Pluto's fifth moon, reading German, Duolingo founder's IAMA

Three things to note today, just like yesterday:

-- A fifth moon around Pluto has been discovered by the Hubble Space Telescope. While the smallest of the lot, having a full five moons (and easily more than five at this rate) is extremely interesting. This new one is some 5 to 13 km in diameter, making it maybe about a third the size of the asteroid 433 Eros. Finding out as much about the Pluto system as we can is very important right now as the trajectory of New Horizons depends on where in the system is deemed to be least hazardous. A hit with a single pebble would be fatal.

Expect a lot more debate about the definition of a planet, though a fifth moon doesn't have anything to do with it. I remain of the opinion that hydrostatic equilibrium should be enough, and silliness about clearing a planet's orbit is not practical when applied to extrasolar planets. We could keep the dwarf label but simply have it blend into the background. In very general terms we have gas giant planets, terrestrial planets and dwarf planets, and the first part of the term should not be the one we fixate on any more than we do with humans.

-- Reading German: this textbook on reading German looks to be very good. Notice how it starts right at the beginning (chapter 1, page 16) with identifying cognates, which I believe should be listed first in most German textbooks for English speakers. Next time I have a few days to spare I'd like to read the whole thing through once and see what other goodness lies inside.

-- The founder of Duolingo is doing an AMA (ask me anything) on Reddit right now. I came in late and asked about European Portuguese. With 700+ comments at the moment I'm not confident I'll get a response.