28 September 2020 Issue #257
|Phil Plait||Sep 28|| 5|
[Spiral Galaxy M81 image credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona]
Subscribers keep me gravitationally bound in their Hill spheres.
As Dave Barry said, “Poli” = many and “tics” = blood-sucking parasites
I remember watching an episode of a crime drama TV show called “The Streets of San Francisco” when I was younger. The episode was called “Dead Air” (it took me a little while to find this info online). Larry Hagman was a guest star, playing a popular radio advice guy who took calls from women about romance and their needs. The ladies loved him, as he was supportive and attentive to their calls.
I don’t remember the main plot at all, but what I do remember, vividly, is that this character was a dick, just a really awful and duplicitous guy, who hated the women he took calls from every day to help. At the end of the show, we see him in the booth, the phone lit up with calls from women waiting to talk to him. But, while off the air during a commercial, he launches into a long, sneering rant talking about how terrible these women were, how shallow and beneath his contempt they were.
What he didn’t know is that the sound engineer surreptitiously switches him to being on-air during the diatribe. He realizes this, and looks down in a panic at the phone, and we see the “on hold” lights flick off, one by one.
I remember thinking how great it was that when people saw who he truly was, saw through the façade, they dumped him like moldy bread.
Too bad it was only fiction. My, how the world has changed.
A brief synopsis of some interesting astronomy/science news
My friends at Astronomers Without Borders have started a new website devoted to reviews of astronomical equipment for amateur astronomers and astrophotographers. Called AstroGear Today, they review telescopes, eyepieces, camera lenses, and more. The reviews are done by experienced skygazers and cover a lot of ground. Um, sky.
They also feature observing tips, beginners’ guides, and citizen science projects. They just got started a few weeks ago, so if you don’t see what you need there drop them a note… and for writers: They take pitches.
I have this bookmarked and I hope you’ll do that too. It’s already useful, and it’ll get even more so as it grows with time!
Oh: They’re on Twitter, too.
Astro Tidbit II
Yes, another one
If you woke me up cold out of a deep sleep and asked me which planet in the solar system had the largest Hill sphere, I’d say, “How did you get in my bedroom?”
But then after that I’d say “Neptune”, and I’d be right.
This came up recently when I was writing about a pair of Neptune’s moons, which do a fun little dance to keep from crashing into each other every time they pass each other. I had to look up some data about them, and wound up on Wikipedia’s page on Neptune’s moons.
I found what I needed there, but as I looked over the table of the moons’ physical and orbital characteristics I was shocked to see that two of the distant moons orbited Neptune at a distance of more than 45 million kilometers.
Holy crap. That’s a long, long way. The Moon orbits the Earth at less than 400,000 km, for comparison. Heck, Mercury orbits the Sun at about 58 million km, so these moons are on that same sort of orbital scale.
After three or four seconds staring at those numbers in the table, I literally then thought, Oh, well OK, that’s because Neptune’s Hill sphere is obviously huge.
Just in case you were curious about how my brain works.
So what’s all this then? Simply put, if you have, say, a planet orbiting a star, the planet’s Hill sphere is the volume of space around it where its gravity dominates over the star. It depends on the star’s and planet’s masses, and their distance. If you have a moon orbiting a planet, it pretty much has to be inside the Hill radius or else the star’s gravity will yank on it hard enough to dislodge it.
Close in to a star the Hill sphere for a planet is small, because the star is right there, and can yank pretty hard. Farther out and even a small planet can have a big Hill sphere, because the star’s gravity is much weaker.
Neptune is both massive (17 times Earth’s mass) and far from the Sun (4.5 billion km), so its Hill sphere is enormous, bigger than any other planet. Now there are complications, like if another planet is nearby, but even then Neptune wins. Uranus never gets closer than 1.4 billion km, so Neptune really is the rear guard of the solar system.
[The Hill spheres of the planets (and a few smaller worlds). Note this is logarithmic, so differences are larger than they appear; Neptune’s is by far the largest. Credit: Fuhghettaboutit / Wikipedia]
Oh, and this is also why it’s hard for moons to have moons. The nearby planet makes that pretty difficult, though not necessarily impossible. Moons tend to have small Hill spheres.
It’s funny what concepts follow from math. You can apply a little math to the physics you know, and suddenly you find a relatively simple concept for how far out a planet can hold on to a moon without losing it. It’s not exact, but it doesn’t need to be; it’s a concept, not a definition. The Universe is pretty loosey-goosey about a lot of things, where “close enough” suffices to understand something. Sure, you dive into the precise gravitational interaction of all the planets and moons and show that the Hill sphere changes as the planetary configuration changes with time, but that’s not necessary to “get” the idea.
I try to keep that in mind in life, too. I don’t always need to be exact when good enough is, well, good enough.
I could say more, but hey… you get the idea.
What I’ve recently written on the blog, ICYMI
[The International Space Station passes in front of Mars in a composite series of frames from a very carefully planned video taken in San Diego by Thomas Glenn using off-the-shelf equipment. From Thursday’s article. Credit: Thomas Glenn]
Monday 21 September, 2020: Saturn's moon Enceladus shows fresh ice: More geysers on the tiny iceworld?
Tuesday 22 September, 2020: Cosmic flowers bloom as stars begin to die
Wednesday 23 September, 2020: The ring of material around M87’s supermassive black hole is changing with time
Thursday 24 September, 2020: Incredible video: The International Space Station transits Mars!
Friday 25 September, 2020: So, um, maybe the Sun will eventually swallow the Earth. Bummer.
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