Is Pluto a planet, or not? Maybe you caught the news last month that Illinois, birthplace of Clyde Tombaugh, has declared Pluto a planet. It joins New Mexico, Tombaugh's longtime home, which made a similar declaration two years ago.
When I first heard about the New Mexico resolution, I was told that they had declared that Pluto would be a planet within the state's boundaries. That made me a bit curious: would Pluto even fit inside New Mexico? I looked it up: Pluto has a diameter of 2300km, while New Mexico is about 550km in longitude and a bit more in latitude. Not even close (see Figure 1). Too bad -- I liked the image of Pluto and Charon coming to visit and hang out with friends. Though at Pluto's orbital velocity (it takes it just under 248 years to complete its 18 billion kilometer orbit, meaning an average speed of 23 million km/year or 63,000 km/day) and its current distance of about 32 AU (4.8 billion km), it whould take it about 207 years to get here.
But it turns out that's not what the resolution said anyway. Both states' resolutions said roughly the same thing:
BE IT RESOLVED BY THE LEGISLATURE OF THE STATE OF NEW MEXICO that, as Pluto passes overhead through New Mexico's excellent night skies, it be declared a planet and that March 13, 2007 be declared "Pluto Planet Day" at the legislature.
RESOLVED, BY THE SENATE OF THE NINETY-SIXTH GENERAL ASSEMBLY OF THE STATE OF ILLINOIS, that as Pluto passes overhead through Illinois' night skies, that it be reestablished with full planetary status, and that March 13, 2009 be declared "Pluto Day" in the State of Illinois in honor of the date its discovery was announced in 1930.
So the law applies to anyone (though it's probably not enforceable outside state boundaries) -- but only when Pluto is overhead in New Mexico or Illinois.
But wait -- does Pluto ever actually pass overhead in those states?
New Mexico stretches from 31.2 to about 37 degrees latitude, while Illinois spans 36.9 to 42.4. Right now Pluto is in Sagittarius, with a declination of -17° 41'; there's no way anyone in the US is going to see it directly overhead this year. Worse, it's on its way even farther south. It won't cross into the northern hemisphere until the beginning of 2111. But how far north will it go?
My first thought was to add Pluto's inclination -- 17.15 degrees, very high compared to other planets -- to the 23 degrees of the ecliptic to get 40.4°. Way far north -- no problem in either state! But unfortunately it's not as simple as that.
It turns out that when Pluto gets to its maximum north inclination, it's in Bootes (bet you didn't know Bootes was a constellation of the zodiac, did you? It's that 17° inclination that puts Pluto just past the Virgo border). That'll happen in February of 2228.
But in the Virgo/Bootes region, the ecliptic is 8° south of the equator, not 23° north. So we don't get to add 23 and 17; in fact, Pluto's declination will only be about 7.3° north. That's no help!
To find the time when Pluto gets as far north as it's going to get, you have to combine the declination of the ecliptic and the angle of Pluto above the ecliptic. The online JPL HORIZONS simulator is very helpful for running data like that over long periods -- much easier than plugging dates into a planetarium program. HORIZONS told me that Pluto's maximum northern declination, 23.5°, will happen in spring of 2193.
Unfortunately, 23.5° isn't far enough north to be overhead even from Las Cruces, NM. So Pluto, sadly, will never be overhead from either New Mexico or Illinois, and thus by the text of the two measures, it will never be a planet.
With that in mind, I'm asking you to support my campaign to persuade
the governments of Ecuador and Hawaii to pass resolutions similar to
the New Mexico and Illinois ones. Please give generously -- and hurry,
because we need your support before April 1!
[ 20:09 Apr 01, 2009 More science/astro | permalink to this entry | ]