It's Only a Sliver Moon (Daily Astronomy Log with Mike Brown)

Sunset_view_at_Paranal_with_Moon,_Venus_and_an_AT.jpg (photo via Wikipedia)

This past Sunday began the last lunar cycle of the year, which, for the first time in almost 20 years, includes a full moon on the winter solstice itself. I'm going to spend some of the time between now and then using the moon as a guide for a little tour of the night sky.

Tonight, if your clouds part enough in the evening to reveal the just-set sun - very nearly as far south as it will ever get - you should also get to see a tiny sliver of a new moon hanging like an ornament above it in the not-yet-dark skies. This sliver moon is, to my mind, one of the most impressive sights to periodically grace our skies. To me, the ethereal part is not the sliver itself, looking like a razor sharp sickle glowing in the sky, but the ghostly outline of the rest of the moon that can be faintly seen.

What is that ghostly outline? If you pay close attention you might even noticed that it disappears after a few days. By the time the moon is up to first quarter all you see is that bright sunlit half of the orb. It's hard to tell precisely what is happening, because as the moon waxes towards full it gets brighter and brighter and you might just think that you're having a harder time seeing that ghostly outline in the presence of that brighter moon. But, no, the outline is indeed getting fainter.

What's going on? With a little thinking about what is illuminating the moon we can figure it out pretty easily and even make sense of the little details of when it is brighter and when fainter.

First, a few well known simple concepts. The moon goes around the earth (counter-clockwise when viewed from above the north pole), and half of it is always illuminated by the sun while the other half is not. The fact that we see sliver moons, quarter moons, and full moons is not so much because the moon is changing, as that our vantage point is changing. On those bright full moon nights we are seeing all of the illuminated side and the back is dark. When the moon is new we're seeing the unilluminated half, but if we could fly in to space to see the back side we would see it look full from there. Just like the earth, the moon always has a day side and a night side.

Now, let's assume that we're looking straight down at the north pole of the earth and that the sun is off in the distance at the 6 o'clock position. The moon is there in its counter-clockwise orbit. How can we see a full moon? First, the moon had better be in the right place. If the moon is at the 12 o'clock position, the part of the moon visible from the earth is fully illuminated by the sun, making it full. But that's not all you need; you will also need to be on a spot on the earth where you can see the moon. The best spot to be would be would be standing at the 12 o'clock position on the earth. That 12 o'clock position is in the middle of the dark side of the earth. It's midnight. If you're outside and you look up and see the moon straight overhead, you know it must be midnight.

You can also tell from this general idea when the full moon must rise and set. The earth, again viewed from above the north pole, also rotates counter-clockwise. Where are you standing when you see the moon on the horizon? At the 3 o'clock and 9 o'clock positions. But notice at these positions you can also see the sun in exactly the opposite direction. If you're standing in the 3 o'clock position and the earth is rotating counter-clockwise, though, the sun is soon going to disappear. Sunset! In the 9 o'clock position the sun is just appearing. Sunrise! The moon, when it is full, is doing just the opposite of the sun. So the full moon rises at sunset and sets at sunrise.

Let's try next Monday's first quarter moon as an example. The first quarter is after the new and before the full. Where is the moon? In the mental picture we have been painting ourselves it should be easy to see. If we see the moon only half illuminated, it must be in either the 3 or 9 o'clock positions. But we know the moon is moving counter-clockwise in its orbit and that it will be full soon, so it must be in the 3 o'clock position.

Knowing where the moon is immediately tells us when we will see it. Next week, as the sun sets, look for the moon. The sun setting means that you are standing on the 3 o'clock position on the earth. The moon will be right over head in the sky (note, though, that I'm ignoring the effects of latitude here; if you're at the north pole, the moon will never actually be overhead in the sky. So when I say overhead, you should take me to mean "as straight over head as it ever gets from where you live")

When does next week's first quarter moon rise? You will first see the first quarter moon - which is at the 3 o'clock position - when you are standing on the earth at the 6 o'clock position, directly underneath the sun. Noon. Next week, I'll remind you to see if you can go find the moon rising in the east a little after noon. To most people the appearance of the moon in the daytime sky is always a bit of a mystery. If you have ever felt this way, make next week the day it is no longer mysterious by setting out to find it by knowing where it should be.

All of this brings us back to sliver moons. How can we see just a sliver of light? We must be seeing mostly the night side, but just a tiny bit around to the day side. If the moon were at the 6 o'clock position, we would see only the night side, and it would be new moon (and we would have the possibility of an eclipse; the reason they don't happen all of the time is that the moon goes around the earth on a circle which is slightly tilted compared to where the sun is, so most of the time the circles actually don't cross. ). A few days after new moon, though, when the sun is at, say, the 5 o'clock position, we should see just the tiniest sliver of the sunlit side against a mostly dark moon.

When does a sliver moon set? If you're standing at the 3 o'clock position, the sun itself has just set, and the sliver moon is low in the sky in the same direction that the sun just set. The sliver moon is always close to the sun in the sky, so it must set soon thereafter.

But wait. What about the glow? If the sliver moon is caused by just seeing a little of the sunlit side of the moon but mostly seeing the dark side, how could there possibly be a ghostly glow coming from the dark side of the moon? The side that is glowing cannot see the sun at all. How can we see it?

The answer comes from thinking about what the earth looks like from the moon. If you were standing on the moon and the moon were full, what would you see? You would be looking at the dark half of the earth. The lights of the major cities would fill the otherwise dark void.

What if there moon were at first quarter and you were looking down? You would see half of the earth illuminated, the other half dark. The people right at the line between light and dark would be the people for whom the sun were just setting. Those people could look straight up in the sky and see you standing on the moon.

Finally, let's look at the sliver moon. When we see only a sliver of light on the moon, people standing on the moon see only a sliver of dark on the earth. The earth itself is almost full.

When the moon is full the nighttime is so bright that you can walk around the wilderness without carrying a flashlight. If you were on the moon and the earth were full, the light in the sky would be nearly 60 times brighter (the earth is both bigger and more reflective). You could read your spacesuit repair manual without carrying any lights at all. The landscape would be illuminated the same as if it were twilight on earth.

And you would see that illuminated landscape from the earth. The part of the moon that should be dark would glow in earth light. When the people on the earth see a sliver moon, the moon sees an almost full earth. The dark side glows with earthshine.

The earthshine also explains, of course, why the glow starts to fade as the moon goes from sliver to quarter. By the quarter moon, like tonight, the earth appears quarter, too. There is only half as much light to illuminate the dark side. And in another week, when the moon becomes full but the earth is new, the earthshine is totally gone.

So while the crescent sliver is light that comes from the sun, reflects off of the moon, and then arrives at the earth, the ghostly glow is light that comes from the sun, reflects off of the earth, arrives at the moon, reflects off the moon, and then arrives back at the earth. The glow is the ghost of us. Tonight, when you look up at the sliver moon, look for that glow of earthshine. And wave at it. The light from your wave will travel up to the moon and be back again 4 seconds later and you'll be part of what I think is the prettiest treats to grace our skies.

Die, Pluto, Die!
Mike Brown, astronomer and "pluto killer," welcome to the Boing Boing guest blog!