Tag: crepuscular rays

A ray of coincidence

By Phil Plait | July 11, 2012 2:32 pm

I love a good coincidence!

Yesterday, I posted this for the daily #BAFact:

It says, "#BAFact: Face east just after sunset. That dark band across the horizon? The Earth’s shadow on the sky." It also has a link to an article I wrote that mentions this shadow of the Earth on the sky. It’s called the Belt of Venus, and I wrote a somewhat longer explanation on my Google+ page.

So last night, the same day I posted that, I’m coming home from shopping. My wife is driving, and I glance out my window, to the east. Guess what I saw?

Yup. I snapped this with my phone (which is why the foreground is motion blurred). That dark band on the horizon is the shadow of the Earth itself being cast on the sky. Well, technically, cast on junk in the sky like haze. Also, technically, those are the shadows of the Rocky Mountains to our west, but they still count as being the Earth.

Also, there were clouds that night to the west also casting shadows and letting rays of light through. Those are called crepuscular rays – one of my all-time favorite terms. They can actually stretch all the way across the sky and reach the horizon again. A funny thing: those rays are parallel! Perspective makes them appear to diverge away from the sunset, like railroad tracks appear to converge in the distance. But they also reconverge on the point on the horizon directly opposite the Sun. That’s what you’re seeing here; the brightest one is coming down from the upper left.

Even better: tomorrow I’m heading out to Comic Con, where I will see my pal Gail Simone. She’s a comic book creator, one of the best. She likes putting scientific and skeptical quotes in her books, and in 2007 she quoted me… about crepuscular rays!

COINCIDENCE??? Well, yeah.

You may take this as a giant reminder: always keep perspective on coincidence. It’s like a ray of hope.

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CATEGORIZED UNDER: Cool stuff, Pretty pictures, Science

Crepuscular rays are parallel!

By Phil Plait | November 2, 2011 7:00 am

When you go outside at sunset, many times you’ll be greeted with spectacular rays of light and shadow stretching across the sky. These are called crepuscular rays, and are caused by clouds blocking the sunlight, their long shadows cast on haze and other particulates floating in our air.

Those rays fan out, spreading away at different angles… but that’s an illusion! The rays are parallel, and I offer this photograph as proof:

[Click to penumbrenate.]

That shot was taken on October 18, 2011, by an astronaut on board the International Space Station as it passed over India. Towering cumulonimbus clouds threw their long shadows back, away from the Sun. Note that the shadows from different clouds are parallel to each other! That’s because the Sun is very far away compared to the distance between the clouds.

Here’s a picture I found on Flickr showing what we see from the ground, though (it’s not of the same clouds, but just a typical display of crepuscular rays). The fanning out of the rays is actually an illusion, caused by perspective! It’s precisely the same thing that makes railroad tracks or long roads appear to converge in the distance. Things farther away look smaller, so the parallel rails of a railroad track appear to get closer together as you look farther away. For railroad tracks you look down to see this; for cloud shadows you look up! Other than that, they’re the same.

So why do the shadows in the first picture look parallel? It’s because the astronaut was looking straight down on the clouds and shadows, so his distance to any part of the shadow was roughly the same; the shadow near the cloud and way downstream (so to speak) were both about the same distance away from him. That negates the perspective effect, and the shadows are revealed for what they truly are: parallel.

Astronauts have said it for years, but it bears repeating: exploring space gives you perspective. And in this case, it’s literally true.

Image credit: NASA; Elsie, Esq.’s Flickr Stream

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CATEGORIZED UNDER: Pretty pictures, Science

A sun pillar gooses the sky

By Phil Plait | December 28, 2009 7:18 am

Regular readers know I’m fascinated with sky phenomena. On December 26, 2009, around 4:30 p.m. local time, I happened to look out my office window and saw a gorgeous beam of orange light shooting straight up into the sky out of the sunset. I rushed outside with my camera and got this shot of it:

[Click to embiggen.]

Isn’t that pretty? I think it’s a sun pillar, a vertical column of light caused by the setting sun’s light being reflected off of the flat surfaces of hexagonal ice crystals in the air. Given that it was several degrees below freezing when I shot this, ice crystals in the air isn’t a totally crazy idea.

However, I can’t be sure this isn’t just a run-of-the-mill crepuscular ray, just those normal beams of light you see coming from the setting Sun as it’s broken up by clouds near the horizon. But it’s rare to see a single ray like that, and much rarer to see it beaming straight up like that.

As I was taking this shot, I heard honking coming from the north that I instantly knew was coming from geese. I waited a moment, and then hundreds of geese in formation flew right into the shot! Awesome, and very scenic. I was able to get some pictures, but had to stop down the camera to get the geese, and that introduced noise (the speckly grainy look in digital images) into the sky. I fiddled with the contrast a bit to show the geese, and the picture still looks a little "grainy" but cool:

There were several V-formations of geese this size going by over the course of a minute or two, and during that time the pillar faded, as you can see in this shot. Still, what a sight!

This is the kind of thing you see when you open your eyes and look up. If we could take every human on the planet one at a time, hold their hand, and simply show them this, then I just bet this world would be a better place.

Life is beautiful. You just have to notice it.


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