So, You’re Lost in Time: Determine When You Are

Your adventure through time has begun! So when are you? This first entry in the history survival guide will help you (and any unwitting time travelers with you) estimate your time period and how far off course you are with nothing more than the stars and gumption!

 

The first responsibility of any sensible chrononaut is to know when they are! You are, of course, free to be eaten by any timeline’s combination of wolves, saber-toothed cats, and hyperintelligent lizards, but please be sure to throw your chronometer and guidebook free of the blood for ease of retrieval. We thank you.

 

In that case, chrononaut beta, throughout the history of Earth, the landscape has undergone fantastic changes. So the intrepid explorer must look away from familiar natural landscapes to place themselves in time–specifically up to the stars.

 

The night sky can be thought of as two hemispheres combined to form a sphere around the Earth. Star are embedded in the “celestial sphere” like careless adventurer reentering a timestream within the ceiling. A circumpolar constellation will always remain above the horizon because of its proximity to the center of the planet’s rotation axis, making them an ideal reference point to observe the gradual wandering of stars. Since this movement is incredibly small each year, the degrees of movement are represented with equally small units: the arcsecond*.

 

Consider the plight of Edmund Halley (1718 A.D) when he noticed that the three bright stars Sirius, Aldebaran, and Arcturus were almost half a degree (~1800 arcseconds) different than Hipparchus (190 B.C) described! The only way to make sense of this was for him to realize that stars must be in motion, and over thousands of years, they had moved far enough to be apparent to the naked eye. So don’t hold your breath waiting–without a proper oxygen unit attached.

The stars’ movement (here, proper motion [μ]) is the sum of its own motion and relative position to Earth. This proper motion is only apparent over hundreds of thousands of years. Also, as stars are nocturnal, this strategy will require waiting until night; but patience is the virtue of non-digested explorers.

One familiar circumpolar constellation is the Big Dipper within Ursa Major. Most of the stars within the Big Dipper are believed to have formed at the same time and move together as a group–except, that is, for the stars Dubhe and Alkaid. These stars are further away from the main cluster, and are moving in a different direction, which will act to emphasize how the shape of the Big Dipper will change predictably over time. To estimate your epoch, compare the degrees of shearing to your expected time period.

With no instruments, the degree a star moves can be estimated within a few degrees of errors by hand. For reference, the angular distance between Mergrez and Dubhe during the years around the first moon landing in the 20th century is ~10.3°.

Southern Hemisphere

If the night sky does not include the Big Dipper, it is more likely that you have found yourself in the southern hemisphere of Earth than another planet (probably). Fear not! The same principles apply for the circumpolar constellation Crux in the southern hemisphere (where the angular distance between Mimosa and Delta Crucis is ~4.3°). Otherwise, you are free to panic for the standard allocated time of three minutes.

*Remember, of course, that an arcsecond is a unit of length–not time. One arcsecond is a fantastically small (1/3600th of a degree) slice of a circle, and it is used to describe the arc of a star moving across the celestial sphere

1 degree = 3600*arcseconds and 1 arcsecond = degrees/ 3600

Consider a star with a proper motion of 0.1 arcseconds per year: After 100 years the star moved 0.1*100 = 10 arcseconds

 

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