Category: Space

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One of the things I find fascinating about the Tunguska and Chelyabinsk impacts is that in one case it took decades of scientific research and multiple theories to settle on what probably caused it, while in the other we have video footage and the actual meteorite.

But there were eyewitnesses to Tunguska despite its remoteness, and somehow I’d never read their reports before.

As for the debate about what caused the Tunguska event: it was clearly something from space, but no one has ever found an impact crater or an actual meteorite, just damaged forest. Plus the scientific expeditions weren’t carried out until years later. Current consensus is that it was a meteor, but it exploded in the air before impact, causing the visible fireball across the sky, intense heat, shock waves, atmospheric disturbances and so on but no crater.

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Huge swaths of orange in a night sky view that wraps around the constellation Orion.
Photo by Andrew Klinger via Astronomy Picture of the Day

The first time I saw a picture of Barnard’s Loop (the arc running through Orion), I was astonished at the scale of it in the sky. I always had it in my head that (aside from the Milky Way, anyway), most of the astronomical features we see in photos are not just too faint but too small to see with the naked eye. The fact that I could instantly see the scale, because it wraps through a constellation I know, really drove home the fact that a lot of features don’t need telescopes, just long exposures. (And the right film/sensors and filters.)

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An orange ring in the blackness of space

How cool is it that we now have an actual image of the event horizon of a black hole! More precisely: it’s the glowing accretion disc of matter falling into the black hole, and the event horizon’s silhouette.

The Event Horizon Telescope, actually a worldwide array of telescopes, used interferometry to effectively create a planet-sized telescope to see the light around the supermassive black hole at the center of M87, a galaxy 55 million light years away.

I remember talking with a college classmate about giant interferometry telescopes back in the late 1990s. It’s incredible to see the technique actually making discoveries like this!

What we’re seeing in this image isn’t a top-down view of the accretion disc, but an angled one — think of Saturn’s rings — and the gravity of the black hole is bending the light from the disc. Phil Plait has a great article on the science behind the image. Katie Mack has a Twitter thread on how the image was produced, why the ring looks the way it does (it tells us which direction the disc is spinning!), plus simulations of this type of black hole seen from different viewing angles.

Here’s a paper talking about the history of black hole images, with a detailed discussion of what you should expect for the “shadow” image we’ve just seen. Check Fig 12, with renderings of shadows for disks at different angles arxiv.org

4x4 grid of black circles with red/yellow outlines, with bright and dark spots in the outlines.

— Katie Mack (@AstroKatie) April 10, 2019

Direct links to the articles she mentions:

And this Mastodon thread by @SohKamYung@mstdn.io collects some more articles worth checking out:

Forbes makes the excellent point that, while we’d seen a lot of circumstantial evidence for black holes over the last few decades, this confirms that they exist.

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Full moon, mostly red except for a whiter edge at the upper left.

The evening was hectic, and I almost forgot. I had literally just put my son to bed when I remembered, “The eclipse!” We went out to see if the sky was clear.

Clouds were rushing across the sky, but for the most part, it was clear, and we had a perfect view of the moon looking like a dark brown chunk of rock in the sky.

Update: It wasn’t quite this red to the eye, it was more of a deep brown, maybe slightly brick red. Probably a matter of retina sensitivity vs. camera sensors.

(Then I spent 10 minutes fighting with camera settings while he went back to bed.)

Update: I went back out about an hour later to check out the view as the moon left the earth’s shadow, and caught these two photos, taken about the same time with different exposures so that you can see either the lit portion of the moon, or the part that’s still in the earth’s shadow.

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I woke up way too early to see if the Super Blue Blood Moon* eclipse would be visible or blocked by clouds. (You never know, and I didn’t want to wake up the kiddo in the middle of a school night if there wasn’t anything to see.) I had a clear view, but the street lights were too bright to see the red color. It just looked dull brown.

So I took a couple of pictures, then went back in to wake up the kid. He still wanted to go out and see, but only for one look. Totally understandable. I carried him out, we looked at the darkened moon, then I carried him back in and put him back to bed.

It’s the third lunar eclipse he’s seen, though one of them we didn’t get to see much of since it was so cloudy. And while it’s not as cool as a total solar eclipse, it’s something you can see with no special equipment by walking out into your front yard anywhere in the world that has a view of the moon during the several hours it takes the Earth’s shadow to move across it.

I went back out one last time to try for some photos of totality, but they didn’t come out any better than the ones I took the first time. I looked around for a spot that might be darker and still have a view of the moon (without trespassing in someone else’s back yard), but didn’t see one — there are way too many lights at night these days. Then I tried to place the constellations I could see, and failed. Then I went back in to go back to bed.

Unfortunately I don’t think I really got back to sleep, so I’ve been dragging all morning. The interruption didn’t help the kiddo either, so getting him to school was a bit of a challenge.

*Blood moon: lunar eclipse, Earth’s shadow darkening the moon & turning it red. Cool to see, widely visible.
Super moon: full moon during closest point in orbit, looks slightly bigger.
Blue moon: 2nd full moon in a calendar month, otherwise no difference.