Roadside Picnic – A Nitpick on How to Shoot the Earth

This blog post is all about a line from the Russian scifi novel “Roadside Picnic“, written in 1971 by the Strugatsky brothers. The book itself went on to influence huge amounts of science fiction and cinema, including the 1979 film “Stalker” by Tarkovsky. (And later a number of video games along the same lines).

The rough premise of the novel is that Aliens have briefly visited earth, before departing again. We get this beautiful framing of the Aliens from the learned professor character:

A picnic. Picture a forest, a country road, a meadow. Cars drive off the country road into the meadow, a group of young people get out carrying bottles, baskets of food, transistor radios, and cameras. They light fires, pitch tents, turn on the music. In the morning they leave. The animals, birds, and insects that watched in horror through the long night creep out from their hiding places. And what do they see? Old spark plugs and old filters strewn around… Rags, burnt-out bulbs, and a monkey wrench left behind… And of course, the usual mess—apple cores, candy wrappers, charred remains of the campfire, cans, bottles, somebody’s handkerchief, somebody’s penknife, torn newspapers, coins, faded flowers picked in another meadow

And the novel, set 30 years after the visitation, covers the lives of the various people who now live in a world with dangerous and incomprehensible artifacts, originating in these bizarrely contaminated “zones”.

The quote in particular that I was struck by is this (emphasis mine):

“The Pilman Radiant is simplicity itself. Imagine that you spin a huge globe
and you start firing bullets into it. The bullet holes would lie on the surface in a smooth curve. The whole point of what you call my first serious discovery lies in the simple fact that all six Visitation Zones are situated on the surface of our planet as though someone had taken six shots at Earth from a pistol located somewhere along the Earth-Deneb line. Deneb is the alpha star in Cygnus. The Point in the heavens from which, so to speak, the shots came is the Pilman Radiant.”

and later on (emphasis mine):

“It suddenly occurred to me that Harmont and the other five Visitation Zones — sorry, my mistake, there were only four other sites known at the time — that all of them fit on a very smooth curve. I calculated the coordinates and sent them to Nature.”

at first glance that seemed to make perfect sense to me. I could imagine in my head a diagram much like this one, which shows the ecliptic and the tilt of the earth offset from that. What could be more natural than for an external celestial event than to lay upon a Great Circle path across the Earth?


But the more I thought about it, the more I realised that it doesn’t make sense. If you fired shots from Deneb at Earth, then there’s no reason why they actually need to lie on any single curve.

There’s at least 3 ways I can think of to interpret the professor’s discovery.

The first way is that all three shots were fired at the same time, and they happened to intercept Earth. (i.e. they were not aiming at Earth’s centre of mass or anything). This makes most sense if the aliens didn’t even stop, and the unearthly condition of the “zones” is just what happens when ships powered by some reality-alterning warp drive technology passing through ordinary matter.

Roadside picnic interpretation 1 - v01

This would result in 6 sites, all along parallel lines. However there is no curve which explains or allows you to predict the others. (Hypothetically, if you knew about site E, F & D, this would allow you to calculate C. However A & B would not be determinable without knowing at least one of the pair. )

So this interpretation is not a good fit for the story.

The second way to interpret the scientists words are that all the “shots” were aimed at the centre of the earth, but fired at different times. This is consistent with the mentions of spinning in the description. And perhaps the alien technology relies on using the earth somehow. (e.g. they need to aim the teleporter at our gravity well, or something) The results would look like this;

Roadside picnic interpretation 2 - v01

This would result in all the sites having the same latitude North or South. All the “entry wounds” would be in one hemisphere, and all the “exit wounds” would be in the opposite hemisphere.

However this doesn’t fit the story at all, since all the sites having a common latitude is not something you need to do any indepth calculations on, or fit a curve to. And there’s no real need to write to Nature to inform them of your amazing discovery, since everyone with a table of the coordinates would immeidately see the pattern themselves.

The third way to interpret the scientist is this. The shots are fired at different times, and intersect the earth without being aimed at the centre. This results in all the lines having the same angle to the axis of Earth’s rotation, but that’s about it:

Roadside picnic interpretation 3 - v01

There’s no curves, or great circle routes, or really anything you can do with this model. True, if you were informed of the location of a new site [and the time of the hit], it allows you to infer that there is a sister site on the other side of the planet to investigate. But that’s not the sort of calculation the professor described.

My conclusion is either;

  1. This was something quickly thought up for a scifi-story, and 50 years later I’m over-analyzing it.

2. The Dr Pilman character made a mistake. And that his greatest achievement “The Pilman Radiant” is actually a coincidence or misinterpretation, and humanity knows less than we think about the aliens visitation. In fact, given how apathetic and nihilistic his character is about the whole situation — He says at one point that most important discovery that Humanity has made is just that the visitation has occurred. And that it’s not important who they were, or where they come from, etc. — I think he would be rather delighted to know that his main idea is wrong.

Edit — 1 hour after posting the original. Jess and Scott helped me realise that if there was a laser beam stretching across the cosmos, and Earth “wandered into” that beam as part of its orbit around the sun, then it would indeed make a smooth curve. 

So if we have the point of view of the “giant matter transmitter canon” on Deneb, and Earth moves into that beam from left to right: 

Roadside picnic interpretation 4 - v01

Then subsequent shots will hit the earth at the same angle, and different latitudes, and the longitude will depend on how far the Earth has rotated in the day/night cycle compared to how fast it moved in the annual orbit around the sun. 

Roadside picnic interpretation 4b - v01

If we look up the speed of the Earth at the equator, it’s about 0.47km/sec. And the speed that the Earth orbits the sun is around 30km/sec. So clearly the picture on the right is much closer to the truth. 

Darn. So the events described in the novel actually make sense. I’m kind of disappointed. 

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