Best astronomy post of the year – where halos around the head come from and why the full moon is so bright

full moon by *L*u*z*A*

Yelping at Saints
[Via Boing Boing]

If your December evening skies have been clear recently you probably can’t help but have noticed the slow growing of the moon as it has risen from being for a twilight sliver almost two weeks ago, to a half-illuminated disk passing Jupiter to an almost-full orb rising only an hour or two before the sun sets. There’s nothing new here. It does essentially the same thing every 28 days, but it is still a show worth watching.

On Tuesday, 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.


As he discusses, each of us carries a halo around our heads every day. We only have to take time to see it.

We do not spend as much time outside at night as we used to or we would know how much brighter the moon is when it is full. People before TV and indoor plumbing knew.

The idea that our personal halos and the bright full moon are linked is one of the things that makes science such a wonderful endeavor.

It is also wonderful to read a scientist who can weave such an amazing story.

And the really cool thing is that there will a lunar eclipse this month, fully viewable in North America. So watch the eclipse* and then notice how really bright it is afterwards.

*Of course, living in the Pacific NW, it is highly unlikely that the sky will be clear enough to see anything but I can hope.