Why we can see the moon during the day – now. Powered by Northrop Grumman

Light tricks aside, for a celestial body to be visible during the day, it must eclipse the sun. Few objects can surpass our main sequence star in its own system. A few supernovae have been so bright that they have been visible during the day. Meteors, if large enough, flare up like giant torches as they burn through the atmosphere. And here and there throughout history, a comet rivaling the sun in brightness streaked across the sky, terrifying the inhabitants of Earth.

Then there is the moon. He breaks all the rules. Long before humans understood the orbital plane of the Earth and stars, how many ancients looked at the moon and thought, what is this thing?

Trickster or Rockstar

It’s not hard to see why the moon has kept our species scratching their heads for millennia. It’s almost constant, going in and out every 27 days. But in most cases, the moon makes less sense than anything else in the sky. How can it look as bright as the light of a fire, but as solid? Why can we sometimes see the moon during the day, sometimes see it at night and sometimes not at all? What are these strange patterns on the surface, and why do they never change? Why doesn’t the moon follow the same path in the sky as the sun, planets and stars? And why does it sometimes turn red, seemingly out of nowhere?

In earlier eras of human history, the amount of information about the moon held 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 thrown from the Earth. The sun, likewise, was strong and also scorching. Rather than producing its own power, the moon reflected the scorching light of the sun. This celestial configuration allowed Anaxagoras and other like-minded people to correctly predict that when the orbital planes of the three solid objects aligned, solar and lunar eclipses would result.

Seeing is believing

Suffice to say that the Athenians were not entirely of the same opinion. Many ancients found it easier to imagine a god stationed in the sky than a highly reflective rock – a rock placed up there for unclear reasons by known forces. Why this rock never descended, why it kept going out of phase, and how it remained unphased through time seemed equally unfathomable. Nowadays, basic knowledge of astronomy is a birthright. But 2600 years ago, it was easier to arrest Anaxagoras for heresy than to understand that the lunar phases are an optical illusion: that when the sun sets in the west while the moon rises in the is, the moon only appears full because we are standing in-between two distant bodies of different sizes. Telescopes, invented more than a thousand years after Anaxagoras died in exile, have allowed us to easily see craters, rocky planes and other imperfections in the sphere once believed to be a god.

Then, of course, we went.

After Apollo, lunar knowledge is commonplace. Using a tennis ball and a flashlight, two school children standing in a darkened room can easily demonstrate the phases of the moon. The child playing the sun stands still with the flashlight shining directly in front of him. The other child playing Earth faces the flashlight, trying not to squint too much. They hold the tennis ball in front of them with their left arm extended, then they turn 90 degrees to the left. Presto: They can only see 25% of the illuminated moon. The moon/tennis ball is 50% lit, but the child only sees a lit quarter from his position. We see the same quarter illuminated from Earth during the first quarter moons.

During this time, any children standing around the child with the flashlight can see 50% of the illuminated moon, as is always the case. As the child with the tennis ball continues to spin to their left, they will also be able to see more of the illuminated moon. Everything looks the same from the point of view of the sun. From the perspective of the child with the tennis ball, the day goes from noon until sunset, and the moon waxes towards the full as time rapidly advances through an entire 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 of this can be demonstrated in just a few minutes. But don’t let the humility 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 displays like this were deemed more compelling than pantheons filled with drama, intrigue, and interpersonal conflict.

Speaking of which: the moon’s orbital plane relative to Earth can be demonstrated with the same simple setup. If the moon’s orbital plane were exactly the same as the one Earth uses to travel around the sun, we would have lunar and solar eclipses every month. Instead, we only have them once in a while. This is because the moon’s orbital plane is about 5 degrees off Earth’s orbital plane. To demonstrate this, instead of standing up straight, the child holding the tennis ball in front of him should sag a little to the left, as if one ear is heavier than the other. Or they could let their arm holding the ball swing as they spin it. Either way, the ball should drop slightly below the line of the flashlight beam and then float slightly above it. As the child spins in place with the slightly offset tennis ball, it will only occasionally create a solar eclipse, when its wobbly orbit comes directly in front of the flashlight beam.

new gods

Simple as that, two lightly equipped children can blow eons of mythology from 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 figure. In the age of live streams from lunar cameras and constellations of satellites, it’s easier than ever to see how the moon is 50% illuminated at all times. Yet wherever you are on Earth, day or night, northern or southern hemisphere, the same rules apply now as they did then. Whatever phase you are in – be it crescent, first quarter, gibbous crescent, full, waning gibbous, third quarter, waning crescent, or new – you will see the part of the moon half-lit facing 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 fill the sky and snow falls on our roofs, we can call a perfectly clear view of the lunar surface. How, at any time, we can ask a light box in our hand to show us the Anaxagoras crater as seen by an orbiting spacecraft. Part-lit by the sun, part-hidden in shadow, the rock cast into space continues to challenge us to rise above assumptions, superstitions, and the comfort of not knowing, and to look back the whole, imperfect shape of the Earth as it hovers above the horizon.

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