What is the Draconitic Cycle?


In my reading I just came across a mention that the moon has something called the draconitic cycle, or sometimes, the draconitic period (or “draconic” if someone is getting all tense about “draconitic” as a word). The term is a bit of a holdover from medieval astronomy, when everybody was worried all the time that an eclipse might sneak up on them, and so you can see the use in a term which represents how long it takes between successive passages of the moon through its ascending node. Really, it’s amazing they thought they needed to name that at all; surely the idea of successive passages through the ascending node is so common that it barely needs a word, the way we express such concepts as “       ” or “      ” or even “        ” by leaving a gap hanging in our conversations and just waving our arms frantically at people who don’t know what we’re going on about. Anyway, it picked up the name “draonitic cycle” just as you might expect, by astronomers watching the skies night after night to see how long it takes the Moon to be run over by a bicycle, which took until about 1890. Before then bicycles were just those hilarious things with a giant front wheel and a tiny back one that you could ride for parts of a second before tipping over and falling down. The cycles are much quicker these days.

Did We Need Spaceships All *This* Fast, Actually?


So now the space probe Juno’s gone and swung past the Earth, building up a little extra speed on its way to Jupiter and becoming the fastest man-made object that isn’t just trying to escape something embarrassing it said in an online forum, so I hope nobody’s left on it anything they wanted back anytime soon. These planetary flybys are really neat ways of getting a space probe to travel faster even though you can explain why it works to a bunch of freshman physics majors and they’ll still stare at you the way a Labrador retriever stares at the glass coffee table hoping that maybe this time the potato chip you tossed on it will fall through.

If it isn’t going fast enough by this time, though, it’s possible it’s going to go even faster than that. Back in 1990 when Galileo (the space probe) went flying past Earth on its way to Jupiter it got a whole bucket full of extra speed, but it turns out it got about four millimeters per second more than it was supposed to. Maybe that doesn’t sound like too much, since it was already going at 13,740,000 millimeters per second, but when you get down into the grit of the numbers you realize: this isn’t even that much.

But nobody was quite sure where it came from, as the satellite was launched before they had the E-ZPass lanes where you don’t even have to slow down at the toll booths. So in 1992 when Galileo went flying past the Earth again on its way to Jupiter (it was supposed to do that, so this wasn’t just Jupiter being fickle and pretending not to be there) NASA watched very closely and the probe didn’t do anything funny at all except for sticking out its tongue and making a sound which experts still dispute, as they can’t settle whether it was said “nyah-nyah” or “nanny-nanny-boo-boo” or “this is Andy Griffith for the Mutual Radio Theater” (a short-lived 1980 project to revive scripted network radio programming in the United States), but they’re pretty sure it wasn’t that last one.

This is obviously an extremely tiny anomaly in a phenomenon very difficult to precisely measure, or as New Scientist probably billed it, a fundamental challenge to our understanding of physics and a potential revolution in interacting with the world, except for those of us who interact with it using only pointed sticks or sarcasm. But it all could’ve been a mistake, maybe someone failing to keep track of how many millimeters per second they had in petty cash or something, and this only got more interesting in its way when the Near Earth Asteroid Rendezvous, flying by Earth on the way to not flying by Jupiter, got an extra 13 millimeters per second. Obviously, space probes were getting greedy. When Rosetta, which flew past Earth three times over to try getting to a comet, it got a lousy 1.8 millimeters per second the first time around, nothing on the second, and on the third left two and a quarter millimeters per second on the moon just to spite us.

What all this naturally reminds everyone of once they’re reminded about it is the Pioneer Anomaly, where Pioneer 10 and 11 were found to be travelling aster than they were … er … they were accelerating more slowly than … they were accelerating to outer space more than … I’m not sure what it was they were doing, but they were doing it for an awfully long time until someone went back and checked very carefully and, to the delight of popular science magazines the world over, discovered they hadn’t been doing anything funny at all and we should feel bad for suspecting them of it. That’s why in 2012 NASA launched an emergency expedition to send the Pioneer spacecraft some special apology editions of New Scientist, which are going to use these cracks in spacetime that might totally exist and prove the world’s actually a computer simulation of itself to get there sooner.

So overall I’m interested knowing there’s these anomalous millimeters per second being thrown around, since knowing how space probes do it would probably help next time I need a teensy little bit of extra speed and am going to Jupiter.