# How can you find where the Earth is, if you were lost near Neptune?

While on a space mission to Neptune, you accidentally broke your tether on the way. After a couple of hours floating, you were hit by an asteroid.

You passed out.

When you are conscious again, you met me. I offer to bring you back to your home with my ultra-speed spaceship.

The problem is, you don't have any pen, paper and no Pioneer plaque with you. You have to somehow tell me where to take you. I also want to visit your planet.

You know that for you to leave Solar System, you must have been unconscious for a long long time. You feel hunger, but not starving. Therefore, you should not be too far away from the place asteroid hit you.

You can see around, you can point the direction you want to go, and I guarantee that I will avoid any collisions on the way (so you don't have to worry about pointing at the right direction at a wrong time).

You need to observe your surroundings very carefully and tell me which way to go. But be careful, a wrong turn might get both of us lost.

Some clarifications

• Since your tether was broke, you only have what an astronaut brings out of space vehicle.

• You are able to show and tell, but not able to draw and explain the units.

• You need a very precise method to get back to earth. A common way to get lost is "I'll figure out how to get X when I get to Y."

• Even if you memorized the position of all planets during your mission, you have no idea where they are now because you passed out for some time.

• If you assume that you can distinguish the sun from that distance, then you also should show how (either a photo or some mathematical demonstration).

• Even if I know where the Earth is, I cannot know without proper observation. And I do not have that technology near me. Just like you don't have your technology with you to know where any planet is right now.

## More clarifications

• You cannot just say "go to Mars and we will find Earth. I have no idea what is Mars and what is Earth.
• You are near Neptune. Which means, until you provide a solid argument, I will assume you don't know where any planet is. That argument might be "I saw a picture taken by Voyager and in that photo Mars is seen like this." But I will not believe you if you say "I can 100% tell where Earth/Mars/Jupiter is." I don't want to get lost into Solar System.
• I do not have anything with me. You cannot just assume that my species invented any material for you to write. I even don't know this word. We communicate using brain waves.
• For the last time, provide exact series of actions to go to Earth. I will not accept "just go towards Sun, and it is the blue/third one." If you were in my shoes, would you waste your energy and time based on this vague description? I think not.
• You are an astronaut. Therefore, you have no pockets, no bags, nothing. This is how you Earthlings behave. We don't need no space-suit. You need. But you put no pockets on your suits. It is not my fault. So please stop telling me "it is very absurd I don't have anything with me."
• If you know where the planets are right now, then you need to prove it. You may assume that you passed out for a couple of hours or a couple of days, but you have to tell me where is your home based on your current position in space.

To sum up, stop telling me how easy it is and point me the way!

TL;DR You are lost in solar system. How can you find where the Earth is?

• Thank the nice spaceman with the FTL ship, but then say something like, "Oh, I'm not from around here. This is just a lifeless system which my civilization mines for materials. I'm from the fourth planet of the closest binary star system, but I honestly don't need a ride home. My buddies will find me pretty soon. So you can let me out here. Enjoy your visit to my planet and thanks for stopping by." Some sacrifices must be made to keep Earth safe from alien invaders. Feb 16, 2017 at 16:08
• @HenryTaylor "Mines for minerals" Oh so there are minerals here and no life to worry about? I'll let our automated giant space escalators know. They can eat and digest a whole planets worth of minerals in one go! Feb 16, 2017 at 16:13
• @AndyD273. Glad to see that there is someone out there even more paranoid than I am. There is hope for the survival of humanity! Feb 16, 2017 at 16:46
• Reminder to close-voters: The problem cannot be fixed if the OP is not made aware of it. (Also, I don't see how this is story-based...) Feb 16, 2017 at 16:57
• @Hurkyl: Earth can be (and has been) photographed from well beyond the orbit of Neptune, using '70s imaging technology: en.wikipedia.org/wiki/Pale_Blue_Dot So if you can talk to the aliens, just tell them to look around the plane of the ecliptic for a blue planet with a big moon :-) Feb 18, 2017 at 18:51

This is a simple solution rooted simply in the fact that Space is really, really big. In fact, have two solutions...

If you are hungry but not starving, you have been unconscious for a day or so, at most. Certainly less than 3-4 days, or you would have died of dehydration.

Unless the asteroid propelled you to super-luminal speeds or punted you into a wormhole, it is utterly impossible for you to be closer to any other star than you are to Sol. The nearest star to us is Proxima Centauri, at 4.25 light years away.

So, solution 1: Follow the radio garbage. We don't blast as much garbage into space as we used to, but we still toss enough out there that an extraterrestrial civilization could find us using it. In fact, we send out signals expressly for trying to say 'hi.'

But, supposing your rescuer only has short range communication system, Solution 2 is this: Step 1 "head towards the nearest star." Any ship capable of traveling through space must have the ability to tell relative distance to stars in order to navigate. And unless you were unconscious for at least 2.125 years while moving at light-speed....Sol is still the closest star.

Next, you may specify that Earth is 499 light seconds from the center of the star, situated within a 7.155 degree angle respective to the equator of the sun. This is enough to get you close-ish to Earth....though it's still a pretty large area. You might have to explain what our measurements mean.

Or you could simply explain that we are the third planet from that star, and seek it out together.

After that? Well....follow all the radio garbage, or look for the bright bluish 'star.' Earth is visible from Mars by the naked eye, so if you get within our orbital loop, you'll be able to spot it.

If you must point to get close to the star...well, with the amount of time you were unconscious...Sol will still be the brightest star in the sky. Point at that. Wait til it looks about the right size compared to how it looks from earth, then start looking for the bluish dot. And if, by some chance, another planet appeared brighter than the sun from the exact spot you started. Oops. Try again. It's not like you left the solar system or anything. From any individual planet, no other planet appears brighter than the sun. So if you stumble up to Jupiter or something...your next guess will be the right one.

• with a nod to the great Douglas Adams, ""Space is big. Really big. You just won’t believe how vastly hugely mindbogglingly big it is. I mean you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space."
– WRX
Feb 16, 2017 at 16:07
• You're neglecting the effects of time dilation; at relativistic speeds, you could end up very far away in a subjective day due to time dilation. You don't need FTL for that. That said, the speed you'd need to be going at for that kind of time dilation is still higher than is plausible for this scenario. Feb 16, 2017 at 18:17
• @user2357112 - The astronaut was accelerated by a meteor strike, and survived. He's probably not going at relativistic speeds. Feb 16, 2017 at 18:54
• This is a great answer, although for the most part it seems to be ignoring the issue of language barriers. The OP made of point of saying you don't have anything to write with or any kind of "Pioneer Plaque" indicating that how you intend to communicate things should be considered. Granted you can still point at the nearest star, and given that Earth is the only life-bearing planet, it won't take long for an interstellar-capable craft to find your home from there, but still... You talk a lot about things you could tell the alien, but how do you tell him these things? Feb 16, 2017 at 19:08
• @Steve-O: How did the alien offer to take you home? It's not really clear whether a language barrier was intended to be an issue. Feb 16, 2017 at 19:39

First thing: You were not hit by an asteroid.

Space suits have micrometeorite protection, but understand the word micro. Any meteorite/asteroid which is able to knock you out will rip you in pieces. They are moving several times faster than a bullet.

If you must be unconscious, use a faulty valve which allows too much CO2 in your system, but not enough to kill you.

Second thing: You cannot leave the solar system.

Pioneer and Voyager probes were accelerated as fast as possible and they need years to get to the outer planets. So you will always see the tiny yellow ball which is our sun. It is also the only light source (not a reflector) which is not a dot, but a ball.

Third thing: You cannot do wrong turns.

Head for the ball with high speed and photograph the background. Only the planets will visibly move, the big bright dot is Jupiter. All planets move in a plane, the ecliptic. So you only need to get between sun und planet and head in sun direction, if necessary continue the search behind the sun. Now look out for a tiny blue ball. Earth found.

This is by the way the method you can find planets in Elite Dangerous if you do not have the equipment; they did it very realistic.

Some clarifications about misconceptions:
A handgun bullet travels with approx. 1000 kph (330 mps or 1/3 kps), a rifle bullet has approx. 3000 kph (~1 kps). To circle around Earth you need 8 (!) kps, to leave Earth you need 11 kps and to leave the sun from Earth's position you need 42 (!) kps. It should also be said that once a body reaches over 5 kps, its kinetic energy is comparable to the energy of the best explosives (meaning it does not matter if it is a shell or not). While the necessary speed is much lower at Neptune's orbit, it is still several times higher than a bullet ! And every body entering the solar system will be necessarily accelerated to such speeds. So yes, an asteroid/meteor who could knock you out is deadly, no discussion. If your ship/astronaut is fast, it either get hit much worse (wrong direction) or it does not get hit at all (right direction). That the speed cancels out that only a tiny, hurtful fraction remains is astronomically unlikely (you do not know the component of its speed apart from solar acceleration).

Second, the gravity well of the sun is massive. It has thousand times the reach of the Kuiper Belt so it really does not matter that we start from Neptune. As the highest rated answer says so wonderfully: Space is big.

• I am highly suspicious if an asteroid or comet can only be as slow as a bullet, which is approx. 2500 kph. Second, I never claimed that you are out of solar system. You can surely go towards the sun, but how long will you go? is the question. Feb 16, 2017 at 21:12
• Note about your second thing: Pioneer and Voyager started from Earth. This unfortunate astronaut is starting from Neptune. Feb 16, 2017 at 21:20
• Being hit on the head by an apple did wonders for Isaac Newton. Being hit on the head by a relativistic asteroid should bootstrap you considerably higher! The way home should be trivially explained as one of the solutions to the Grand Unified Theory. ;-)
– SRM
Feb 16, 2017 at 23:27
• I'd disagree that you would necessarily be ripped to shreds getting hit in space. Comets, asteroids, and meteors may be going faster than a speeding bullet in their orbits relative to the Sun, but if you're outside of a space ship which is traveling between planets, you would surely be traveling extremely fast, too. If you were very, very lucky, it might just be like getting hit by a fast ball. Feb 17, 2017 at 1:00
• Not the eclipse: the ecliptic - en.wikipedia.org/wiki/Ecliptic Feb 17, 2017 at 10:57

Given the clarification in comments that this isn't meant to be about the language barrier, it's not terribly hard, even without being able to translate units (as the question specifies).

The question says a "mission to Neptune", but doesn't specify that I actually arrived at Neptune before the accident. So I'm assuming that I might be anywhere between Earth and Neptune, plus the (negligible on this scale) distance of no more than a week's drifting (given "hungry but not starving").

Finding the Sun

The Sun will be clearly distinguishable by its brightness.

Even at Neptune's orbit, while the Sun's disk is probably not distinguishable, the Sun will be vastly brighter than any other astronomical object.

(By the inverse square law, the Sun at 30 AU will be 1 / 900 of its brightness at Earth. This is still hundreds of times brighter than the Full Moon seen from Earth for example...)

Finding planets

The next step is to ask to be taken near the Sun. Without units, I can't specify a precise distance, but I can say to keep going until I say the Sun looks large enough. (This doesn't need any instruments to determine, as I know the Sun's apparent size at Earth will fit behind the tip of my thumb when held at arm's length; so I just wait until it's significantly larger than that.)

This gets me inside the Earth's orbit, which does two things for me:

• I don't have to worry about phase effects;
• I know I'm relatively near the Earth, within 1 AU plus my distance from the Sun.

The combination of these things means the Earth will be very bright - brighter than any real star. The constellations will look the same from anywhere in the Solar System, so the next step is to find Sirius (this is easy because it's next to Orion, which is very recognizable). Anything brighter than that has to be either Mercury, Venus, Earth, Mars, or Jupiter.

Distinguishing the Earth

The Earth will probably be easily visually distinguishable from the others by spotting the Moon - it ought to be visible from anywhere in this region (it will always be 'full' phase, since I'm closer to the sun. The Moon is a quarter million miles from Earth. At 200 million miles away, farther than I can be if I'm closer to the sun, that's 800 times the distance or 1 / 640,000 times the brightness. That sounds tiny, but 1/640,000 of a full Moon is still brighter than magnitude 2 - a relatively bright naked-eye star.)

Fallbacks

However, what if I'm incredibly unlucky? The Moon could happen to be behind the Earth or so close it can't be distinguished, or another planet could be right next to a reasonably bright star (so I see two pairs that could plausibly be Earth and Moon).

No visible pairs: Ask to be taken maybe 1/4 of the way around the Sun (the exact arc length isn't important, so units aren't necessary, this can be demonstrated with hands) and look at all the plausible candidates (really bright things) again. This should change the angle of view.

Two visible pairs: Check the colors of the brighter member of both pairs - if it's red (or orange/pink), it's Mars, so the other one is Earth.

If neither is reddish, it gets trickier.

-- For the Earth and Moon, there should be a quite obvious difference in brightness between the two members of the pair. The Earth is more reflective than the Moon, and it's about 4 times the diameter, so 16 times the reflecting area. This is a difference of several magnitudes at least.

-- The two members of the pair should be quite close. If neither object looks like a disk rather than a star*, they should be within a degree or so. They should both fit behind a thumb tip at arm's length, or at least be very close to doing so.

*If you can distinguish a disk, even a tiny one, you should be able to clearly tell whether it's Earth or not by color (specifically the presence of both very bright, white clouds and relatively dark, bluish seas -- Mars is reddish, Mercury has nothing that could be taken for clouds, Venus and Jupiter are all clouds).

I'm assuming I do not have access to a gravity based compass. That leaves visual navigation. I Look for the brightest point around you, all 360 degrees. here is the problem. I am far enough out that the Sun will be a very small bright spot, and I also run the risk that another planet of some sort with sufficient albedo might actually be brighter.

What's left? Prayer and Charades. I mime the sun then the orbit of each planet. Stress the third planet and then sequentially mime each planet's orbit until I get to Neptune, stressing Neptune's orbit. I then mime a line going from me back to the third orbital ring. I Hope like hell this makes sense to the visitor. The Visitor, if he has managed to navigate the gulf between stars, should be able to get the concepts of the relative orbits, will have a gravity compass or some such, and should be able to extrapolate from that that there is only one planet in the goldilocks zone in the Sol system and that I therefore should come from there. Of course this would be soooo much easier with a bablefish.....

• It could be just your bad luck that the alien hates miming with a homicidal fury. But, probably, not if it's so nice as to pick up a wandering spaceman. Feb 17, 2017 at 1:34
• In my defense, in space, you can't apply mime makeup and the space suit hides the black and white striped shirt and beret Feb 17, 2017 at 13:58

Reverse your trajectory. if you were hit by an asteroid - and somehow not pulverized - you are basically a cue ball on a pool table. Have the aliens go back along the vector you were on when they picked you up. That should get you back to within a few floating hours of where you started, which should be negligible by space travel standards. You can likely easily demonstrate this to the aliens with a simple demonstration.

• except your table is an orbital plane around the sun, which makes all trajectories rather complicated Feb 16, 2017 at 22:46
• No, it really isn't. Its literally orbital mechanics 101. They wouldn't be in space if they haven't already mastered it. Plus, if he's close enough that a planet has enough impact on his trajectory over such a short period, he already knows where he is - at that planet. Feb 16, 2017 at 23:06
• orbital mechanics are slightly more complicated for most people (xkcd.com/1356) Feb 17, 2017 at 2:29
• Luckily, our adventurer here is running into a space-faring race, not 'most people' from contemporary Earth.. Feb 17, 2017 at 4:26
• pilots don't necessarily understand fully how their ships work (cf. Sirens of Titans partly about that) Feb 19, 2017 at 1:23

Ok, first some assumptions...

1. You're alive and travelled in a Newtonian rather than Relativistic fashion
2. Your rescuer possesses, at the very least, the ability to operate an interstellar ship.
3. Your rescuer has sensory organs providing sight or a similar function.

Assumption 1 restricts the travel to an area within our Solar neighbourhood. I doubt you'd survive long enough to even reach the Oort clouds.

Assumption 2 provides that the rescuer understands the concept of a planet and understands that human-like life can't develop on anything other than planets in the Goldilocks zone, can calculate where that zone is, and identified the number of planetary bodies around Sol in the process of travelling here. Since we can do all that pre-FTL, a post-FTL civilisation would be able to as well, and also the most probably reason for a visit to Sol is to look at a potential life supporting planet.

Assumption 3 provides that the alien can see you moving and identify simple motions such as waving your arms.

So...

1. Point at me
2. Point at alien ship
3. Point at sky... I don't think you'd actually be able to single out the Sun from Neptune but the sky should be just fine
4. Mime a large circle in the air to convey that you want to go to a planet or describe an orbit
5. Tap something 3 times to count out which one I want to go to.

If he/she/it is smart enough to master FTL, that should be sufficient hint.

Go with the constellations

Ok so you don't have paper, but you can pee on the floor or something.

Take me to the place where I can see the big dipper, the little dipper, Orions belt, and Leo, Vergo, etc etc what ever you can remember.

You will need at least 3-4 to triangulate the position, but the more the merrier.

The FTL ship would then need to take the constellations and map them to get a pretty good guess.

Once your in the neighborhood, turn on the radio and follow the oldies station the rest of the way home.

• i do not think that pee will drop down to the floor in the space Feb 16, 2017 at 21:34
• The constellations will look more or less the same from any point in the solar system. I doubt you'd visually notice the differences. Feb 16, 2017 at 21:37
• Yeah, but "Once your in the neighborhood, turn on the radio and follow the oldies station the rest of the way home." Feb 16, 2017 at 22:22
• @padawan I was assuming the FTL nice dude had gravity of some kind. If not, then I suppose you can use #2. Feb 16, 2017 at 22:23
• I also assumed that "out of the solar system" meant far enough away our sun is a tiny spec and not just the one big honking giant thing in the viewer. Feb 16, 2017 at 22:26

Since, in previous answers, the respective authors took the liberty to modify the parameters in which the question was made, here's my take on it.

Why not take advantage of that situation. It's has all the conditions for a long and exciting adventure, between two potential friends, one helping the other find his/her home.

Good writing! ;)

I have no idea where the planets are around the sun. But I would kinda assume an astronaut might.

If I was taking a trip I would expect to have glanced at a map. (Possibly when trying to line up the radio to talk to earth) If it was a long trip I might even look out the window now and again and note some constellations and how they arrange themselves relative to the map.

Or if your spaceman isn't into science or looking at stuff; Astrology has that information per-compiled. I think we are in the sign of Pisces so I find its constellation and the Earth will be between that and the sun. And if our astrologer knew which constellation Neptune (it doesn't change very fast) was in it would be as simple as finding it and pointing backward.

Earth is 100 about solar diameters from the sun, but that doesn't help unless I have an idea how far out I am or I can communicate enough that the point is moot.

## Follow the Galactica.

But not all the way. It's going to veer off course to lead the Cylons away from Earth.

## Or give them the location

Tell them 93 million miles from the sun, third rock, between Venus and Mars, and planar with most of the other planets in the system. They should find it in one loop. It will help a lot if you can give them the units of measure in kilometers. They should already know it because everyone uses the Metric system. That will spare you taking your shoes off to explain what a mile is. If it helps, a kilometer is 1/40,000 of Earth's circumference.

If you find Mars, look for a functioning satellite and follow where the antennas are pointed.

Assuming your just barely out side the solar system, and not "far away". Then you just need the third thingy from the sun. The only one with H2O.

So,

O . . o . . . . . .

should work. Like my previous answer your don't need paper to make marks, just write on the walls with what you have on hand.

You can also do

O - OOOO - O
OOOO


Or

+-----E-E----------+
|  +-------------+ |
E  E  NPNPNPNP   E E
|  |  NPNPNPNP   | |
|  +-------------+ |
+------E-E---------+


Were also the only planet in the solar system with life on it, so you could go that route.

In The Dish, the Aussies found the Apollo 11, which they'd lost track of, by looking near the moon. In this scenario, looking near the sun will work well.

If you know that you are somewhere near (within 1 light-year) of the Sun, then you can do this. The Earth is at distance of 215 Sun-radiuses from the Sun.

You can then identify the 12 star-sign constellations, then you can say that the Earth will be in a straight-line between the correct constellation and the Sun. Remember, that the constellation is overhead at midnight (and thus the one you want to pick), 4 months before the assigned birth month. Thus for December, you want to find Gemini or Taurus (and not Sagittarius).