Teena Meets The Eclipses

“Don’t look up until it suddenly gets really dark, Teena.  I’ll tell you when it’s time.”

“OK, Uncle Sy.  Oooo, look at the house where our tree makes a shadow!  It’s all over crescents!”

“Yep, wherever leaves overlap to make a pinhole, it’s like the one we made in our cardboard.  See, those crescents are just like the one our pinhole beams onto the sidewalk.”

“Yeah.  ‘Cause it’s the same Sun, right?”

“Sure is.”

“Are other little kids seeing the eclipse all over the world?  They’ve got the same Sun, too.”

“No, just the ones who happen to be on the shadow stripe that the Moon paints on the Earth.”

“How many kids is that?”

“Hard to tell.  Some families live where the shadow passes through, some families travel to be there, lots of other families just stay where they are.  No-one knows how many of each.  But we can make some not-very-good guesses.”

“The crescent’s going so slow.  Let’s make guesses while we’re waiting.”

“OK.  Let’s start by imagining that all the world’s people are spread evenly over the land and sea.”

“Even on the ocean?  Like everyone has a little boat?”

“Yep, and sleds or whatever on polar ice, people everywhere.  In our city there are eight blocks to a mile, so if we spread out the people there’d be one person every other block.”

“Every other block.  Like just on the black squares on our checker board.”

“Uh-huh.  The Moon’s shadow today will be a circle about 80 miles across and it’ll travel about 2500 miles across the whole country.  The stripe it paints would cover about 6½ million spread-out people.  Maybe 10 million if you count the people in little boats, ’cause the eclipse starts and ends over the ocean.”Local eclipses

“Lots of people.”

“Yes, but only about one person out of every thousand people in the world.”

“We’re pretty lucky then, huh?”

“Oh, yeah.”

“Are there eclipses on other planets?”

“Of a sort, but only for planets that have a moon.  Poor Mercury and Venus don’t have moons so they never see an eclipse.”

“Aww. … Wait — you said ‘of a sort.’  Are there different kinds of eclipses?”

“You’re very alert this morning.  And yes, there are.  Two that get the publicity and two that we never see on Earth.  It has to do with perspective.”

“Per … perspec…?”

“Perspective.  The word originally meant very careful looking but it’s come to be about how things look from a particular point of view.  See that tree across the street?”


“Think your hand is bigger than the tree?”

“Of course not.  I climb that tree.”

“OK, put your hand between your eyes and the tree.”

“Oh!  My hand covers the whole tree!”

“Yup.  Nearer things look big and farther things look small.  That’s perspective.  Eclipses are all about perspective.”How big is the Sun

“How come?”

“The perspective principle works in the Solar System, too.  If you were to travel from Earth to Mars to Jupiter and so on, the Sun would look smaller at each planet.”

“Like the far-away trees look smaller than the close trees.  But what does that have to do with eclipses?”

“A planet gets an eclipse when one of its moons comes between it and the Sun.  That’s what’s happening right now here.  Our Moon is moving between us and the Sun and blocking its light.”

“But I don’t see the Moon, just the carved-out piece.”

“That’s because we’re looking at the unlit side of the Moon.  It’s so dim compared to the rest of the sky.  Anyway, the Moon’s width we see is just about the same as the Sun’s width.  The moons on the other planets don’t match up that well.  On Mars, for instance, its moon Phobos appears less than half the width of the Sun even though the Sun appears only 2/3 as wide as we see it.  Phobos can never cover the Sun entirely, so no true eclipse, just a transit.”

“Can the planet’s moon be bigger?”

“Sure.  On Jupiter, Europa’s width completely blocks out the Sun.  That’s called an occultation.  You can look up now.  Jupiter people can never see that corona.”

“Oooooo, so pretty.  We’re lucky, aren’t we?”

“In more ways than you know, sweetie.”

~~ Rich Olcott

The Luck o’ The (insert nationality here)

“Afternoon, Al.  What’s the ruckus in the back room?”

“Afternoon, Sy.  That’s the Astronomy crew and their weekly post-seminar coffee-and-critique session.  This time, though, they brought their own beer.  You know I don’t have a beer license, just coffee, right?  Could you go over there and tell ’em to keep it covered so I don’t get busted?”

“Sure, Al.  … Afternoon, folks.  What’s all the happy?”

“Hey, Sy, welcome to the party.  Trappist beer, straight from Belgium!”

“Don’t mind if I do, Cathleen, but Al sure would like for you to put that carton under the table.  Makes him nervous.”

“Sure, no problem.”

“Thanks.  I gather your seminar was about the new seven-planet system.  How in the world do the Trappists connect to that story?”

“Patriotism.  The find was announced by a team from Belgium’s University of Liege.  They’ve built a pair of robotic telescopes tailored for seeking out rocks and comets local to our Solar System.  Exoplanets, too.  Astronomers love tying catchy acronyms to their projects.  This group’s proudly Belgian so they called their robots TRAnsiting Planets and Planetesimals Small Telescopes, ergo TRAPPIST, to honor the country’s 14 monasteries.  And their beer.  Mainly the beer, I’ll bet.”

“So the planets are a Belgian discovery?”

“Well, the lead investigator, Michaël Gillon, is at Liege, and so are half-a-dozen of his collaborators.  Their initial funding came from the Belgian government.  But by the time the second paper came out, the one that claimed a full seven planets spanning a new flavor of Goldilocks Zone, they’d pulled in support and telescope time from over a dozen other countries — USA, India, UK, France, Morocco, Saudi Arabia… the list goes on.  So it’s Belgian mostly but not only.”

“I love international science.  Next question — I see the planets are listed as TRAPPIST-1b, TRAPPIST-1c, and so on up to TRAPPIST-1h.  What happened to TRAPPIST-1a?”

“Rules of nomenclature, Sy.  TRAPPIST-1a is the star itself.  Actually, the star already had a formal name, which I just happen to have written down in my seminar notes somewhere … here it is, 2MASS J23062928 – 0502285.  You can see why TRAPPIST-1 is more popular.”

“I’m not even going to ask how that other name unwinds.  So what was the seminar topic this week?”

7 planets
TRAPPIST-1’s planets,
drawn to scale against their star. The
green ones are in the Goldilocks Zone.

“The low probability for us ever noticing those planets blocking the star’s light.”

“I’d think seeing a star winking on and off like it’s sending Morse code would attract attention.”

“That’s not close to what it was doing.  It’s all about the scale.  You know those cartoons that show planets together with their host sun?”

(showing her my smartphone) “Like this one?”

“Yeah.  It’s a lie.”

“How is it lying?”

“It pretends they’re all right next to the star.   7 planets perspectiveThis image is a little better.”  (showing me her phone)  “This artist at least tried to build in some perspective.  Even in this tiny solar system, about 1/500 the radius of ours, the star’s distance to each planet is hundreds to a thousand times the size of the planet.  You just can’t show planets AND their orbits together in a linear diagram.  Now, think about how small these planets are compared to their sun.”

“Aaaa-hah!   When there’s an eclipse, only a small fraction of the light is blocked.”

“That’s part of it.  Each eclipse (we call them transits) dims the measured brightness by only a percent or so.  But it’s worse than that.”

eclipses“How so?”

“All those orbits lie in a single plane.  We can’t see the transits unless our position lines up with that plane.  If we’re as little as 1½° out of the plane, we miss them.  But it’s worse than that.”

“How so?”

“During a transit, each planet casts a conical shadow that defines a patch in TRAPPIST-1’s sky.  You can tile TRAPPIST-1’s sky with about 150,000  patches that size.  There’s one chance in 150,000 of being in the right patch to see that 1% dimming.  In our sky there are over 6×1015 patches the size of TRAPPIST-1h’s orbit.  The team had to inspect the just right patch to find it.”

“With odds like that, no wonder TRAPPIST uses robots.”


~~ Rich Olcott