BAN #431: Mea culpa, Daisy ears, Gravity slingshots

Subscribers are, on average, closer to Earth than the planet Mercury.

Mea Culpa

Oops

Two weeks ago I wrote an article about how Mercury is the closest planet to Earth on average, even though Venus does get closer at times.

Let me be clear: This is correct. Mercury is closest if you average over time, as the links and videos in that article attest.

However, I gave a simplified example of Mercury and Venus with a diagram I drew showing how this works, and that example is — arg! — wrong.

This was pointed out to me by a friend of mine who is much smarter than me and also has a phenomenal ability to synthesize information into a bigger picture that is far better than mine.

The problem with my simplification is that I take the times when Mercury is close to us (called perigee) and assume it’s also as far from the Sun as it can be (called aphelion) — that minimizes the closest approach. That artificially decreases closest approach, so of course the numbers wind up showing it’s closer to us than Venus on average.

So my simplification is wrong, even if the general answer it gets (“Mercury is closer”) is right. It gets it right for the wrong reason. Aggravating. I didn’t think it through all the way, and when I got to the right answer I stopped. That’s a fun critical thinking fallacy, and a trap that can be difficult to avoid.

In fact this gets more complicated the more you dig into the geometry, so go back and watch the videos or read the paper I linked to for the facts. It’s still interesting to me that Mercury is closer on average, which isn’t at all obvious. The Universe is a weird place.

So, my apologies for steering y’all wrong there. I’ll hopefully do better in the future.

Unicorn Chaser

A palate cleanser (definition here)

I just realized I haven’t posted a picture of our puppy Daisy in a free BAN issue since we got her! There have been a few in the paid issues, but none here, so as another apology here is an adorable photo of her ridiculous ears.

She’s just as cute behind her head as in front.

Astro Tidbit

A brief synopsis of some interesting astronomy/science news

[To make up for my error, here’s another cool fact that I bet you’ll enjoy.]

Have you ever wondered just how we use the gravity of a planet to slingshot a space probe to the outer solar system, or inward toward Venus, Mercury, or even the Sun?

This technique is actually called a gravitational assist, and it’s a funny term. It’s not just gravity at work here! Galileo showed (by example) that an object entering the gravity of a planet will leave with the same velocity, and Newton codified it with math, so there must be more to this. And there is: velocity. The space probe steals from (or gives to) the velocity of the planet around the Sun, using that to head outward (or inward) without having to use a lot of extra fuel.

Back in 2012, when cameras had a resolution of like 5, I made a video describing this:

I bring this up now because another newsletter called First Excited State actually goes through the math. It’s technical, but the point of the newsletter is to go through an explanation using roughly first-year University level physics. Not for beginners, but not hugely advanced either. This particular one has some vector analysis in it, which you have to use to really get the math right for the slingshot. I know a bunch of folks here love this kind of math, so here you go.

I found out about the newsletter because they listed mine as a recommended read, and I appreciate that! So back atcha.

Incidentally, this concept comes up when talking about Jupiter’s reputation as a protector Earth when it comes to inbound comets, flinging them away so fewer hit us. However that reputation isn’t actually deserved, since it actually does send comets our way that would have otherwise missed us. I wrote about this on the blog a while back, in fact.

Again, Universe = weird.

Blog Jam

What I’ve recently written on the blog, ICYMI

Monday 24 May, 2022:  Stellar hypoxia may help make more massive black holes

[This one was praised by some of the scientists who did the work!]

Tuesday 25 May, 2022: Breaking down the structure and history of a nearby — and weird — galaxy

Wednesday 26 May, 2022: Scattered X-rays from space: Turning noise into signal

Thursday 27 May, 2022: A weird four-star system may lead the way to catastrophic supernovae

Friday 28 May, 2022: Companion star survives the hell fury of a supernova

Et alia

You can email me at [email protected] (though replies can take a while), and all my social media outlets are gathered together at about.me. Also, if you don’t already, please subscribe to this newsletter! And feel free to tell a friend or nine, too. Thanks!