# Tag: BAFacts

## BAFact Math: Jupiter is big enough to swallow all the rest of the planets whole

By Phil Plait | August 22, 2012 9:46 am

[BAFacts are short, tweetable astronomy/space facts that I post every day. On some occasions, they wind up needing a bit of a mathematical explanation. The math is pretty easy, and it adds a lot of coolness, which I’m passing on to you! You’re welcome.]

Today’s BAFact: Jupiter is so big you could fit every other planet in the solar system inside it with room to spare.

Volume is a tricky thing. Our brains are pretty good at judging relative linear sizes of things: this thing is twice as long as that thing, for example. But volume increases far more rapidly than linear size. Take a cube where each side is one centimeter. It has a volume of one cubic centimeter (cc). Now double the length of each side to 2 cm. It looks twice as big, but its volume goes up to 8 cc! The volume of a cube is a the length x width x height, so there you go.

Spheres are the same way: the volume increases with the cube of the radius. Specifically, volume = 4/3 x π x (radius)3. So one sphere might look slightly larger than another, but in fact have a lot more volume.

Such is the way of Jupiter. I see pictures of it compared to the other planets, and honestly Saturn looks only slightly smaller – Saturn’s radius is about 60,000 km compared to Jupiter’s 71,000. But that turns out to make a huge difference in volume!

Here’s a table I created to compare the planets. The first number column is the planet’s equatorial radius in kilometers (the biggest planets aren’t perfect spheres, but as you’ll see this doesn’t matter). The second number column is the volume in cubic km based on that radius. The third is the volume of the planet divided by the volume of Jupiter (so that ratio = 1 for Jupiter itself). The last column is the same, but rounded to two decimal places to make it easier to read.

The big conclusion here is pretty obvious when you look at that last column. Even though Saturn is only a little smaller than Jupiter, it only has 60% of the big guy’s volume! Uranus and Neptune together are only another 9%. If you combine all the planets in our solar system, they add up to only about 70% of Jupiter’s volume. That leaves a lot of room left over for all the moons and asteroids in the solar system, too!

So Jupiter really is a monster. There’s a half-joke astronomers say: The solar system consists of the Sun, Jupiter, and assorted rubble. As you can see, that’s really not that far off from the truth!

Image credit: NASA

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MORE ABOUT: BAFacts, Jupiter, math, Saturn, volume

## BAFacts: Halfway there!

By Phil Plait | July 16, 2012 10:30 am

On January 4, 2012, I posted my first BAFact: a short astronomy fact that was brief enough to put on Twitter but informative enough to be interesting. I posted the first one on perihelion – the point in Earth’s orbit when it’s closest to the Sun – and the last one will be a year later.

Because 2012 is a leap year with 366 days, July 5th was the 184th day: the first day of the second half of the year. That means I’m more than halfway done!* Appropriately enough, here’s the July 5 BAFact:

I post the BAFacts on Twitter, Google+ (where I can flesh them out a bit more – and add pictures – since there’s no character limit), and have a complete archive of them on the blog as well. With 180+ already in the bag, reading those should keep you busy for a while!

I generally link them to previous blog posts dealing with the topic in question, but not always. I’ve actually been surprised at how difficult it can be to reduce a topic to 100 or so characters (leaving room for the leading "#BAFact: " and shortened link, plus room for retweets), and how that limits some topics. I have also been surprised to find out I haven’t written about some topics! For example, I was thinking recently of making a BAFact about the nearest known black hole, Cygnus X-1, and discovered I had literally never even mentioned it in a blog post! That’s weird… but by coincidence that got fixed just this last weekend.

So this exercise in brevity has given me new things to write about. I’ll note that there have been arguments over the accuracy of some of the BAFacts, too. Sometimes that’s just due to having to be so brief that the description might be misleading if you don’t click the link; I struggle with those but usually make them as clear as possible, and hope people actually read the post to clarify. And once I really did just make a mistake; as I recently mentioned I didn’t know that recent research had found that zodiacal light is mostly from comet dust and not asteroid collisions, and had to post an immediate correction! But that’s OK; I love learning new things, too.

So as we enter the second half of these, I hope you keep up with them and enjoy them. And if you have a beef with them, find a mistake, have something to add, or know of a good picture or story relating to them, follow it up with a tweet of your own! The whole point here is to have fun and learn things. Which, when it comes to science, are exactly the same.

* Well, kinda. Perihelion is actually on January 2, 2013, roughly a day earlier than usual because we have an extra calendar day this year. The Earth orbits the Sun not caring at all for our calendrical contrivances, so when the time comes I’ll decide whether to post the last BAFact based on the Earth’s orbit our roughly-hewn measurement of it.

## Announcing BAFacts: a daily dose of sciencey fun

By Phil Plait | January 4, 2012 11:16 am

I’m happy to announce I’m rolling out a new feature: BAFacts, a short daily factoid about this strange and fun Universe we live in. Every day in the mid-afternoon GMT (in the morning for most of the US) I’ll tweet something I find interesting, cool, or gee-whizlike. They’ll all be about science, mostly space and astronomy, but really anything that catches my fancy is fair game.

Some will have links for more info (if the tweet itself is short enough to accommodate one). I’ll also post them in my Google+ stream, and I’ll include more info there when I can. I’ll use the hashtag #BAFacts to make them easy to find. I have also created a BAFact archive where I’ll list the previous BAFacts.

I started thinking about doing this months ago, and always found some reason to delay the launch. Maybe, I would think, it would be better to do it this way, or post it that way… but I decided that the best way to do something new in social media is to do it. Get it out there, and fiddle with it later if something comes up that can improve it.

So maybe I’ll figure out how to add more links, or pictures, or math, or whatever. I’m happy to take suggestions. But for now, BAFacts launches today…

And just why am I starting BAFacts today? As I wrote earlier, today is perihelion, when the Earth is closest to the Sun in its orbit. It’s something of a coincidence that it happens so close to New Year’s (according to the standard Gregorian calendar most of the planet uses these days). It’s funny: the first day of the year is pretty arbitrary when you think about it, but the point of perihelion is an actual, physical thing, not arbitrary at all. It would actually make a kind of sense to start our year on that day… except that the Earth’s orbit isn’t like a racetrack; it changes shape every year due to the influence of the other planets, so the precise time and day of perihelion changes by a day or so every year. Oh well.

Still, it’s something of a milestone in our orbit, and since it’s close to New Year’s day it’s an appropriate time to start something new. It was either today, or wait until the Vernal Equinox in March, and I didn’t want to wait that long!

I hope y’all enjoy it, and get as much of a kick reading them as I do writing them.

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