The 6pm moon

A month ago, just as the sky softened into evening, I looked up—and there it was. The first quarter moon, exactly where I’d predicted, peeking through the clouds. I felt a small, quiet thrill: I was right.

Until then, the moon always seemed so random – sometimes it’s on one side of the sky, sometimes the other, sometimes missing altogether.  I could only catch glimpses out my city windows or between buildings. What it looked like, and where, and when, was unpredictable, like the weather or the news.

But surely there’s a pattern, like clockwork – like calendar-work. It’s just a sphere flying around another sphere. Of course there is. So here’s the secret that will brighten your day next time you look up: you will always see the same phases of the moon at the same part of the sky at the same time of the day, anywhere in the world.

Try it. 

At 6pm, when you are on your way home or heading out the door, look for the “waxing crescent,” the first sliver of light after the black “new” moon. It will be on the western horizon in the setting sun.  The new moon will have just passed in front of the sun (if it lines up perfectly we get an eclipse); it makes sense that the early crescent phase is still hugging close to where the sun has just set. 

A week later, as the moon continues its journey around the earth, the half-lit “first quarter” moon will be overhead at 6pm.

A week later, at 6pm, the full moon can be seen rising on the eastern horizon.  That makes sense too – for the moon to be full, it must face the sun head full on, from the opposite side of the earth. As the sun sets, the full moon rises, like in a puppet show.

And then for two weeks on your way home from whatever you are doing at 6pm, you won’t see the moon at all.  That 6pm moon is blocked by the earth under your feet.  Goodnight moon.

The moon is still there, of course.  If you miss seeing it, just wait until later in the night or get up early to watch it glide.

You don’t have to look for the moon only at 6pm. You can trace its chariot across the night sky. It will, always, rise in the east. Just like the sun. Of course it does — the earth rolls it into view, and the earth only rolls in one direction.

The moon won’t be perfectly overhead, both thanks to where you are standing, the moon’s askew orbit around the earth, and the sharp 23 degrees of earth’s tilt.  It will instead travel near the same seasonal paths that the sun travels – but half a year behind.  The low December sun in London dots behind buildings near my home even at midday; the moon will sneak behind those same buildings this June and be hard to spot in the bright summer evenings.

Just as the moon is lowest in the summer, it rises highest in the winter. I suspect that’s part of the excitement of October’s Halloween or autumn harvest moons: not only are the days suddenly much darker as the full moon rises in the east, but the moon also shoots up higher in the sky.

Summer is coming now, and I thought I shouldn’t publish this post; I thought you won’t be able to see the moon. But the emoji culture of the moon being a night-time-only phenomenon is just wrong. Yes, the full moon sets as the sun rises, so isn’t visible in the day. Yes, the sun is so bright that it makes seeing a new moon impossible and a nearby crescent moon hard.

But our eyes are amazing at detecting contrast. The moon’s edge is sharp; the sky is uniform; our brains are built for that. Not all parts of the day sky are equally bright, as you’ll kind of know if you’ve squinted taking a photo facing the sun. A blue sky on the other side of the sky might be a hundred times dimmer than near the sun. Moisture in the atmosphere smears light across the sky, sometimes smudging the line between the moon and the sky, sometimes blocking it entirely. 

But if the sky is crisp and dry, if the moon’s path rises high enough to clear buildings or nature near you, and if the moon is not a full moon or too blindingly close to the sun, you will see it at some point during the day.  The moon is not just a night-time friend.

At twilight, we notice it the most, as our eyes adjust and the contrast between day and night starkly flips. As your light-sensing street lights flick on, the moon pops into life. Your brain hardly registers how dramatic that drop in light is; you find yourself sitting in a dark room squinting at your screen, unaware daylight has faded. But the light levels outside have plummeted a million-fold. By night, even a dim crescent moon is ten times as bright as the rest of the night sky and a full moon a hundred times. 

Now you know where to look for the moon, look more closely at its shadows.  The bright half faces the sun, which sets to the west.  Waxing (i.e. getting bigger) evening moons, like the crescent or half moons, will always be bright on that western right side (in the northern hemisphere). That evening crescent C moon you’ve been drawing for years? It should always be a backwards C!  (The American children’s book “Guess How Much I Love You” gets this wrong. It should really say, “I love you to the moon and … backwards!”)

The reflected light tells us where to find the sun; the shadow tells us where to find North. The moon’s shadows cut across from its south pole to north pole.  Since our north pole is roughly in line with the moon’s, you can trace the moon’s pole north up to find our own north. You can find your way home.

Sometimes, you can see not just the shadow as a cut in the night sky, but a dark orb showing our entire rocky space buddy. That is the marvellous “earthshine”: light that bounces off our clouds and oceans and ice caps, rebounds off the dusty soil of the moon, and ends up back in your eyeball. All that photon pinballing, just for you. Wicked!

As for the dark side of the moon: there is no dark side. All sides of the moon rotate into the sun, once a month. (To find true darkness, space explorers need to find shadowed craters.)

The weather, politics, trees, health, people – these are complex things, hard to understand, hard to predict. But the moon?

The moon is always there for us, predictable, a partner to our dance. It’s more than a sibling or pet, it’s really a clone: 4.5 billion years ago, a Mars-sized mass hit our planet, splitting us into the earth and the moon, and knocking us askew. (NASA has an amazing simulation of this event here.) We are all moon dust.

So now, I hope you share my comfort and joy at knowing right where the moon is and where it’s going. At 6pm, it’ll either be a setting backwards C in the western sunset, a right-handed half circle above our heads, a full moon rising in the east, or sleeping under our feet. It’ll rise high in the winter and low in the summer. It’ll be there, day and night. We’ll see it, even during the day, unless it’s blocked from view or glared out by the clouds and sun.

We always know where to find it. And even if we can’t see it today, it’ll always rise again.