With the naked eye, can one identify a comet that might impact earth?

[toadspittle]

Say you're the Professor on Gilligan's Island. You have a thorough knowledge of orbital mechanics, but don't have a telescope or computer. One day in the sky, you spot a comet. Over many months, you track and record its movements, through naked-eye observations, as it appears to approach the sun and later swing back out. Could you determine by these observations alone that it might be going to impact earth? What if you had the Skipper's sextant and your trusty slide rule?

Or would it be so obvious at some point ("Boy, that thing looks BIIIIIGGGG...") that anyone could tell it was going to hit? If so, how much advance notice would the Professor be able to have over his dimwitted fellow castaways? ("Hey, Professor--why are you climbing to the top of the mountain?" "Never mind, Gilligan. But if the tide suddenly goes way out to sea, be sure to walk out and pick up some pretty shells, hmm?")

[dracoi]

The orbits of the planets and some comets (Halley's, for example) were laid out using only the instruments you describe, and they were pretty darn accurate. There's a lot of math that goes into this and the math is limited in accuracy only by the data.

With only the naked eye, you would simply draw each night's sky, and note the distance from one star to another using whatever measurements you could make. (For example, you could hold a ruler up and measure distance from nearby stars). I'm not sure of the limits of accuracy on this, but clearly it worked well.

As for knowing whether it will hit Earth... even a small error becomes significant. Let's say you could eyeball it and calculate the orbit with only a 1% margin of error. Well... the Earth is about 150 million km away from the sun. So your 1% error translates into +/- 1.5 million km. The Earth itself has a 6,400 km radius. So your error margin is about 230 times larger than the Earth, and (ignoring Earth's gravity) that translates into something like 1 in 50,000 odds of a collision, even with 99% accuracy.

(One other limitation: when the comet is coming toward us from the sun side, it would only be visible on the day side of the planet, probably briefly at sunset/rise like Mercury and Venus. This would limit the number of reference points you could measure it next to.)

[CalMeacham]

Eventually.

[Anne Neville]

Originally Posted by dracoi

With only the naked eye, you would simply draw each night's sky, and note the distance from one star to another using whatever measurements you could make. (For example, you could hold a ruler up and measure distance from nearby stars). I'm not sure of the limits of accuracy on this, but clearly it worked well.

Tycho Brahe, who made some of the best pre-telescopic observations, generally managed to get within 1 arc minute with planetary data. Of course, even if he didn't have a telescope, he did have equipment much more elaborate than holding a ruler up to the sky.

Edmund Halley, when he predicted that Comet Halley would return, only predicted the year of its next return.

[billfish678]

My WAG

Predict an impact with any degree of certainty? Particularly months in advance? Pretty much no.

Predicting way in advance that the comet is going to get pretty darn close and that there is some very small chance it might actually hit the bulls eye? Yes IMO.

You could probably do it just knowing a few things. Knowing the simple rules of planetary (and cometary) motion. Knowing how to draw circles, ellipses, and parabola's (not necessarilly even the math for it, just the tricks for how to draw them by hand).

You probably wouldnt even need the fancy math "real" astronomers would use to predict an impact.

If you gave me a really big drawing surface, I think I could draw out what was going to happen where the accuracy of my eyeball observations was as much of a problem as drawing accuracy.

And depending on the encounter geometry, I think there might be some other visual/geometry clues in the few days/weeks before the encounter that might hint further between darn close and really darn close. Note that I am not as sure about this point. And this is still IMO not going to predict an impact with much reliability.

The last few days might give some hint of it being really really close, but even then the prediction accuracy would probably only be a few percent.

[Triskadecamus]

You mean like, What the fuck is . .

Sure.

Tris

[Chronos]

In principle, you could do it with as few as three observations. For best results, though, you'd want significantly more. And you're not going to make the discovery, at least, with the naked eye: The Professor would probably hear about the possible impact on the radio months before he even knew it was there.

[rbroome]

Originally Posted by CalMeacham

Eventually.

I think the key word is might. No doubt one could spot a comet that is going to strike the earth, but while it is far enough out to still be in doubt, it depends.

[qazwart]

Well, the 1910 pass of Halley's comet panicked a lot of people, but mainly because astronomers already knew the Earth was going to pass through its tail. People were worried about the gasses from the tail poisoning the Earth.

There were actually pictures of the comet as it passed by the Earth. Here's one taken from Fifth Avenue and Broadway.

Okay, it's taken in Gary Indiana and not New York.

But you can see even though Halleys comet passed close to the Earth, it didn't look a whole lot bigger than normal.

[qazwart]

Well, the 1910 pass of Halley's comet panicked a lot of people, but mainly because astronomers already knew the Earth was going to pass through its tail. People were worried about the gasses from the tail poisoning the Earth.

There were actually pictures of the comet as it passed by the Earth. Here's one taken from Fifth Avenue and Broadway.

Okay, it's taken in Gary Indiana and not New York. But you can see even though Halleys comet passed close to the Earth, it didn't look a whole lot bigger than normal.

I'd say, "Yes, you could see a comet with the naked eye that will strike the Earth." But no, without some really good instruments, you wouldn't be able to say whether or not it will hit the earth until it casts a shadow.

Besides, if you were on Gilligan's island, there wouldn't much you could do except run around at really high speeds while Yakety Sax was playing into the commercial break.

[sittininlab]

Okay, if observing comets by eye is possible, and given that many people were looking up in the early 1900's, does that mean the the Tunguska event (http://en.wikipedia.org/wiki/Tunguska_event_ is more likely caused by something that doesn't reflect as much light as a comet? Like a meterorite?

[Chronos]

No, the Tunguska event was probably caused by a (relatively) very small object. Even if it was a comet, it might very well not have been visible until it reached the atmosphere.

[Anne Neville]

Originally Posted by sittininlab

something that doesn't reflect as much light as a comet? Like a meterorite?

Comets don't all reflect a lot of light. Some comets emit gases and dust when they get near the Sun, and those gases and dust reflect light. The gas and dust spread out quite a bit from the comet. A comet's coma (a cloud of stuff surrounding the nucleus of the comet) can be quite literally larger in diameter than the Sun. The comet nucleus is a lot smaller than that, of course (Hale-Bopp is a large comet, and its nucleus is estimated to be about 60 km in diameter).

This is important because some comets produce a coma and tail, but others don't. When a comet produces a coma and tail, it does so by having material melted off of it. That material doesn't get replaced, and if a comet comes near the Sun too often, it runs out. The comet doesn't just dissipate at the point when it can't produce a coma and a tail any more- there's still stuff left. Comet Encke isn't generally visible to the naked eye when it comes around (it has a period of 3 years), but it's estimated to be about 4.8 kilometers in diameter. That's quite big enough to ruin your whole day if it hit the Earth.

[toadspittle]

Originally Posted by qazwart

There were actually pictures of the comet as it passed by the Earth. Here's one taken from Fifth Avenue and Broadway.

Are you sure that's a real photo of the comet? Because it looks nothing like the other photos of the 1910 pass. It looks to me like it was painted on the negative.

[Chronos]

Now that I look at it, that Gary, IN photo doesn't look like any comet photograph I've seen.

[Ludovic]

Originally Posted by toadspittle

Boy, that thing looks BIIIIIGGGG...

TWSS!

[Zebra]

Originally Posted by qazwart

There were actually pictures of the comet as it passed by the Earth. Here's one taken from Fifth Avenue and Broadway.

Photoshop. Well, not with actual photoshop but you could alter photos even back then. I think all the stars are fake, the flag is fake and the lamps on the right seem odd as well.

[Nametag]

Yeah, that's painted; I don't think it's even possible for a comet's tail to look like that. I agree with Zebra about the stars and lamps, too. And the flag! Bright red and blue? at night, in 1910?

[Der Trihs]

Originally Posted by Nametag

Yeah, that's painted; I don't think it's even possible for a comet's tail to look like that.

Here's a comet that looks rather similar in shape. But yes, the Halley's picture looks painted.

[qazwart]

Originally Posted by toadspittle

Are you sure that's a real photo of the comet? Because it looks nothing like the other photos of the 1910 pass. It looks to me like it was painted on the negative.

The picture I linked to was the third one returned by your Google Image query, and I did a similar query too. I selected that picture because it was the only one that showed a landscape which gives you an idea of perspective of the comet's actual size.

Is it a real picture? Of course it is! I found it on the Internet.

You actually might be right about it being painted on a negative, but I'm assuming that the post card is showing more or less what the comet did you like in the night sky.

[Chronos]

I selected that picture because it was the only one that showed a landscape which gives you an idea of perspective of the comet's actual size.

You can't use terrestrial objects to give a scale to astronomical objects in a picture, because it depends on how close the camera is to the foreground object and what the zoom is. You have to either compare it to other astronomical objects, or to a phenomenon like a rainbow that always has the same angular size.

[j_sum1]

Back to the maths. Don't you also need to record the exact time of the observations? I'm not sure that would have been real easy on Gilligan's island.

[Chronos]

You do need times, but you can make your observation at any time that you can know. Observations made three nights in a row, at the moment when Sirius rises each night, for instance, would work fine. Or more realistically, every night for a month at that time.

You would still need ephemeris tables from a book, but this being the Professor, I think we can assume that he's memorized all of them.

[toadspittle]

Originally Posted by Chronos

You would still need ephemeris tables from a book, but this being the Professor, I think we can assume that he's memorized all of them.