Racecars, Retrograde Motion, and the Red Planet
To our ancestors, the stars moved from East to West across the sky just like the sun, completing one crossing every night. It appeared that the heavens orbited the Earth, so it was only natural to suppose that we are the centre of the universe, and everything moves in perfect circles around us.
However, something threatened this perfect view: the planets. Although they looked like points of light just like the stars, the planets wandered in their own paths against the background sky. Sometimes, their motion even seemed to reverse direction for a while, and they’d loop around before continuing on their normal path. This is known as retrograde motion, and astronomers tried extremely hard to make it fit into their geocentric model of the universe.
Ptolemy proposed the concept of epicycles, where the planets completed orbits within their orbits, moving in smaller, looping paths on their way around the Earth. As you can imagine, this model got extremely complicated very quickly, creating layers upon layers of epicycles to explain the complication of the heavens. Astronomers realised that it wasn’t entirely accurate, because it often failed to predict the movements of the planets as precisely as it should. Nonetheless, it was the best model they had for centuries.
Cue Copernicus, who suggested a teeny adjustment to model: how about we put the Sun at the centre of the solar system, instead of the Earth?
After much grumbling, astronomers realised that this actually simplified all their complications. They could scrap the epicycles entirely, and with Kepler’s refinements—realising that planets orbit in ellipses, and not circles—we suddenly had a precise model that explained away retrograde motion.
So if planets aren’t moving in orbits within orbits, what’s happening when they loop around?
Simple physics tells us that planets further from the sun orbit more slowly than those closer to the sun. Because Mars is further out than Earth, it orbits slower. Imagine Earth and Mars are racecars, with Earth on the inside lane and Mars on the outside. Earth is travelling more quickly, so at regular intervals, it overtakes Mars on the inside and zooms on around the Sun, while Mars continues along behind, never to catch up.
The trick with retrograde motion is that it’s all in the perspective. So let’s think about what this overtaking motion looks like from Earth. At first, as Earth approaches to overtake, Mars looks like it’s moving the same way as Earth is. As a passenger in Earth’s car looks forward at Mars, they see—in a snapshot of time—Mars set against a particular part of the crowd. We’ll use a person as a landmark: say there’s a guy with a hotdog.
But Earth is moving so fast that it overtakes Mars before it reaches the hotdog guy itself. At the moment Earth overtakes, our passenger sees Mars against a different crowd—a lady draped in a flag, sitting a whole block of seats before hotdog guy. To the passenger, it seems like Mars has moved backwards a little bit, relative to the crowd.
Then, as Earth completely overtakes and zooms past, the passenger must look back at Mars. This time, they see Mars against yet another crowd: there’s a yelling toddler on her dad’s shoulders, who is sitting a whole block of seats before the flag lady. Mars has moved even further back again.
When Earth and Mars both speed on a little longer, things return to normal, because to our passenger, Mars seems to pass the yelling toddler, the flag lady and the hotdog guy, and then it’s moving forward relative to the crowd again.
Because we understand how cars move, we know that Earth and Mars are both moving forward the whole time, and it’s only the passenger in Earth’s car who thinks Mars is looping back on itself. The backward motion is only an illusion—a trick of perspective from the limited view of Earth’s passenger seat.
For the real Mars, the illusion is the same: we only think Mars is moving backwards. From Earth’s faster vantage point, we’re looking at Mars against not a different crowd, but a different set of background stars. This 2-minute video will help you visualise it.
One last thing: if Earth and Mars orbited on exactly the same plane, the retrograde motion of Mars would just be the red planet moving back and forth along a straight line. But because Mars is orbiting around the Sun at a slightly tilted angle relative to Earth’s orbit, we see this beautiful, curved looping motion that so fascinated those who came before us.
this really is a perfect example of a few things that are extremely common for Black people:
1. code switching. yes, Black people communicate and relate to one another differently than they do with White people. This is learned behavior. It has to do with combating white privilege in this country and having to assimilate in order to be labeled as socially acceptable, professional, polite, etc. Another good example of this is one I got from both a family member in the military and nilesheron (hopefully he doesnt mind me sharing this). Both talked about how they had to stop drinking Hennessy because of the way they were viewed at work functions. Both started drinking scotch or sticking to beer. Why? Because Hennessy is a “Black man’s drink” and therefore inappropriate.
2. Black communities emphasize and value familial relationships among their members. Play cousins, and play brothers, and older men sayin “what’s up baby?” when they bump into each other, and women sayin "hey sis" and President Obama embracing Kevin Durant are culturally specific practices. They are important. They are valuable. And they are one of the few areas you can look at and find pure Black culture and love.
I think in the US we tend to forget to see race as culture and neglect to look at it in this penetrating and eloquent way.
Ernest Hemingway would have died rather than have syntax. Or semicolons. I use a whole lot of half-assed semicolons; there was one of them just now; that was a semicolon after “semicolons,” and another one after “now.”
And another thing. Ernest Hemingway would have died rather than get old. And he did. He shot himself. A short sentence. Anything rather than a long sentence, a life sentence. Death sentences are short and very, very manly. Life sentences aren’t. They go on and on, all full of syntax and qualifying clauses and confusing references and getting old. And that brings up the real proof of what a mess I have made of being a man.
never loved a gif so much
this is kinda creepy tho
The story behind this painting is very interesting, though. The artist’s mother was very sick, so the house was always filled a suffocating silence. This painting depicts him wanting to scream, but having to be silent, hence the anguished expression. In the whole painting you see figures in the background, one of which is his sister.
The many pro-surveillance advocates I have debated since Snowden blew the whistle have been quick to echo [Google CEO] Eric Schmidt’s view that privacy is for people who have something to hide. But none of them would willingly give me the passwords to their email accounts, or allow video cameras in their homes.
I wouldn’t fine it far fetched at all
I had a great story recently — I love telling it — of a little girl who was in a drawing lesson. She was 6, and she was in the back, drawing. The teacher said this little girl hardly ever paid attention. In this drawing lesson, she did. And the teacher was fascinated.
She went over to her, and she said, what are you drawing?
And the girl said, I’m drawing a picture of God.
And the teacher said that nobody knows what God looks like, and the girl said, they will in a minute.
Kids will take a chance. If they don’t know, they’ll have a go… They’re not frightened of being wrong… If you’re not prepared to be wrong, you will never come up with anything original… And by the time they get to be adults, most kids have lost the capacity. They have become frightened of being wrong.
In an altogether excellent episode on the source of creativity, NPR’s TED Radio Hour revisits the most popular TED talk of all time, by Sir Ken Robinson, author of the indispensable The Element: How Finding Your Passion Changes Everything.
Half a century earlier, the great social scientist John W. Gardner made an exquisite case for what kids can teach us about risk, innovation, and the fear of failure.
Also see Debbie Millman’s indispensable Fail Safe.(via explore-blog)
One the best TED talks of all time, with a genuinely brilliant man. Has such a good sense of humor and brings a new way of seeing old problems.
The world’s 85 richest people have as much wealth as the world’s 3.5 billion poorest.
This statistic was recently released in United Nations report that uses Oxfam figures. It’s also a huge wake-up call for anyone who doesn’t consider income inequality a major issue in global politics. (via micdotcom)
(We want to think that based on our technology we live in a Renaissance, but it’s more like a Dark Age. Global Capitalism = Feudalism.)(via notational)