Hacker News Reader

Comment by psychphysic | Hacker News Reader

This is a great trick and with practice you don't even need a watch, heck you don't even need an object to cast a shadow.

The sun is over there, a shadow would be that way, it's about 5 o'clock. That must be north!

What I've always thought of as witchcraft is this.

Jab a stick in the ground, mark the end of the shadow. Come back some time later say 15-30mins. Mark the new end of the shadow.

Draw a line between marks and that is an east-west line!

In the Northern hemisphere the stick is south of that line, in the southern hemisphere it's North!

What!!

View on HN

That works the best close to noon, but I don’t think that works so well at around dawn or dusk.

Latitude also matters. The closer it is to the arctic or antarctic circle, the iffier that gets.

This isn’t a trick so much as just how the sun and the earth defines the plane of the ecliptic and how we experience it because of Earth’s rotation.

There are some interesting things that can be done with starlight navigation. The Polynesans, for example, might navigate by latitude based on how far up Sirius is in the sky, or using the little dipper (and Polaris) in the Northern Hemisphere as a clock.

Starlight navigation is incredible. I was in the marines with a helicopter pilot who had a second set of wings on his uniform, an odd pair I'd never seen. Turns out he was a starlight navigator on C-130s before becoming an officer and getting his pilot's wings. He used to wear the starlight navigator ABOVE the pilot wings, which I always thought was incredible. The training and skills you need to get this set of wings is quite an undertaking.

I can navigate with a map and compass, but you're using terrain to help you locate yourself on the map. Being able to do it in the expanse of the ocean or the air, with only the stars is pretty amazing.

The concept of celestial navigation is actually pretty simple. Actually doing it requires physical skill.

The general idea is this. Take for example the star Altair, at any given instant it is directly above exactly one spot on Earth. If you were at that spot and measured it's elevation above the horizon, it would be 90 degrees. That point is called the "Geographic Position (GP)". So, if you were at some unknown location and measured Altair to be 45 degrees, you would know you were 45 degrees from Altair's GP, and all of the points satisfying that can be plotted as a circle on a map. Repeat that for another star and you end up with two circles that intersects at two points. You could repeat with a third star, but in practice you have a rough idea of where you should be, and you choose stars that produce a large enough circle that you can disregard one of the two points. And taking that "in practice" one step further, the diameters of the circles are so large, you really just need to draw part of the circle as a straight line, and that lets you plot in a small enough area that map distortions are of no significance (unless your very close to the poles). There's a standard form called a "VP-OS" which makes the process a lot easier. A video showing how to do it is [1].

To determine the GP of a star (or planet) you need an ephemeris of some sort, there's software to do it, but a copy of the Nautical Almanac provides enough information to make the process simple enough to do with paper and pencil.

Actually measuring the elevation of a star on a moving, bobbing ship is where the real skill comes in. At the equator, being off by one degree means your result will be off by about 60 miles. Skilled navigators can fix their position within a few hundred feet!

[1] https://www.youtube.com/watch?v=bVIPorb_aMM

Leave it to hacker news comments to summarize something I thought was black magic into paragraph that I completely understand. Not sure I could replicate it but it makes sense to me now.

So, this has been around a lot longer than we had accurate star charts that actually understood the motion of planets and stars, right? Do you have any summary of how these charts that describe where a certain star is above on the earth on a specific day came from before we understood the motions of the stars?

I'm not too familiar with the ancient history of it, but it requires an accurate clock. There's a good book called "Longitude" that goes over the development of the clock that could be used at sea. But, before then, it was well known that you could measure the altitude of Polaris to get your latitude, so ships would sail to the latitude they wanted, then turn East or West.

We've had accurate star charts for a long time, Hipparcus had made accurate enough measurements to detect the Earth's slow 26,000 year wobble called precession. The key piece missing for navigation was a clock.

I assume that doing this technique with a computer and a camera could achieve really good accuracy - you really just need a zoomed image showing the horizon and some stars in a few directions, and by averaging across hundreds of stars you ought to be able to achieve perhaps 1/100th of a pixel of precision, which works out to 1.8 meters of position accuracy. Modern tech can see stars even in daylight too.

I wonder why that technique isn't used instead of GPS - back in the 70's before GPS was deployed, it surely must have been an attractive option.

Even today, in warzones with GPS blocked, it seems like a good option.

On land, it can be hard to very accurately measure the horizon (mountains etc).

However you can use a similar technique by using a weight on a string to make a perfectly vertical line, and seeing the alignment between stars and that.

https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird#Astro...

Uncertainty in atmospheric refraction limits exactly how accurate you can get. Celestial Navigation books say it's better to make several measurements of the same star than to measure a bunch of different ones.

From what I understand, some land surveyors often still use a sun sighting to determine true North.

GPS has the advantage of working when it's cloudy, or when the sky is blocked by trees or buildings. And a GPS receiver is pretty cheap, durable, compact, and foolproof compared to a telescope and camera system which will have to be recalibrated regularly.

Something I only learned recently: Boeing 747's had a sextant port, for use in navigation. This is only one of several aircraft variants (including military aircraft) which used sextant navigation.

<https://virginflightdeck.blogspot.com/2010_09_01_archive.htm...>

Mentioned at least once on HN: <https://news.ycombinator.com/item?id=5719703>

OK, rather more than once: <https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...>

IIRC, ICBMs also use starlight navigation.

They do, you may also find this [1] interesting, it doesn't cover the starlight navigation system but the 19,000 piece Advanced Inertial Reference Sphere (AIRS)

https://www.thedrive.com/the-war-zone/30254/this-isnt-a-sci-...

Why does it work worse in higher latitudes and closer to dawn and dusk?

The shadow of the tip of the stick will always move in a straight line from east to west right? At high latitudes I could imagine the shadow gets so long that finding the tip might become hard. Around dawn and dusk, what is the problem? Does the shadow become too faint? Does the shadow move to slow to give a big difference in position?

At dawn and dusk, aka twilight, the sun is below the horizon. So you only get indirect illumination. Contrast is terrible, and shadows will be diffuse. The direction should still be mostly right, but you will be hard pressed to define the end of the shadow.

The position of the sun can be a bit weird depending on the season the further away from the equator you are. Especially in the periods where the sun never rises above the horizon. Other than that I don't know exactly what problems you might have. The midnight sun perhaps flips the direction?

As someone living in the eternally rain-drenched and cloud-wreathed north of Europe, you lost me at "The sun..."

Normally an upvote is enough but I just have to say that your enthusiasm about this made me smile so wide. This is great.

You’ll love Jeremy Tan - https://youtube.com/c/JeremyTanMagic

Read that whole thing in his voice having seen the end.

Oh boy, I got sucked into those shorts real hard. How the hack (heck) does he guess the card those people on the street seemingly all by themselves picked?

It’s a rabbit hole but you can find some of the tricks online - you either do slight of hand, control the card picked (a force), or you control the card once in the deck.

Combining them and being really good at it is what makes it “what”!

https://youtube.com/c/Jasonmahermagician Is another one I enjoy and he has longer videos showing some of the tricks.

Wait, how does that work?

The shadow is always going to be opposite of the sun, so it does not provide any extra info which the sun doesn't already give you.

In order to estimate the current time ("it's about 5 o'clock") using the position of the sun/a shadow - you need to know the cardinal directions. So, only if you already know where north is, you can tell the time from the shadow.

In order to estimate the cardinal directions from the position of the sun/a shadow - you need to know what time it is. So only if you know the time, you can tell where north is from the sun/shadow. And to know the time you need a watch or something similar?

But if you can deduce both time AND cardinal directions just using the sun/shadow and no other information... yeah, I have to agree, that seems like witchcraft!

> In order to estimate the current time ("it's about 5 o'clock") using the position of the sun/a shadow - you need to know the cardinal directions

On this planet the sun is always moving from East to West. And assuming you suddenly didn't wake up on the other side of the planet you know if you are in Northern or Southern hemisphere.

So if you are in Northern one, then shadows always point in the North direction, the difference between marks on the ground show you the sun direction (E-W).

More so, in the morning shadows point to NW, in the evening to NE, so you can have a vague estimate even without waiting.

> And to know the time you need a watch or something similar?

You don't need the time per se, you need to know what sufficient time passed, it's just hard to make a precise line with a sticks. But if you have something long and thin then you can make a line in a couple of minutes.

Just take a sheet of paper, anything sticky (even a pencil, just make it stay vertical), mark the shadow, browse Reddi^W HN for a couple of minutes, mark the new shadow, make a line.

We just don't notice it, but Sun is hurling through the skies with astonishing speed.

PS I can guess the current time with ~1-1.5h accuracy just looking through the window to see how the buildings are lit by the sun. Works only from my windows, of course.

The shadow measures not just the horizontal angle change of the sun, but also the vertical angle change of the sun.

A very common way to measure time is to reference the horizontal angle change w.r.t. noon. In that case you need two moments where you measure the horizontal angle. Where, for the first moment, you need to measure two vertical angles (probably more) and deduce that the sun has reached its highest point.

In this witchcraft, you still measure two vertical angles and two horizontal angles. You just split up when you take the vertical angles.

Knowledge like “the sun rises in the East and sets in the West” goes out the window when you live at Around 60 degrees or further North.

I highly recommend spending 12 months way up north to have your mind constantly blown by the path of the sun. Every year it’s still a shock to watch.

It's because the actual true statement is that the sun is in the East at 6 o'clock in the morning and in the West at 6 o'clock in the evening, and of course, if the sun rises at 3 or 9 (and perhaps you've got a timezone that's more than half an hour off of 'local sun time' and one more hour shift caused by daylights savings time) then it makes quite a difference.

But if you would have an analog watch with a 24-hour dial tuned to your local sun time, then you should be able to effectively treat it as a compass everywhere, also way up north.

> The sun is over there, a shadow would be that way, it's about 5 o'clock. That must be north!

That works for example if you have a path and you're trying to find out which way goes north and which south, but a watch can give you an actual quantifiable bearing that is actionable if you have no other ground orientation.

If you dig that, you'll dig a ottomani sun compass.

https://thankfulslave.blogspot.com/2012/06/sun-compass-ottom...

And read The Natural Navigator by Tristan Gooley. You will be amazed at what you can use to navigate when you can read the signs.

https://www.naturalnavigator.com/tristan-gooley/

Or, if you’re in the Pacific Northwest and mistake the diffuse daylight for direct sunshine, you’ve probably started tracking crow movements by accident.

Or if you're on the east coast and you realize that the bright star you've been following was the shuttle from La Guardia!

Isn't this incorrect? If it's summer, south of the Tropic of Cancer, the stick should be North of the line. It's the Tropic of Cancer where the sun is overhead, not at the equator. Or did I understand basic astrophysics wrong.

Similarly, in the winters, I'd imagine North of the Tropic of Capricorn would play the role of the Tropic of Cancer.

I have a wonderful book called the “Rudge Book of the Road” from the 1920s that’s full of stuff like this.

It was written as a companion for Rudge-Whitworth motorcycles and tells you how to determine direction of travel by looking at trees among other things.

Exactly. It depends on where you are and your concept of time.

In my long distance cycle touring days in places I had not been before, in open terrain, I knew what time it was and where north was just by where the glowy thing was in the sky. I had no need for a compass or watch, I was used to the movement of the glowy thing and it never moved unpredictably on a day to day basis. For a second opinion I could consult the current status of my sunburn and what hurt the most. For a third opinion I found surrounding flora was easy enough to read.

At night, under clear skies, it was the same story, the heavens always moved predictably to me, my in built 'star nav' worked great, I never overshot my destination or ended up going in the wrong direction.

This is all well and good, however, this innate ability can easily be defeated in a city. I am not sure maps helped, over reliance on such 'technology' rather than intuition has led me to head off in completely the wrong direction, often with two or three corrections needed.

I have a Rudge bicycle from the 1990's, a rebranded Raleigh import, notable for being an early foldable mountain bike.

Speaking of antiques, remember Scribd? There is your "Rudge Book Of The Road" there to download. Auction sites have the original for sale, 1927 vintage possible for a small fee.

Reply on news.ycombinator.com