Time travel only works if you plot the movement of planets, stars and galaxies. For a time machine to stay in one spot on Earth as it travels through time, it must also follow the Earth's trajectory through space.
This instructable lists a few, but not all of the mathematical projections you will need to take into account before you set off on your trip across time.

## Step 1: First, You Need to Know Where You Are Going.

For instance, it's May 9th 2008, you want to go back to Nov 4 2007.

If you just pop back in time, without moving in space, you'll wind up on the other side of the sun from where you want to be. Due to the Earth being in a different place on Nov 14th

The Earth spins counterclockwise at approximately 1675km per hour, looking down from the north pole.
The distance from the Earth to the Sun changes due to it being in an Elliptical orbit. The Average distance is 149.6 million km with the Perihelion (shortest distance) being on January 3rd 147,300,000 Km, and the Aphelion (longest distance) being 152,100,000 km on July 4th.

You'll need to find a way to move through the Suns gravity well to the point where the Earth was before.

## Step 2: Then You Need to Know Where Our Solar System Has Moved.

Then the difficulty increases when you look into how the solar system moves in our galaxy.
As our solar system orbits the galaxy at a speed of almost 220km per second, it moves in a sine wave that has a 64 million year cycle.

So the entire solar system on November 14th 2007 would be over 3,364,416,000 Kilometers from where it was on May 9, 2008.

## Step 3: Where Has the Galaxy Gone?

To further complicate things, The Milky Way Galaxy is even moving.
From Wikipedia;
Many astronomers believe the Milky Way is moving at approximately 600 km per second relative to the observed locations of other nearby galaxies. Most recent estimates range from 130 km/s to 1,000 km/s. If the Galaxy is moving at 600 km/s, Earth travels 51.84 million km per day, or more than 18.9 billion km per year, about 4.5 times its closest distance from Pluto. The Galaxy is thought to be moving towards the constellation Hydra, and may someday become a close-knit member of the Virgo cluster of galaxies.

Another reference frame is provided by the Cosmic microwave background (CMB). The Milky Way is moving at around 552 km/s[33] with respect to the photons of the CMB. This can be observed by satellites such as COBE and WMAP as a dipole contribution to the CMB, as photons in equilibrium at the CMB frame get blue-shifted in the direction of the motion and red-shifted in the opposite direction.

## Step 4: So Before You Set the Date in Your De Loren and Take Off.

So in order to land on the same spot on Earth at any time in the future or past you would have to be able to map the movements of the Earth while taking into account all this spatial movement from the rest of the universe. Otherwise you will pop up in the right time, but in empty space, the sun, or in solid rock.

These calculations would show you where you need to move to, but you'd still have to find a way to move through the time barrier so that you and the Earth would meet up.

If you think of how this would actually work, you would actually be able to move anywhere in the Universe, in time. Only if you can do the calculations.

To all your budding time travelers, good luck!

Update: I changed the title to Mechanics of Time Travel from your suggestions. Thanks!
6.21 MHz
i like this very much, it will aid me in building my time machine- however my thery is that i would place a sort of beacon deep in the antartic, then when travelling through time it can be used as a sort of locater, so the time machine can lock onto it and place the machine any where on the earth.
I think last time a beacon was buried in the Antarctic James Arness wound up running around and infecting the scientists, while Kurt Russell went around torching things with a flame thrower.&nbsp; It may be better to place the the beacon on the Moon, perhaps in a simple geometric shape so it won't attract any attention.<br />
the thing refrence?
OK first of all if you were to travel through time you would have to move at a speed close to the speed of light. the closer you get to the speed of light the further you would go in a shorter amount of time. nothing is faster than the speed of light so trying to go faster than it is not in the picture. second, what you are saying is good food for thought if you were trying to time travel in space. I like where you are going with that, keep up the thoughts. reply and we will continue the conversation.
Actually, you don't need to travel even close to the speed of light to time travel. The point is that we are already moving forward in time right now. But some scientists have managed to slow down light to about 80% of it's speed. So I wonder what nifty things that experiment will bring us.
the speed of light varies with the medium that it travels through. but say if you were also able to travel though a transparent block glass with the light, you wouldn't be able to travel faster than the light in the glass. Neat.
i like the way you think. i didn't know that about the slowingof light. my want is to move back through time, now that is going to be an interresting thing to do. i'm in the works at this time to figure out the mechanics, and the math.
I can travel through time. Forward. At the same speed as everything else.
Someone finally gets it.
I quote Dimitrie Martin (I think I spelled his name right.) "I have a time machine, it only go forward at normal speed, its essentially a cardboard box and on the outside I wrote, TIME MACHINE in sharpie!" PS. I to have built a time machine. It takes me forward at 60 seconds a minute !
Me, too, it's a dna-based computer in my body that looks like blood veins, skin, bones, and vital organs in the shape of a human body.
I just use the power of my mind.<br />
It is fun to walk in the opposite direction of the sun and after a long time your watch says you have traveled back in time.
black holes are the only thing that lacks any holes.nutron stars come close.
I think in a Larry Niven novel they did an experiment where they drilled a hole in a neutron star so it would bend the fabric of space and create a wormhole. I would love see the technology we come up with in the next 2000 years. I'm still waiting for my floating buggy from the Jetsons though.
<p>Yeah, a few decades ago we were supposed to have those a few years ago, and we only have a compact airplane and a really expensive and not even on the market folding car-plane.</p>
Maybe that will have to be my next instructable.&nbsp; How NOT to build a flying car. :)<br />
The Theory of Relativity states the the faster you go, the more time you gain (The earlier it actually is)... but only if you're travelling west.
You also gain mass, which makes you need more energy to move the mass, which makes you gain mass. It's a vicious circle. Technically once you achieve the speed of light, you have become energy, and are stuck there. So you'd want to get as-close-to the speed of light as you could.
It's a quote from Calvin and Hobbes I believe, with the Hobbes talking about crossing time zones ("the more time you gain... but only if you're travelling west") But yes, I too have heard the impossibility of travelling at the speed of light due to the necessity for an infinite amount of energy to do so...
Love Calvin and Hobbes, can't believe I missed that reference.
don't sweat it, it's a ridiculously obscure reference... "Indispensible Calvin and Hobbes" book if I'm not mistaken...
If you travel faster than the speed of light traveling west @ say 32,000 feet, how fast would you have to be going to pass yourself before you left?
There is no West in space. I believe that technically you would be moving away from yourself.
"@ say 32,000 feet" not in space.
never herd of Larry Niven.dose he have a brother named david? what if you could dig that hole to china straight threw would you only fall to the middle?
first time is something man invented. our planet once had about 6 hour days from spining faster so aplied to that light moved less distance. now its incresed the distance.time is mans tool. the universe is'nt bound by mans tools. you can move forword in time with speed imposible to go backwards
You Realise Traveling To The Future Would Be Much Easier Off The Planet, Think Of Forward Time Travel As, The Faster You Are Going The Slower You Age Compared To The Rest Of The Universe Around You...
Actually traveling to the future doesn't take any thought at all. See, while reading this you have traveled to the future a few seconds from when you started. If we had a way to freeze yourself, and thaw out safely, we could all travel as far into the future as you wanted.... in real time. It's going to the past that takes all the work.
who wants to go faster thean light, as fast a you could posibly imagine. simple answer cancel mass. and to go through time you need to rise to the 2nd dimesion in time (we are curantly in the 1st)
Very interesting information, but the title of the instructable lead me to believe that you would be giving examples on how to time travel. You might want to change it to something like "The Dangers of Time Travel", or "Time Travel Tips".
Here is a very easy example of time travel. sit in a chair and face a clock. close your eyes. open you eyes much later, and you will see that you have moved forward. :) Actually, next time you are taking a jet, Sync your watch with someone on the ground. When you land compare the time. You'll see that your watches don't match anymore.
ok isnt there a theory that if u travel fast then light (wich i dont belive is posable in a univerce that has friction,the law of conservation of energy/limmited energy, and leaders who would kill each other in the past, and more that i dont understand)but! so far i found one way to travel to the future and past, pass the international date line! also if a time machine is made it will need to be a deloriean because of its steel construction (and looks good), with a flux capasiter that needs 1.21 giawatts, (bolt of lightning?, and or plutonieum) and need to reach 88 mph. also needing to know what is standing in the way of it in the present/future/past so you dont hit anything.
oh opps i forgot the second part if u travel faster then light the theroy is that you yourself stop ageing at light speed, and you become younger at faster, wile going slower (like we are now) you grow older. also i think you need to know absolute speed, because of the relitivity theory everything is relitive, size, weight, who can eat the most hotdogs in a min, light, speed, time,
Well, you really don't stop aging, or become younger. The only way to stop aging is to either be frozen, or die. Aging is a biological process. If you figure out how to reverse that, you will be a very rich person. :) I wouldn't worry about hitting anything. If you can generate enough power to push yourself and a few hundred metric tonnes of machinery through time and space, moving a planet or two out of the way should be easy.
HELLO TO THE FUTURE! I AM COMMUNICATING TO YOU FROM THE PAST! I'm currently existing in what we currently refer to as "eleven AM, Friday morning". However, I don't know if future races will understand the concept of "AM". In fact, you may no longer use "hours" or "weekdays". Perhaps I should indicate my temporal existence by counting the number of Planck time-units since the creation of the universe: approximately 8Ã10<sup>60</sup> t<sub>p</sub> (Planck time units). In any case, I hope you are able to translate my undoubtedly primitive American English into whatever verbal (or non verbal?) language you use while flying around your Utopian cities in your Wankel-engine powered rocket cars. Looking forward to a time when cel phones don't suck, -Vax
You are not. Oh why dont you send a message from an alien while your at it. LOL
Perhaps in the future we will be able to use simple HTML in our comments?<br/><br/>Current time: ~8 x 10<sup>60</sup> t(p) (Planck time units)<br/>
If nobody tried it, I will
Turns out my explanation is not quite right, at least not at visible frequencies in ordinary matter. The wavelength of the photons is large relative to the atoms; visible light is in the range of 600 nanometers while atoms are typically a few hundred picometers (over a thousand times smaller). So you have to treat the light as a wave, at least with classical physics. But the light does interact with the electrons in the atoms in such a way that their progress is slowed. However, the light itself is not slowed, as you can see when it comes out the other side (if it comes out the other side). It still travels at c, so the material did not slow light down. The electrons in some materials are so loose and interact with light so strongly as to reduce its "speed" to zero or even lower. The light cannot enter these materials so it is reflected. We call these materials "metals".
Light always, *always* travels at a constant speed, 299792458 m/sec, regardless of the observer's reference frame.<br/><br/>However, in a solid, or in any dense material, photons don't go very far before they hit an electron and are absorbed as they knock the electron into a higher energy state. A moment later the electron re-radiates a new photon and drops into its original energy state.<br/><br/>This process continues many, many times until a photon finally escapes out the other side of the solid. Because of the tiny delay added every time a photon encounters an electron and a new one is radiated, the &quot;speed of light&quot; in the material seems to decrease. But it really hasn't. Even within the material, every photon always travels at the speed of light. It's just that any one photon can't go very far.<br/><br/>
I suppose the real problem would be if you were just a bit off on your calculations and you ended up in empty space stannding completely still. That would mean that the milkyway would soon smack into you at it's incredible velocity. Count me out of that one. I'll wait until the software for that calculation comes out and if Microsoft makes it, I'll just have to wait for the opensource.
I'm assuming that a real time travel device would be a type of capsule that protects the passengers from things like this. The old pc game Journeyman used something that looked like a diving suit. A planet ramming into you would still hurt though.
That is, if you don't burn up on re-entry.
Taking motion into account when traveling back in time is a point very well taken. However I take issue with: "The Milky Way is moving at around 552 33km/s with respect to the photons of the CMB." You can't measure your absolute velocity by looking at the speed of photons flying by. Regardless of your velocity relative to their source, photons always fly by you at the same speed (c).That's a basic facet of special relativity. However, having the same cosmic background red shift in all directions is as good a definition as any of a universal inertial reference frame.
I've never believed that light travels at just one speed. Recently some physicists even managed to freeze a light beam. But I could just be crazy. :)
MMMMM , Very interesting , unfortunately you forgot to mention , that the time travel is possible if we go back through the time divisory line ( the one that divides The Pcific ocean (japan and america )) at a ridiculous speed , im telling about 12 km/sec And Give a couple of turns around the earth in a single minute , bad thing is founding a 2.2 gigawatt power supply
look up on youtube timetravel discovery channel he explains it better but the therory is that an object moving at the speed of light like light itself will move another object on it like marshmellows in hot cocoa after you stir, the marshmellow isint moving through the cocoa the cocoa is moving the marshmellow if you get a powerful enough laser moving in a spiral it will move a partical at lightspeed but to people out side it would be moving faster thus traveling back in time.