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IdahoBaird
2009-Feb-22, 09:53 PM
First, hello everyone. I'm new to this group. I found out about it at the Astronomycast site. I'm a regular listener of Frasier's and Pamela's podcast.
Here's my two-part question: Part I: If I was to suddenly wake up in a totally unfamiliar place, on Earth, could I use what I see in the night sky to figure where I was at that moment? What sort of knowledge would I need? Would having a pair of binoculars or a good telescope offer more help? If not those, what other tools would help? A sextant, perhaps? Part II: If I woke up on another "environmentally-hospitable celestial body" how could I tell where I was and what tools, if any, would help me?

Many, many thanks for contributions to this discussion.

Daryl
Sagle, ID

Seeka
2009-Feb-22, 09:56 PM
If you knew the night sky well then i would say sure you could definitely tell if you were northern or southern hemisphere :) After that i am not sure.

Whats a sextant?

IdahoBaird
2009-Feb-22, 10:26 PM
Okay, I could pin down which hemisphere I was in because some constellations, etc. are only visible in the southern hemisphere. Got it. A sextant is a device used by sailors to navigate and know their locations at sea. I don't know how to use it but I have "googled" it and see several websites to aid in learning more about it. I wonder if a sextant can be used on land. Thanks, Steffanie.

- Daryl

m74z00219
2009-Feb-22, 10:29 PM
Welcome IdahoBaird!

IB, if the sky is clear, you would need no more than the naked eye to guide you. Let's assume that you're in the northern hemisphere first. You want to find the big dipper first.

http://www.truthbook.com/images/site_images/Big_Dipper_North_Star_Polaris_Cassiopeia_450.jpg


Roughly, the two stars on the left point towards the north star (polaris). If you face this star, you are facing north. Once you know which way north is, you have a convenient "compass".

A sextant would be useful, if you had one, because you could more or less accurately measure the angular height of the north star. say, it's 45degrees above the horizon. This equates to you being on the +45 degrees (or 45 degrees north of the equator) latitude line.

As far as longitude goes...I'll leave that to someone smarter to explain (I don't know).

Hope that helps some.

m74

Tzarkoth
2009-Feb-22, 10:35 PM
Welcome to BAUT, IdahoBaird.

As Steffanie said, Northern or Southern hemisphere would be fairly easy to detect just by looking up into the night sky. If you had a sextant handy and knew how to use it, you could probably pinpoint your position on the planet down to a square kilometer. But if you were going to be using tools I recommend thanking Einstein and relying a good GPS device.

If you were somewhere else in the universe, other than Earth, beyond a few light years constellations will be almost completely unrecognizable. If you were somewhere in the Milky Way and were on friendly terms with an ET civilization at least as advanced as ours, then you could work out where you were in relation to the centre of the Galaxy and hence have a good idea of where Earth might be.

EDG
2009-Feb-22, 10:56 PM
Whats a sextant?

http://en.wikipedia.org/wiki/Sextant

IdahoBaird
2009-Feb-22, 11:36 PM
Okay, this is interesting. Seems that if I was heavy on observational skill I could tell if I was on Earth and I could approximate where I was. Now, could I get an idea of what YEAR it was? That is, allowing that I could have freely traveled forward or backward in time, would there be something in the night sky that would tell me what year it was?

Again, thanks for indulging this inquisitive noob.

-Daryl

Seeka
2009-Feb-22, 11:43 PM
Cheers for the sextant explanation:)

I don't think you could tell very well what year it was. What comes to mind is if you were on earth then i would look for Betelguese the red supergiant in Orion thats due to explode any day between today and 1,000 years time, if it's not there i know it has exploded so i must be roughly 1,000 years from 2009:D
But then it could have exploded already and we have not seen the result yet. D'oh.

Tobin Dax
2009-Feb-22, 11:59 PM
Okay, this is interesting. Seems that if I was heavy on observational skill I could tell if I was on Earth and I could approximate where I was. Now, could I get an idea of what YEAR it was? That is, allowing that I could have freely traveled forward or backward in time, would there be something in the night sky that would tell me what year it was?

Again, thanks for indulging this inquisitive noob.

-Daryl
I don't think you could without more information. Due to precession, the daily times that stars rise and set changes a little bit, but you'd have to know what time of the year it is along with the positions of stars in the sky. For short amounts of time, that could be hard to do. If it's 70 degrees out at 10 p.m. and Orion is in the sky [in the northern hemisphere], you're probably around 13,000 years before or after now. This could also affect your location approximation, making your longitude estimate incorrect.

Jeff Root
2009-Feb-23, 12:20 AM
If you have either the Astronomical Almanacs for the range of years
that you might have moved to, or equivalently and more conveniently,
a computer and software that has that data and can display the sky
for any time and place on Earth, then you have what you need.
If you are on Earth, but don't know where or when, you would start
by trying to identify constellations. If you can, you know you aren't
too far away in time. You might need to compare the positions of a
star over a period of several hours to determine whether the celestial
poles are in their familiar positions or elsewhere. Once you have found
the poles you can find the ecliptic, which shouldn't be too far away
from the celestial equator. Then you identify as many planets as you
can. If you can find Jupiter and Saturn you will be able to compare
their locations in your tables or computer program to greatly limit the
number of possible dates that you are at. Jupiter is in the same part
of the sky every 11.86 years, and Saturn is in the same part of the
sky every 29.46 years. So a combination of both could tell you what
year you were in over a very long stretch. Adding in Mars or Venus
could reduce or eliminate any possible ambiguity.

-- Jeff, in Minneapolis

Hornblower
2009-Feb-23, 01:50 AM
None of this will do you any good in finding your longitude unless you know what time it is at the reference longitude at the moment you do your star sightings. In the absence of a chronometer showing accurate Greenwich Mean Time (GMT), a good ephemeris of the Moon can be used for a rough estimate. That means knowing what year it is, and having a copy of the Astronomical Almanac or the equivalent for that year.

The Moon moves about one arcminute in two minutes of time. One arcminute probably is about the limit of measuring accuracy with a sextant. If that is the case we can determine the standard time within a couple of minutes, and thus find our longitude within about 1/2 degree, or about 30 miles in equatorial regions.

If we were content to estimate the longitude within a thousand miles or so, we could make do with rougher time estimates. Suppose we were in a vast stretch of desert somewhere along the Tropic of Cancer. If we somehow knew that our local time was about the same as GMT, that would place us in the western Sahara. If our local time was three hours later, we would be in Saudi Arabia.

If we had just awakened from a prolonged sleep like Rip Van Winkle and had no idea what year it was, our longitude would be indeterminate for all practical purposes.

IdahoBaird
2009-Feb-23, 03:39 AM
Bravo. Bravo, all! I read through this thread but once and will go back and peruse, I promise. These things take me a little while to grasp. Sometimes I have to play the Astronomycast episodes three or four times for them to start to sink in.

Again, thanks. Any Dr. Who fans among you?

- Daryl

Murphy
2009-Feb-23, 05:08 AM
I don't know much about finding your way around the Earth, but I think it would be possible to find out where you were in the universe if you found yourself on an extra terrestrial planet.

You will need; a deep knowledge of all aspects of Astronomy, catalogues of all the stars and galaxies currently know to Humanity and several powerful telescopes.

With you're telescopes, in conjunction with the star catalogue’s data, it should be possible to analyse the stars you see in the alien night's sky (i.e. discovering their distances, brightness, composition, spectrums, classes, etc.), if any of these stars match ones in the catalogue, then you will start to know where you are in the universe.

Try looking for really obvious objects first, like Giant star, Pulsars and Galaxies, if you can see the Andromeda Galaxy close by for instance, then you are probably still in our Milky way galaxy (you could confirm this if it appears to be at approximately the same distance and angle as it does from Earth). Then, if you know you're in our galaxy, try searching for well know Pulsars and giant stars that are a few thousand light-years from Earth, if you can find them, then you've got to create a 3D-star map (presumably with computer software) which should be then able to interpolate where you are taking the measurements from in relation to where they were taken the first time (i.e. on Earth).

So once you know roughly where the Sun should be, you search that area for a G2V type main sequence star matching the know properties of the Earth's Sun. If you can find it, then just set course for Home! (Assuming you have some method of interstellar travel of-course).

If, on the other hand, you cannot locate any of the stars or nearby galaxies close to Earth, then you may be in very distant part of the Universe indeed, where Human telescopes have not yet penetrated, so your catalogue data will be useless and you'll probably be searching in vain forever.

Jens
2009-Feb-23, 05:38 AM
About the initial question, where are you on earth, there is a lot of interesting history to it. Finding latitude is quite simple: all you need is a sextant and some knowledge of the stars. But longitude is tricky: it's best to have a good clock. Umberto Eco's novel The Island of the Day Before is sort of about the quest to find latitude. It's a good read too.

Jeff Root
2009-Feb-23, 05:46 AM
Murphy,

If he ended up very far outside the local group of galaxies, I think it is
unlikely that he would be able to identify anything, because everything
would look different. Both because of the different perspective, and
because of the difference in time.

Even if he just ended up on the far side of our galaxy, if he happened
to be in a place where he couldn't see the Andromeda galaxy or the
Magellenic Clouds, he might not be able to place where he is at all,
and would not be able to tell that he was still in the Milky Way.

-- Jeff, in Minneapolis

Jeff Root
2009-Feb-23, 05:55 AM
Jens,

You certainly don't need a sextant to determine latitude to, say, half a
degree. If the pole is in its usual place, and you know where the pole
is in relation to either Polaris in the north or Crux in the south, or you
take a few hours to watch the stars move so you can find the pole by
observation, getting your latitude doesn't require any instrument.

Of course, if you wake up and see Polaris directly overhead, you know
pretty quickly that you are probably in big trouble.

-- Jeff, in Minneapolis

antoniseb
2009-Feb-23, 06:03 AM
Sorry I missed the beginning of this discussion.

If you are interested in knowing where on Earth you are based on the information available in the sky, and you have no ability to make lenses or magnifying mirrors, but do have neolithic tools and skills, and the year is within ten thousand or so years of now... you can determine your latitude very quickly (within a few miles). You can determine what year it is within about 5 years (requiring several years of measurement). It is doubtful under those circumstances that you could determine longitude... However, if you did have a perfect almanac of solar and lunar eclipses, you might be able to determine your longitude as well (and the exact year).

Is this what you were asking?

لطفيّ
2009-Feb-23, 07:51 AM
The story of determining longitude is a fascinating one. There were some abortive attempts to use astronomical phenomena, like the positions of the moons of Jupiter, but it was really only well solved when clocks that could keep time accurately on a sea voyage were developed.

eburacum45
2009-Feb-23, 10:56 AM
The method for discovering longitude favoured by the Astronomer Royal, Nevil Maskelyne, back in the 18th century, was the Lunar Distance method mentioned by Hornblower; more details here
http://en.wikipedia.org/wiki/Lunar_distance_(navigation)
it could of course, be used today, but looks very tricky.

slang
2009-Feb-23, 11:34 AM
Now, could I get an idea of what YEAR it was? That is, allowing that I could have freely traveled forward or backward in time, would there be something in the night sky that would tell me what year it was?

Depending on how much data you took with you, and what kind of equipment is available, and how far you freely traveled in time, I think you might be able to pin down the time somewhat by studying proper motion of nearby stars (http://en.wikipedia.org/wiki/Proper_Motion).

chornedsnorkack
2009-Feb-23, 11:55 AM
Let´s consider questions I, II and III then.

I:
I suspect that the quickest way to figure out your latitude might be to check the constellations at the zenith. If you want to measure the height of Polaris or, worse, Sigma Octantis, you are dealing with measurement errors of a large angle. If you want to check the constellations at horizon, you need to establish the location of true north and south first. But zenith... if you identify a constellation around zenith and the star at zenith, you know your latitude, and time of clock.

Regarding longitude - no wonder it was the bigger problem for sailors. And yes, the one obvious and conspicuous way of measuring true time is the position of Moon against constellations.

II:
If you are on an extraterrestrial planet, first of all, can you observe the whole sky?

If you can, then start checking whether you are in Milky Way.

The Big Magellanic Cloud should be visible everywhere in Milky Way. It is far enough from Milky Way plane that nothing can obscure it, far enough from centre that distance from its plane is about the same anywhere in the disc, it is bright, it has distinctive shape and orientation against the Milky Way, it is accompanied by Small Cloud in the fixed relative position... you would have to be massively unlucky to have anything that could conceivably be mistaken for the Magellanic Clouds. Certainly the inner satellites of Andromeda are rather different from Magellanic Clouds. How does Triangulum look from Andromeda, or Andromeda from Triangulum?

If you really are in Milky Way, you know exactly which angular distance bands from the disc you should be seeking the Clouds from. Even if you do have something else in these bands, you should still see the Clouds as well.

And what else can you have there? Diffuse nebula? Planetary nebula? Supernova remnant? Open cluster as big and bright as Big Magellanic Cloud should resolve into stars. And a globular cluster would not have the shape of the Big Magellanic Cloud.

Once you do find the Clouds, you know which hemisphere of Galaxy is which. Can you find the centre of Milky Way simply by looking for the branching of Milky Way and the brightest part? The angle between direction to centre and to Magellanic Clouds would tell you which part of the disc you are in.

Speaking of globular clusters, they make nice yardsticks for orientation. Even if they are not awfully distinctive, they are visible from far away and are far from Milky Way where the other nebulae (open clusters, diffuse and planetary nebulae and supernova remnants) concentrate. For example, 47 Tucanae is nicely in a set with the Magellanic Clouds. If you find the Magellanic Clouds, but no 47 Tucanae in the set, you know you must be far away. You might get a wrong globular cluster visible to eye - but which ones?

And if 47 Tucanae is slightly displaced from its normal position, you can quickly measure which way the solar neighbourhood is.

Now, stars are nondescript and mostly white dots of light. They become unrecognizable some way away from Sun. But the bright stars consist of a mixture of stars that are dim and nearby and those which are bright and distant. This means that if you make a small shift, the groups of distant stars would be only slightly distorted, while the nearby stars are scattered in unrecognizable ways.

Where are groups of distant stars? Orion for one. Orion´s belt, Rigel and Betelgeuse are hundreds of ly away. They should stay conspicuous and together as an only slightly distorted group. The only possible change is getting a bright nearground star into the constellation, like Sirius which is near Orion already. But being alert to the possibility should help spotting stars that do not belong and identifying the rest, just as a constellation can also contain a planet or a few.

Also, look round for dense groups of stars. They are likely to be open clusters, and there are not very many open clusters around, so identifying one would give you good baselines. Like Pleiades... and Earth has a very nice alignment of Pleiades, Hyades and Aldebaran, so slight distortions of the pattern should give away where you have been displaced.

III:
Periods of a few centuries or millennia could best be identified by the precession. Is it Polaris an North Pole? How far does Polaris stray from a fixed point?

If you have been displaced by a few tens of thousands of years, and potentially wound up in another precession period, check proper motion of the fixed stars. Could someone, say, illustrate the shape of Dippers 26 000 years ago and 26 000 years in future?

For periods of few tens of years, Jupiter and Saturn should supply a 60 year cycle.

ngc3314
2009-Feb-23, 01:57 PM
For the time - if you have the equivalent of an almanac, the collective locations of the bright planets will pin down the time if you know the date within a few millennia.

Seeka
2009-Feb-23, 03:52 PM
Of course, if you wake up and see Polaris directly overhead, you know
pretty quickly that you are probably in big trouble.

-- Jeff, in Minneapolis

At the risk of sounding stupid, why would you be in trouble if you woke with Polaris overhead?

Nick Theodorakis
2009-Feb-23, 03:54 PM
At the risk of sounding stupid, why would you be in trouble if you woke with Polaris overhead?

Because you'll be really cold! (It means you are at the north pole, unless some time travel was involved.)

Nick

grant hutchison
2009-Feb-23, 04:02 PM
The method for discovering longitude favoured by the Astronomer Royal, Nevil Maskelyne, back in the 18th century, was the Lunar Distance method mentioned by Hornblower; more details here
http://en.wikipedia.org/wiki/Lunar_distance_(navigation) (http://en.wikipedia.org/wiki/Lunar_distance_%28navigation%29)
it could of course, be used today, but looks very tricky.Yeah, Maskelyne has suffered a bit from being portrayed as the baddie to Harrison's goodie in Dava Sobel's book Longitude. But Maskelyne's method worked considerably better than chronometers that were not hand-crafted and maintained by Harrison (ie, pretty much all of them), so even after ships were routinely equipped with chronometers, it was for a long time considered good practice to take the occasional "lunar" in order to check and reset your chronometer.

Grant Hutchison

Seeka
2009-Feb-23, 04:10 PM
Because you'll be really cold! (It means you are at the north pole, unless some time travel was involved.)

Nick

You know Nick just as i posted my question it hit me that perhaps you would be in the north pole:wall:

Thanks for the answer though:)

eburacum45
2009-Feb-23, 04:59 PM
And someone's pinched your tent...

IdahoBaird
2009-Feb-23, 06:49 PM
Okay, slightly overwhelmed but soooo pleased with the response and added discussion. I feel like it did back in college when leaving the library with both arms filled with books. I see I have some wikipedia work to do. Jeff et al, thanks for the guidance. Chornedsnorkack, bravo!

Wishing you all clear skies.

Nick Theodorakis
2009-Feb-23, 06:58 PM
Interesting related article: The Dimension of Our Galaxy:
Finding Earth from the center of the Milky Way, in a lifetime or less (http://www.astronomycorner.net/games/dimension.html), by Brian Tung.

Nick

JohnD
2009-Feb-23, 10:55 PM
Re: books on longitude.
I've never read (or heard of) Eco's book.
The rather better known one is "Longitude" by Dava Sobel, about the Harrison Chronometers.
It's a lot more entertaining than that rather dry synopsis!

John

mugaliens
2009-Feb-23, 11:06 PM
Whats a sextant?

It's an optical device used in celestial navigation (http://en.wikipedia.org/wiki/Celestial_navigation)to measure the height of a star above the horizon. By means of a very precise clock, simple addition and subtraction, a good chart (map), and using tables which provide information on the movements and positions of the Sun, Moon, planets, and stars, one can calculate one's position with reasonable precision - less than a mile for a ship, and about 3 to 5 miles for an airplane.

Of course, GPS pretty much put an end to celestial navigation, other than as an hobby. Those who do this, however, cite other reasons (http://www.celestialnavigation.net/why.html)... :)

chornedsnorkack
2009-Feb-24, 01:03 PM
Interesting related article: The Dimension of Our Galaxy:
Finding Earth from the center of the Milky Way, in a lifetime or less (http://www.astronomycorner.net/games/dimension.html), by Brian Tung.

Nick
Something which I have read, and find a number of faults with.

Starting with Andromeda:

The distance to M31, even now, isn't precisely known, but based on the best available figures, it's about 2.5 million light-years away. Since our estimated distance from the center of the Milky Way galaxy is only about 25,000 light-years, or just about 1 percent as great a distance, M31 is pretty much in the same direction in the sky, no matter where we are in the Milky Way. It therefore serves as a convenient and usable beacon for plotting the first main leg of our trip.

From the center of the Milky Way, M31 is probably not visible--it's lost in the glare of the dense galactic nucleus. We have to go "up," out of the plane of the Milky Way, in order to find it. M31, as it happens, is a bit south of the galactic plane, by about 20 degrees. From the center, we won't necessarily know which way is south, so we'll have to guess. If we guess correctly, then after travelling about 1,000 light-years or so, we'll have cleared the nucleus by enough to see M31.

If, after travelling that 1,000 light-years, we don't see M31, we have to conclude we went up on the north or "wrong" side, and sink back down to the south. By the time we get to the right point, we'll have travelled 3,000 light-years total distance. Since there's a 50-50 chance of going either way, the average distance it takes us, just to find our first beacon, is 2,000 light-years.

Since the Magellanic Clouds are about 6 times further from the centre of Milky Way than Sun, they are ALSO pretty much the same direction no matter where we are in the Milky Way. They are a bit further from the Milky Way plane than Andromeda - 30 degrees or so. And they are far brighter. If you are looking for Andromeda, it is a pretty inconspicuous, dim and fuzzy patch. You may not see it at all, lose it in the glare of the dense Milky Way or confuse it for some other nebula - dim open clusters like Praesepe, or bright globulars like 47 Tucanae, or diffuse nebulae or planetary nebulae or supernova remnants.

But the Magellanic Clouds are far brighter. They are bright and shaped enough that any other nebula seen close enough should be looking different - e. g. open clusters should resolve into stars.

Since the Magellanic Clouds are far enough from the Milky Way, chances are they can be seen through the Milky Way disc. In which case spotting the Clouds and verifying that you have the right clouds will tell you where the north and south side of Milky Way are. And since they are far from Milky Way pole, too, it also identifies the sides of the disc.