A thoroughly sporadic column from astronomer Mike Brown on space and science, planets and dwarf planets, the sun, the moon, the stars, and the joys and frustrations of search, discovery, and life. With a family in tow. Or towing. Or perhaps in mutual orbit.

Encore: Yelping at Saints

[I've been watching the moon, which made me remember a much earlier column that almost no one read. Forgive the rerun, but watch the moon!]
If your skies have been clear for the past week you might have been noticing -- as I have been -- the slow but unstoppable growing of the moon. There's nothing new here. It does essentially the same thing every 28 days, but it is still a show worth watching.
In my backyard I see this: each night as the moon moves further and further in its circle around the earth and we see more and more of the illuminated half, the moon is getting just a little brighter. In a few days, as the moon finally goes from just-barely-not-full to finally-completely-full, the moon will finally brighten its last incremental amount and it will be its brightest of the month, though only a little brighter than it was the night before.
This gentle brightening to a muted peak sounds prosaic and reasonable. But it is not true.
I remember once being out on a backpacking trip in the wild mountains inward of the Pacific coast south of Monterey. Some friends and I had hiked all day to make it over a range and down to the bottom of a creek where a little stream of hot water poured out of the earth making a tiny pool in which to soak sore legs and shoulders. We camped a bit away from the hot pool, ate a warm dinner as the sun was going down, and finally began climbing our way to the top of the little ridge separating us from the hot spring. We didn't even bother with flashlights in the dark because the full moon had made the entire woods faintly glow -- plenty of light to get around at night even in the dark of the wilderness. As we had almost reached the top, though, somebody silently flipped a switch and a blinding spotlight was suddenly tracking us from the ridge.
This was miles away from any roads or machinery down a long windy trail, so perhaps I could have reasoned my way out of the situation given a little time for relaxation, but, in the instant, I did what I think most anyone would do when unexpectedly illuminated by a spotlight deep in the woods far from where anyone or anything should be: I yelped. Loudly.
My yelping didn't affect the spotlight, which refused to flinch. It refused to flinch, I realized an embarrassed moment later, because it was no spotlight, it was the moon. It had been hiding behind the ridge until we had gotten near the top, and as we rose over one bump it suddenly revealed itself like the flip of a switch. My credibility as a young astronomer (I had just started graduate school that year) was seriously diminished amongst the friends who had seen me frightened of the moon.
Which is to say that the full moon is really bright.
The fact that the full moon is bright is perhaps not a startling fact, but what is startling is that if I had been coming over the ridge on my way to the hot pool and I had seen the moon a day earlier or a day later, I would never have mistaken it for a spotlight.
You don't have to take my over-tired-from-hiking-all-day's impressions for it. If your skies are clear this week as the moon is finally puffing towards full, go outside and see for yourself. Go out on Wednesday, two days before the full moon, and look around. Check out the barely visible shadows. See what fuzzy shapes you can make out in the distance. Look up and notice that the moon is definitely not fully illuminated, but it is getting close.

Go out Thursday. To really do the job right you should go out an hour later than you did the night before, since the moon will have risen an hour later. Look around. You probably won't be able to tell any difference at all from the night before. Same vague shadows, same fuzzy details. And then look at the moon. Definitely bigger, but one edge is still a little flattened. Tomorrow it will indeed be full.
Finally, go out on Friday, an hour later still if you can. Stare right at the moon, if your eyes can stand it. It does look like a spotlight up there in the sky. It is much brighter than it was just the day before. Look at the now-crisp shadows on the ground and the sharp details on the rocks and the plants that you can now pick out. Now go ahead, if you need to, and let out a little bit of a yelp. I'll be understanding.
What is going on with the moon? How can it get so much brighter in just a day? Who turned on the spotlight?
In medieval paintings, saints and anyone else holy are always depicted with a halo around their heads. Unlike modern halo depictions, which look like a gold ring hovering slightly above the hat line, these medieval halos appear more like a general glow coming from behind the entire head. Whenever I see one of these glowing medieval halos I think about how bright the full moon is.
I have a hypothesis -- totally without the benefit of supporting research, necessary expertise, or, likely, even minor merit -- that the medieval painters painted halos like this because they had seen such halos around their own heads. And I know what the painters saw, because I have a halo around my head, as well.
Here's another experiment to try. Go outside on a bright sunny day and start watching your shadow. Walk along until you find a place where the shadow of your head is falling on grass. Focus on your head shadow while you continue to walk, letting the background grass blur in you vision. You will gradually notice that there is a diffuse glow around the shadow of your head. It won't be around any other part of your body, and you won't see the slightest hint around anyone else's head. Point out your halo to any else and they will see precisely the same thing: a halo around their own heads and nothing around yours. Everyone is holy to themselves.
In reality what you are seeing is not some sort of corporeal representation of your own ego or a mystical aura of self-realization, but simply a literal trick of lights and shadows. When you are looking at the shadow of your own head, you are looking, by necessity, directly in the opposite direction of the sun. Stop focusing on your glowing halo for a minute and now focus on the grass itself. You'll notice that in the region where your halo is you will not see a single dark spot due to a shadow of one blade of grass on another. There can't be any shadows; with the sun directly behind you, any piece of grass that you can see can see the sun, so it can't be in shadow. Start looking away from your head shadow and you notice that you are now starting to see collections of tiny shadows, so the overall scene gets darker and darker even though it, too, is fully illuminated by the sun. Your halo is simply the total lack of shadows that can only occur when you are looking almost exactly opposite the sun. I've seen my halo from many places, on many surfaces: on grass or rough dirt or asphalt while walking, even on the tops of a forest full of trees while looking out of the window of an airplane flying low enough right before landing that I could pick out the shadow of the fuselage and see a beautiful glowing ring around. Anywhere you have sunlight and a surface rough enough to make millions of tiny shadows you get to glow the glow of the saints.
And so it is with the moon. When you look at the full moon you are almost looking at where the shadow of you head would be. The sun, though it has set over the horizon, is directly behind you as you face the full moon. If you could see down to the surface of the moon, you wouldn't see a shadow anywhere, not in the craters, not amongst the craggy mountains, but, more importantly not even at the finest scales of the rocky dust that covers most of the surface. The next day, when the moon is just past full, the shadows will begin to reappear and the spotlight will be extinguished.
It happens every month. It's just a trick of light and shadows. But, every now and then, I still look up at the full moon and think about saints and I feel a little bit of a yelp deep inside.


  1. I read it.

  2. Beautiful post. I had always wondered about those halos, and your explanation makes perfect sense. Thank you!

  3. I always look up...now I'll have to make sure I'm looking down, too. Mike, I love this post.

    Scientifically, what's the effect between the alignment of the Sun, Earth, and Moon? I know there are supposed to be more positive ions streaming at the Earth on the lit side...negative ions on the dark side...mixed where dusk and dawn fall. But is there a web site somewhere that explains it better than Wiki...especially, the halo effect?

    My only astronomy background is trying to teach it to my kids : )


  4. Hi, Mike :)

    Pretty good article. Yesterday there were no distinct shadows, but today just as I called up this screen, sunlight penetrated the blinds. So I decided to go out and see my shadow.

    But by the time I got out, the sun had moved behind a semitransparent cloud. All the shadows were indistinct; I looked around the sky and it was all hazy, the clouds did not have disticnt borders but kind of blended into what little clear blue sky there was.

    I stayed outside for a couple of hours anyway, there was a nice breeze blowing. A couple of times the clouds cleared giving direct sunshine and I could see my halo. It was about a centimeter or two wide at a distance of about two meters. I closed one eye, turned my head, and peered at the little yellow flower next to the shadow of my glasses. This was the maximum effect I could get. It looked like a nice, friendly little flower, brighter than the others. When I moved my head, it became an ordinary flower again.

    It seems the angle at which the effect takes place is about arctan 1/100, about. Has anybody formally solved this? It seems that Kuiper belt objects would often be in that cone of observation.

    Well I hope I can observe my moon halo tonight but the prospects are pretty grim. I have, though, observed bright full moons in the past.

    How did you find that hot spring way out in the middle of nowhere? We have a hot spring in Austin called Stacy Pool, they built a swimming pool around it a long time ago. Homeless bums go there to shower. We haven't had an active volcano in this area for 65 million years so I've always wondered about that. Could radium be transported by groundwater to some rock layer that precipitates it? I don't know. I didn't have a radon detector, those are for people with houses :0


    -Mike Emmert