The Knowledge: How to Rebuild Our World From Scratch

by Lewis Dartnell

  The particular target of most interest for determining the year at an unknown point in the near future is known as Barnard’s star. This is one of the stars closest to the Earth, but it is a tiny, ancient sun, glowing with only a pitiful dim red gleam, and so despite its neighborly proximity still cannot be seen with the naked eye. Barnard’s star can be easily picked out with a modest telescope, with a lens or mirror only a few inches across, though. Despite the slight trickiness in observing this star, it can serve as a natural time marker in the heavens. Barnard’s star, due to its nearness, has the fastest proper motion of any known star in the heavens. It tears across the sky at almost three one-thousandths of a degree every year. This may not sound like much, but compared to all the surrounding stars it’s blistering, and over a human lifespan it races almost half the diameter of a full Moon. So to find the date in the future, all a recovering civilization would need to do is make observations—even easier using photography—of the patch of the sky shown in the figure above, note the present location of Bernard’s star, and read off the current year from the timeline.

  BARNARD’S STAR HAS THE FASTEST PROPER MOTION ACROSS THE NIGHT SKY, AND OBSERVATIONS COULD BE USED TO REESTABLISH THE CURRENT YEAR AFTER A BREAK IN THE HISTORICAL RECORD.

  Over a much longer timescale you can take advantage of the axial precession of the Earth. Like a spinning top, our planet’s axis of rotation topples around in a circle gradually over time. The North Star, Polaris, happens to sit in line with the current orientation of the Earth’s spin axis, and so is the only point that doesn’t seem to wheel around the heavens. Right now, there’s no equivalent South Star, as Earth’s axis is currently passing through a barren region of the southern sky. Within a millennium the north pole will have wandered through blank sky to pass close to other stars, and by 25,700 AD it will have roamed in a complete circle to return to its position at the birth of Christ. (Another consequence of this roving is that the points where the Sun’s path crosses the celestial equator, the vernal and autumnal equinoxes, drift across the sky, and so this process is also called precession of the equinoxes.) It’s a relatively straightforward task to watch where the contemporary celestial pole lies, especially if you’ve redeveloped basic photography and can image the star trails smeared out by Earth’s rotation (with an exposure of a quarter hour or so). Compare it to the star map timelines shown here to read off your current millennium.

  THE CIRCLING OF THE CELESTIAL NORTH (TOP) AND SOUTH (BOTTOM) POLES AS THE EARTH’S SPIN AXIS PRECESSES OVER THE NEXT 26,000 YEARS.

  Recording the different motions of the Earth will allow you to tell the time of day and reconstruct a calendar to anticipate the changing seasons for agriculture. But how do you go about determining where you are on the Earth, and by extension, how can you learn to navigate your way effectively between different locations?

  WHERE AM I?

  Roving across terrain between familiar landmarks, or following the coastline by ship, is easy enough. But away from these comforting guides—crossing the featureless expanse of the ocean, for example—what can you do to make sure you’re heading in the right direction? Chinese sailors first employed the incredible direction-seeking behavior of natural lodestones (in Middle English meaning “leading stone”) in the eleventh century, and later magnetized iron needles. The compass needle works by turning itself to lie parallel with the lines of the Earth’s magnetic field, and so aligning its length between the poles: it helps to mark the north-pointing end of the needle. Not only will a compass enable you to maintain a constant heading in total absence of any other external references, but if two (or more) prominent landmarks are in sight, you can take a compass bearing to them and so triangulate your position accurately on a map or chart. Although you can always find north or south by a clear night sky, the compass is a fantastic navigational tool when it’s overcast. Beware, though: the celestial pole, caused by the Earth’s rotation, and the magnetic pole, caused by the Earth’s churning iron-rich core, do not perfectly match with each other. The discrepancy is only a few degrees at the equator, but as you voyage toward either pole, the compass error toward true north worsens.

  If you’re forced to return to absolute basics and can’t find any relic magnets, you can always create a temporary magnetic field using electricity. We saw in Chapter 8 how a rudimentary battery can be built from an alternating stack of two different metals, which will push current along a lump of copper drawn into a wire and wrapped into a coil to form an electromagnet. While energized, this can be used to permanently magnetize an iron object, such as a thin needle suitable for a compass. (And if you’re really starting from scratch, check Chapter 6 for how to smelt the metals in the first place.)

  A compass will tell you a direction, and, combined with a previously mapped chart and landmarks, can also give your location. But what about a more general system for determining your position at any point on the surface of the Earth? It turns out that the solutions to the two fundamental problems addressed in this chapter—what time is it, and where am I?—are actually more deeply connected than you might have realized.

  The first issue to be resolved in determining your location is to devise a system of unique addresses for all points around the planet. It’s fine to describe the lake as being three miles out of town to the southwest, but what about locating a newly discovered island, or, indeed, your present position in the middle of the featureless ocean? The trick is to find a natural coordinate system for the globe of the Earth.

  Finding your way around a city like New York with a regimented grid layout is relatively easy. The avenues all run roughly northeast with the streets cutting across them at right angles, and most of both are sequentially numbered. Getting anywhere in Manhattan is trivial: you walk up the current avenue until you reach the intersection with the street you want, and then along that street until you hit the destination. The address of a venue in midtown Manhattan can be as simple as listing the intersection it sits on: 23rd Street and 7th Avenue. Or if everyone agreed to a convention of always saying the street number before the avenue, all you would need is a pair: 23, 7 or 4, Broadway. An address here is far more than a label: it is a pair of coordinates that precisely pinpoints the location within the city. And by looking up at the signs on an intersection to see your own current position within the grid, you can instantly work out the direct course to steer to your destination, working along and across the blocks.

  A similar coordinate system works for the entire planet. The Earth is an almost perfect sphere, with the axis of its rotation defining a north and south pole, and the equator as the circular line running around the planet’s midriff. Due to this spherical geometry, it makes sense to divide up the area not with lines spaced at regular distances, as an ideal city grid might be, but with lines spaced at regular angles. So imagine standing at the north pole and shooting a line due south all the way around the planet to the south pole, and then swiveling around 10 degrees and shooting another line, and another, until you’ve turned around through 360 degrees of a complete circle. Similarly, you can start at the equator, already defined as the circle around the planet halfway between both poles, and imagine dropping shrinking rings every 10 degrees toward the north and south, with the poles therefore at 90 degrees.

  These traces running north-south between the poles are called lines of longitude, and the east-west rings encircling the planet on either side of the equator are lines of latitude. All the lines of latitude are parallel to one another, and the lines of longitude cut across them always at right angles. So, near the waistband of the world the latitude-longitude coordinates approximate the street-avenue system on the flat plane of Manhattan, with the square grid increasingly distorted toward the poles by the spherical geometry of the Earth. As with the Manhattan streets, you need to set starting points so you can specify your numbered coordinates relative to them. The equator is the obvious 0° latitude line, but there is no corresponding natural zero mark fo
r the longitude numbering: we happen to use Greenwich, London, as the “prime meridian” purely out of historical convention.

  To specify your location anywhere on the planet using this universal address system, all you need to do is state how many degrees north or south of the equator you are—your latitude—and how many degrees east or west of the prime meridian—your longitude. Right now, my smartphone says that I am at 51.56° N, 0.09° W (I’m in north London, not far from Greenwich).

  So, the original problem we set ourselves—how to navigate the world between known locations—splits neatly into two separate questions: How do I find my latitude, and how do I find my longitude?

  Latitude is actually pretty easy to establish—the richly patterned night sky offers more than enough information. Polaris, the stationary bull’s-eye in the circling star trails, hangs directly above the north pole, so it stands to reason that your angular distance from the equator is the same as the angle between this celestial pole and the horizon. The problem of determining your latitude on the Earth directly translates to measuring the elevation of stars.

  Most simply, you could build a navigational quadrant from odds and ends lying around. A quarter circle of cardboard or thin wood is marked with a subdivided angular scale between 0° and 90° around the curved arc. Two notches are put on the ends of one of the straight edges so that a target can be sighted along it, and a plumb line fixed to the corner dangles straight down to indicate the angle of elevation against the scale. Although not particularly sophisticated, such a basic device will still allow you to sight the polestar and thus discover your latitude on the Earth, to an accuracy of several degrees, which equates to finding how far north of the equator you are to within a few hundred kilometers.

  A far more elegant and accurate instrument was developed in the 1750s, and is still widely used today as a backup navigational device in case of power failure or loss of GPS. The sextant is based on a sector one-sixth of a full circle (and is named for this attribute, fulfilling the pattern set by the earlier quadrant, then octant), and can measure the angle between any two objects. Most usefully for navigation, the sextant can very precisely give you the elevation angle above the horizon of the Sun, or Polaris, or indeed any other star. The design of such a marvelously useful contraption is easy to replicate in retrospect, and as soon as your rebooting civilization has recovered the basic capabilities for shaping metal, grinding lenses, and silvering mirrors, you’ve nailed the prerequisite technologies for the sextant.

  THE SEXTANT, WITH ITS SIGHTING TELESCOPE (A), HALF-SILVERED MIRROR (D), AND ANGULAR SCALE (H).

  The frame of the sextant is a 60-degree wedge of a circle, much like a slice of pizza held vertically, the tip toward the sky. A rotatable arm is pivoted at the tip and hangs down to point at an angle scale running around the curved rim. The key component of the sextant is a half-silvered mirror mounted on the front edge, so that the operator can still look forward through it. An angled mirror on the pivot of the arm reflects the image of whatever it is pointed at down to the half-silvered mirror, and so superimposes the two views for the operator.

  To use your sextant, look through the small sighting telescope and tilt the whole instrument to line up on the horizon through the forward half-mirror. Now rotate the arm until the reflected apparition of the Sun, or any target star, swings down and seems to perch right on the horizon (some darkened pieces of glass can be inserted between the mirrors to safely reduce the glare). The angle of elevation is indicated on the bottom scale by the swing arm.

  Once you’ve relearned the patterns of the heavens and recorded tables of the positions of the brightest, landmark stars for different dates and times, you can then take a sighting off any of these to determine your latitude even if the polestar is obscured. And once you’ve tabulated the noontime height of the Sun for different dates and latitudes, you can use a sextant and calendar to backtrack and find your latitude while you’re on the move during daytime, too. Once you know how to read it, the sky is a fantastic combination tool—both compass and clock for local time.

  The second half of the coordinates needed to pinpoint where you are, the longitude, is unfortunately not nearly as tractable. It’s hard to use the heavens for finding how far east of the prime meridian you are, because the rotation of the Earth is constantly rolling you around in that direction. To extend the New York analogy to the breaking point, seventeenth-century sailors could easily tell which street they were on, but working out the avenue was next to impossible. Their only recourse was to sail by dead reckoning—extrapolating their bearing and estimated speed, and hoping they weren’t being pushed too much off course by unknown currents—to the right latitude at a point they could be confident hadn’t overshot the target, and then sail due east or west along that latitude until hopefully they stumbled into their destination.

  The Earth spins toward the east, causing the apparent motion of the Sun across the sky and the wheeling of the stars at night. The Sun’s position is how we define the time of day (right back to the fundamentals of sundials we saw earlier), and so the problem of finding your longitude—how far around the world you are from some chosen baseline—boils down to finding the difference in time of day at the same instant between the baseline and your current position. The Earth spins 360 degrees in 24 hours, so a difference in the timing of noon of one hour equates to 15 degrees of longitude. Determining your longitude is therefore a measure of time transposed back into space. In fact, you’ve almost certainly felt the solution to longitude acutely yourself: modern high-speed air transport teleports us between remote locations with very different local times before our bodies can adapt. Before GPS, navigators exploited the same principle as that behind jet lag!

  So to find the vital second coordinate to pinpoint your position, you can use a sextant to find the local time where you are now, and compare it to the current time back at the prime meridian. The trouble, though, is how to communicate that baseline time to remote regions of the globe.

  The longitude problem was finally cracked in our history by the invention of suitable clocks: impervious to the pitching and rolling of a ship on the high seas and sufficiently accurate over the months to years of a voyage. Clearly a pendulum and drive-weight system would be useless for a marine clock; it is the spring that provides the solution to both of these functions. A suitable oscillator can be made from a balance spring: a thin strip of metal coiled into a spiral around the shaft of a weighted ring that springs back and forth. The function is similar to that of a pendulum, but with the restoring force at the extremities of the oscillation generated by the tightening of a spiral spring instead of gravity. A spiral spring, wound tightly to store energy in its tension, can also provide the motive force to drive the clockwork. This is a far more compact power source than a steadily descending weight, but employing a spring for this introduces a new problem that must itself be solved with another invention. The trouble is that the force exerted by a spring changes as it unwinds: strongest at first and then progressively weaker as its pent-up tension is released. The best method to even out this power, and so regulate the rate of the clock, is to connect the free end of the coiled spring to a chain wrapped around a cone-shaped barrel known as a fusee. This way, as the spring unwinds it acts further and further up the fatter end of the fusee and so benefits from an enhanced leverage effect that neatly compensates for its reduction in force.

  A suitably complex clock, incorporating mechanisms to automatically compensate for swings in humidity and temperature (which affect the thickness of lubricating oils and stiffness of springs) and other sources of variation, is a miraculous device, an almost magical cage that can hold time itself, perfectly preserved, like a trapped genie.* The trouble, for trying to leapfrog straight to this point during the rebuilding of civilization, is that even knowing the solution to a problem is sometimes not enough. The devil is often in the exquisitely refined detail, and there may not always be shortcuts or o
pportunities for leapfrogging during the recovery of advanced civilization. It took the monomaniacal, obsessive efforts of a single clockmaker, John Harrison, over much of his life, to design and construct a sufficiently accurate marine clock, and necessitated the invention of many new mechanisms along the way, including caged roller bearings to greatly reduce friction and the bimetallic strip to cancel out expansion with temperature.

  So is there a different way around the problem? Obviously, if any reliable clocks or digital watches have survived in the aftermath, all you would need to do is set one to the local time when you head off, pop it in your pocket during your odyssey, and take it out to compare against local time (which you’d still need to determine by sextant observation) to get a fix on your current longitude. But what if there are no relic timepieces?

  The problem in the early eighteenth century was that although you could work out your local time, you had no way of remotely telling the current time back in Greenwich. Harrison’s eventual solution was to take a copy of Greenwich time with you, but it would equally work if Greenwich could somehow regularly communicate its time to ships around the world. One harebrained proposal was to set up a network of signal ships anchored in mid-ocean to relay the sound of cannon blasts indicating the moment of noon in London. But we now know of a far more practical means for signaling over vast distances: radio.

  A post-apocalyptic civilization rebooting along a different pathway through the web of scientific discoveries and technologies may arrive at another solution to global navigation. They might find that building rudimentary radio sets (see Chapter 10) is an easier prospect than the fabulously intricate workings and compensatory mechanisms of a sufficiently accurate timepiece. (But then, this will obviously depend on the rate of reattainment of different technologies—how do you compare the complexity of minute mechanical cogs and springs to that of electronic components?) Regular timing signals can be broadcast from whichever prime meridian is selected as the baseline for longitude, and relayed to far-flung regions by ground stations or other ships. In this way, the sight you could see in the early stages of recovery might be wooden-hulled sailing ships coursing across the world’s oceans, looking very much as they did in the Age of Sail, but with a subtle difference: a metal wire strung up the main mast as a signaling aerial.