At first glance, it might appear that if, for example, Pluto were at both *aphelion and in conjunction, we might not have found out about it for another 40 or so years, by which time it might not have been considered a ‘real’ planet. Or maybe not. Where was it actually located?
I’m guessing that if the math works out, that would have last happened @1865, so it was well past that point in 1930.
Its perihelion was a few decades ago, it wasn’t all that long ago that it was discovered, and its orbit is centuries long, so it would have been relatively close to perihelion when discovered. Conjunction would depend much more on Earth’s orbit than on Pluto’s: If it’s at conjunction at one time, it’s at opposition six months later, so that’s no real barrier to discovery.
Also note that it would have been harder to discover near opposition, as well: Even though it’s closest to us then, it’s only by a couple of AU, and even though it’s at full phase, it never gets very far from full. On the other hand, a planet’s motion against the background stars is minimized near opposition (in fact, right at opposition it’ll appear to be moving the other way), and the motion against the background is the only thing that makes it noticeable at all.
Pluto is a distant planet. It gets close to the sun and farther away than most other planets (that is to say the variation percentage wise is greater). Until 1999 it was actually closer to the sun than Neptune (which is the same year our moon was blasted out of orbit BTW).
But that variation is NOT THAT great. Without doing the calcs my educated WAG is 30 percent brighter/fainter.
Pluto was not that hard to detect brightness wise. It was detected with a rather modest sized telescope IIRC. Once it was found, any place worth a darn that might possibly call itself an observatory could detect it. Heck, many amateur astronomers with back yard telescopes could see it.
The big trick, was finding it and figuring out that is what is was (and even that was basically a one man operation both funding wise and operationally wise).
I’m curious about this. Where in the sky was Pluto when it was discovered? I’ve had no luck finding this.
It was in Gemini when Tombaugh discovered it.
Please explain, I could swear I saw it last night.
That’s just the prop left over from the faked Apollo moon landings.
Seriously, it looks like a flat piece of cardboard.
What the hell—I’ve got a few minutes before dinner, let’s do the calculations.
At aphelion, Pluto is about 49 AU from the Sun; at perihelion, it’s about 30 AU from the Sun. So the amount of light it gets changes by a factor of 49[sup]2[/sup]/30[sup]2[/sup] ≈ 2.66. The light reflected from the surface then has to travel back to us, almost the same distance as it travelled from the Sun to Pluto. So the overall change in luminosity of Pluto between aphelion and perihelion is this factor squared, which works out to about a factor of 7. This corresponds to a change of about 2 stellar magnitudes over the course of the Plutonian year, which is honestly more than I expected.
Note that this doesn’t take into account any changes in albedo during Pluto’s year, which may or may not affect the amount of light reflected. And, of course, the actual observations might be easier or harder depending on Pluto’s motion across the sky.
Jesus, disco music was everywhere in the mid-Seventies, even though everybody knew it was awful.
Stranger
I brelieve he’s riffing on Space 1999.
You got me on that one. And like you, that’s more than I expected factor wise (I was probably thinking of something else astronomy- brightness variation wise).
But, to reiterate my previous point. Pluto was not barely detectable brightness wise.
The bigger problem was it was a point of light in MANY points of light.
To use an bastardized analogy it was a variable needle in a haystack. You can find a needle in a haystack. Whether is it a rather largish needle or a smallish needle isn’t the issue.
It turns out “we” started looking when it on the larger side vs the smaller side.
The more interesting question when I think about it for a moment, is the part of the sky it was/coulda been in.
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Some parts of the sky Pluto passes through over its orbit have a much lower density of stars per “area of sky” than others. It would be harder to detect and or easier to miss in the crowded parts than the non crowded parts.
Those details I do not know and am to lazy to research at the moment.
Well, yes. It’s about the best part of 35 years since I last read Tombaugh and Moore’s highly recommendable Out of the Darkness (and it’s a book I’d loved to have had a copy to hand these last few weeks), but Tombaugh was utterly aware of the difficulties if Pluto had fallen within the Milky Way during the time he was looking for it.
I think another issue was that Pluto’s orbit wasn’t where you’d normally expect planets to be. The original eight planets orbit on what’s essentially a plane; Pluto’s orbit is tilted off of that plane.
That begs the question, what was its position relative to the plane of the ecliptic when it was found?
It seems like it was right in the plane of the ecliptic.
You can go here. Set the date to February 1930 and Heliocentric Lat. to 0
Remember, they only found Pluto because they had (mistaken) reasons to believe that there was another planet out there, acting like a planet. So they were looking where that planet should be.
I wonder how big a role that played on it being found at that time, since it would had been 120 something years since that last happened, and back then telescopes probably weren’t good enough to spot it.
It may have played a role. Tombaugh arrived at Lowell Observatory early in 1929. By September, he discarded Lowell’s predictions of where Planet X might be as useless and began a thorough search of the sky. He exposed plates in the constellation of Gemini, where, on Feb 18, 1930, he discovered the planet. Exposing plates in Gemini suggests to me that he followed the ecliptic. Apart from this, he was lucky - he had only looked at a small part of the sky before finding it. If Pluto had not happened to be on (or near) the ecliptic at the time, who knows?
Go here to see an online demo of the 2 actual photographic plates from 1930 flashing one after the other and pick out Pluto yourself. It also turns out that other plates (from other telescopes as well) had imaged Pluto unknowingly, with the earliest being in 1909.
Here’s a related question: Pluto certainly appears to be shooting away its own weight in nitrogen every couple of years and I’m not sure how that’s possible. Did it capture a nitrogen-ice comet at some point recently and that’s how such a small planetoid can lose such massive amounts of a particular gas in such a measurably short time?