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Aug 1st 2022

Tails From the Orbital Plane: Why We Can See the Moon During the Day


Tricks of the light aside, for a celestial body to be seen during the day, it has to outshine the sun. Not many objects can outshine our main sequence star in its own system. A few supernovas have been so bright that they’ve been visible during the day. Meteors, if they are big enough, flare like giant torches as they burn through the atmosphere. And here and there across history, a comet rivaling the sun in brilliance has streaked across the sky, terrifying Earth-dwellers.

Then there is the moon. It breaks all the rules. Long before humans understood the orbital plane of the Earth and the stars, how many ancient people looked up at the moon and thought – what even is that thing?

Trickster or Rockstar

It’s not hard to see why the moon had our species scratching their collective heads for millennia. It’s nearly constant, phasing in and out every 27 days. But in most other ways, the moon makes less sense than anything else in the sky. How can it look as bright as firelight, but also seem solid? Why can we sometimes see the moon during the day, sometimes see it at night and sometimes not at all? What are those strange patterns across the surface, and why do they never change? Why doesn’t the moon follow the same path across the sky as the sun, planets and stars? And why does it occasionally, seemingly out of nowhere, turn red?

In previous eras of human history, the amount of moon information possessed by modern eighth-graders would have made them legends. Or landed them in jail.

Around 600 BCE, Anaxagoras proposed to his fellow Athenians that the moon was not a deity but rather a rock — one that had been flung from the Earth. The sun, likewise, was solid and also burning. Rather than making its own power, the moon reflected the burning sun’s light. This celestial setup allowed Anaxagoras and anyone of like mind to correctly predict that when the orbital planes of the three solid objects lined up, solar and lunar eclipses would be the result.

Seeing Is Believing

Suffice to say, Athenians were not entirely of like mind. Many ancients found it easier to imagine a god stationed in the sky than a highly reflective rock — one placed up there for unclear reasons by known forces. Why this rock never came down, why it continued to phase and how it remained unphased by weather seemed equally unfathomable. These days, basic knowledge of astronomy is a birthright. But 2600 years ago, it was simpler to arrest Anaxagoras for heresy than to understand that lunar phases are an optical illusion: that when the sun sets in the west as the moon rises in the east, the moon appears full only because we are standing in-between two distant, differently-sized bodies. Telescopes, invented more than a thousand years after Anaxagoras died in exile, allowed us to easily see craters, rocky planes and other imperfections in the sphere that was once believed to be a god.

Then, of course, we went there.

Post-Apollo, lunar knowledge is commonplace. Using a tennis ball and a flashlight, two school kids standing in a dark room can easily demonstrate phases of the moon. The child playing the sun stands still with the flashlight shining straight away from them. The other child playing the Earth faces the flashlight, trying not to squint too much. They hold the tennis ball in front of them with their left arm outstretched, then they rotate 90 degrees to the left. Presto: They can only see 25% of the illuminated moon. The moon/tennis ball is 50% illuminated, but the child only sees one illuminated quarter of it from their position. We see the same illuminated quarter from Earth during first-quarter moons.

Meanwhile, all the children standing around the child with the flashlight can see 50% of the moon lit up, as it always is. As the child with the tennis ball continues to rotate to their left, they will be able to see more of the moon lit up, too. It all looks the same from the sun’s perspective. From the perspective of the child with the tennis ball, the day is going from noon to sunset, and the moon is waxing toward full as time fast-forwards through one whole week.

As the World Turns

Why can we see the moon during the day? Why is it crescent at dawn, first-quarter at noon and full at sunset? All this can be demonstrated in only a few minutes. But don’t let the humbleness of the flashlight and the tennis ball fool you. Understanding this interaction of rotating bodies requires a surprisingly advanced appreciation of geometry. Entire civilizations rose and fell before simple demonstrations such as this were deemed more compelling than pantheons filled with drama, intrigue and interpersonal strife.

Speaking of which: The orbital plane of the moon relative to the Earth can be demonstrated with the same simple setup. If the moon’s orbital plane were exactly the same as the one the Earth uses to travel around the sun, we would have lunar and solar eclipses every month. Instead, we only have them every so often. That’s because the orbital plane of the moon is offset from Earth’s orbital plane by about 5 degrees. To demonstrate this, instead of standing straight up, the child holding the tennis ball in front of them should slump a little to the left, as if one ear were heavier than the other. Or they could let their arm holding the ball wobble while they spin it. Either way, the ball should sink slightly below the line of the flashlight beam, then float slightly above it. As the child spins around in place with the tennis ball slightly offset, they’ll create a solar eclipse only every so often, when their wonky orbit happens to dash directly in front of the flashlight beam.

New Gods

As easy as that, two lightly equipped children can blow eons of mythology out of the sky. At least, the part of moon mythology that explains how the sun is a god in a chariot, a dragon or any other mystical moon persona. In the era of lunar cam live-streams and satellite constellations, it’s easier than ever before to see how the moon is 50% illuminated at all times. Yet, wherever you are on Earth, night or day, Northern or Southern Hemisphere, the same rules apply now as they did then. Whatever phase you are in — be it crescent, first quarter, waxing gibbous, full, waning gibbous, third quarter, waning crescent or new — you’ll see the part of the half-lit moon that faces you now.

Thousands of years ago, even those who understood the physics of the Earth-moon-sun system could never have imagined how we see the moon today. How when clouds crowd the sky and snow falls on our rooftops, we can call up a perfectly clear view of the lunar surface. How, at any given moment, we can ask a glowing box in our hand to show us the crater Anaxagoras as seen by an orbiting spacecraft. Partly lit by the sun, partly concealed in shadow, the rock thrown into space continues to challenge us to rise above assumptions, superstitions and the comfort of not knowing, and to look back at the whole, imperfect form of the Earth as it hovers above the horizon.

Interested in all things in outer space and exploration? We are, too. Take a look at open positions at Northrop Grumman and consider joining our team.