fairly common and thanks to Wikipedia I can firmly use Mars’ two moon system as an example:
Orbital Periods of Mars’ moons:
Phobos: 0.318 days
Deimos: 1.262 days
Thus, it is entirely possible for moons to be out of phase with one another.
As to whether my character could look through a crack in the wall and see both moons simultaneously waxing gibbous and waning crescent, THAT is impossible, but not due to astronomy, but rather his constrained viewing angle. Still, I thank my writing buddy’s astute eye in questioning me on the astrophysics of such a thing, since it forced me to really sit down and think about my con-world as a part of a con-universe, or at least a con-solar system.