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Poof! You're on Earth
#1
Can you find latitude and longitude of your location on Earth without access to surface landmarks or local clocks?

The sci-fi (or sci-fantasy) situation for this question is:

A group of people have a headquarters on an alternate history Earth, accessible via appropriate gateway device. This alternate history Earth has no humans or civilization other than this HQ building, which is sitting on an anonymous patch of desert that could be anywhere from Australia to the US West. The people running the HQ aren't telling their guests where it's located on alternate Earth. 

The gateway device reaches across time in other ways; the alternate Earth might be thousands of years in a (parallel) future or parallel past. Continents are the same shape, but there could be differences in biomes. The stars may be a bit off.

Guests lack transport to travel far enough to find useful landmarks, but are welcome to bring all sorts of binoculars, portable telescopes, sextants, compasses, reference books/computers, etc.

So, for reasonably educated and equipped people, can they figure out roughly their latitude and longitude? If so, how?

It looks like you could find the latitude with the midday sun. Longitude is harder with no calibrated reference points, correct?
Mike Miller, Materials Engineer
----------------------

"Everbody's always in favor of saving Hitler's brain, but when you put it in the body of a great white shark, oh, suddenly you've gone too far." -- Professor Farnsworth, Futurama
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#2
The stars won't be too far off; in fact, using some of the fastest moving stars (especially 61 Cygni) you might be able to calculate the approximate date (within a few decades). From astronomy you could calculate the latitude as well. But longitude is not calculable without a clock, so how could we get a clock?

Having calculated the approximate date using 61 Cygni, the strandees could then search through their database of planetary movements to find a more exact date. Just by observing Jupiter, Saturn and Mars you could find a rough date, and this could be further refined by observing Venus and the Moon. To find the exact sidereal time, you could use the old method of 'lunar distances' which measures the distance between the Moon and various fixed stars.
https://en.wikipedia.org/wiki/Lunar_dist...avigation)

This would need a very good set of reference books, or maybe just an extremely accurate version of Celestia.
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#3
I've often thought that, given modern knowledge of stellar and planetary motions, it'd be "relatively easy" to construct a device which a time traveler could use to observe the night sky and determine the exact time and location anywhere on the Earth's surface so long as they're within a few thousand years of now. Beyond that, errors in our models and chaotic effects in planetary orbits and rotations (like those requiring our current use of leap seconds) would make determining the precise date and time more difficult. Of course, if stars and planets aren't in quite the same locations or orbits, that'd make it quite a bit more difficult, too.
Selden
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#4
If they can't travel very far, why do they need to know their latitude and longitude?
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#5
The error bars would get larger the further away from the present day these people are located. Each 'leap second' would represent a distance of 30 metres at the equator, so the uncertainty in longitude would increase by a few tens of metres every time a leap second needed to be added. This would mean an uncertainty of around ten nautical miles every thousand years- not too bad, assuming the rate of change remains roughly constant.
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#6
Quote:Just by observing Jupiter, Saturn and Mars you could find a rough date, and this could be further refined by observing Venus and the Moon. To find the exact sidereal time, you could use the old method of 'lunar distances' which measures the distance between the Moon and various fixed stars.

Within the limits of detection of hand held instruments, aren't the positions of the planets fairly repetitious? I mean, wouldn't it be possible for Jupiter, Saturn, and Mars to be in the same configuration in (hand wave) 2018BC as 2018CE?

Though...wait a minute...finding longitude doesn't care about the year, just time with respect to Greenwich, correct?

(04-01-2018, 07:46 AM)JohnnyYesterday Wrote: If they can't travel very far, why do they need to know their latitude and longitude?

1) Curiosity
2) Possibility of being stranded and needing to find resources for survival
3) Plot*

*After having just watched The Titan on Netflix, apparently plot can be used to excuse numerous sins against viewers' sanity and scientific education Wink
Mike Miller, Materials Engineer
----------------------

"Everbody's always in favor of saving Hitler's brain, but when you put it in the body of a great white shark, oh, suddenly you've gone too far." -- Professor Farnsworth, Futurama
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#7
(03-31-2018, 12:40 PM)Cray Wrote: Can you find latitude and longitude of your location on Earth without access to surface landmarks or local clocks?

The sci-fi (or sci-fantasy) situation for this question is:

A group of people have a headquarters on an alternate history Earth, accessible via appropriate gateway device. This alternate history Earth has no humans or civilization other than this HQ building, which is sitting on an anonymous patch of desert that could be anywhere from Australia to the US West. The people running the HQ aren't telling their guests where it's located on alternate Earth. 

The gateway device reaches across time in other ways; the alternate Earth might be thousands of years in a (parallel) future or parallel past. Continents are the same shape, but there could be differences in biomes. The stars may be a bit off.

Guests lack transport to travel far enough to find useful landmarks, but are welcome to bring all sorts of binoculars, portable telescopes, sextants, compasses, reference books/computers, etc.

So, for reasonably educated and equipped people, can they figure out roughly their latitude and longitude? If so, how?

It looks like you could find the latitude with the midday sun. Longitude is harder with no calibrated reference points, correct?

Interesting. Without being able to travel significantly, longitude is basically impossible; one has to have a (completely arbitrary) zero reference point and you just plain don't know where you are in relation to Greenwich. If you have a decently accurate clock you can set wherever you are as zero longitude for use from then on. There are methods for longitude involving moonrise and moonset times that are rather obscure; unless you have a professional astronomer in the party, you probably aren't going to be able to use them.

For latitude, the simplest solution is to wait for nightfall when the sky is clear. Even if the pole star is not the same as it is now (by no means impossible - our north pole star will be different in a time of the order of 10k years) you can find a centre of rotation of the sky. Find its altitude and you're done. With a bit of luck, there will be a star reasonably close to it. (Not the case in the southern hemisphere, in the current epoch.) The problem of whether you are north or south of the equator is also quite simple; when facing the pole, the sky turns in the opposite direction in the southern hemisphere from the north.

Even easier, there are several objects visible in the southern hemisphere that are extremely obvious. The main two are the Magellanic Clouds, and for a period of time probably well in excess of a million years they are going to be there.

BTW. over a period of perhaps 100,000 years a compass is actually unreliable; the period of pole reversals is anywhere from .1 to 1 million years. If you're unlucky, there could be no magnetic field at all. In fact, it's thought that we are just at the start of a pole reversal.

To sum up, given the desert location (thus clear skies most of the time) latitude is reasonably easy, but longitude is impossible. Mostly because it's completely arbitrary in the first place.

One more thing: Unless you are a very long way away in time, local wildlife probably gives you a clue as to roughly where you are; which continent, at least. Kangaroos put you in Australia, for example. And the soil colour is different in Australia and South Africa from SW America or the Sahara. (Or even the Gobi Desert.)

And another; a barometer is going to give you a clue. Much of SW North America is a mile up, as is a significant part of South Africa. High enough up, in fact, that the altitude causes a few problems with cooking. Australia is much lower.
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#8
Actually, at no point in a geomagnetic reversal is the magnetic field nullified. You may, however, end up with poles at the equator.

My lifelong goal: To add "near" to my "baseline" classification.

Lucid dreaming: Because who says baseline computronium can't run virches?
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#9
Sailors used to use chronometers to tell what their longitude was by comparing things like sun rise and sunset time to a clock set to GMT. The difficulty in making a clock that accurate that worked on a rocking boat was significant, leading to some really neat engineering:
https://en.wikipedia.org/wiki/Marine_chronometer
OA Wish list:
  1. DNI
  2. Internal medical system
  3. A dormbot, because domestic chores suck!
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#10
(05-03-2018, 07:26 PM)Rynn Wrote: Sailors used to use chronometers to tell what their longitude was by comparing things like sun rise and sunset time to a clock set to GMT. The difficulty in making a clock that accurate that worked on a rocking boat was significant, leading to some really neat engineering:
https://en.wikipedia.org/wiki/Marine_chronometer

Indeed. The problem with that, in this scenario, is that longitude measurement by comparing local time to GMT only works if you have a chronometer set to GMT in the first place - which, in this scenario, you don't.
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