Sextant

Recall …

At my going-away party Pablo said, “You should take a sextant with you and prove you’re at the pole.” And Chris said, “You can have mine.” I have wacky friends. I told that story here, and one of my new friends here at the Pole said, “oh, I know how to use a sextant; I’m an Ensign in the NOAA Corps.” [.. he said violating all the rules of use of quotation marks in journalism.] I hope a future post will feature Ensign Bliss and the Sextant at the Pole.

I’ve gotten it out once or twice since, but yesterday and today made a concerted effort to get four sets of readings roughly 6 hours apart each. This is largely in response to being a short-timer and taking my last shot at a few things I feel an obligation to shoot at.

The sextant, decked out with some welding glass, since I'll be looking at the sun.

The sextant, decked out with some welding glass, since I’ll be looking at the sun.


The sextant is a lovely piece of engineering and craft. The main concept is that there are two mirrors, one fixed, one movable, and an eyepiece which lets you see — at the same time — both the direct view, like a telescope, and the view as reflected in the mirrors. If the thing you are viewing is far away, and the movable mirror is parallel to the fixed mirror, both images are pretty much identical. Then if you move the movable mirror so that the image of one thing — the horizon, say — lines up with the image of another — the North Star, say — the angle between the fixed and movable mirror will tell you the angle between the other two things — how far above the horizon the North Star is, say. And in the “say” world, that will tell you the latitude of your boat and keep you from a death at sea. Or at least help.

Then Ensign Now Lt JG Kel Bliss making sure the sextant is seaworthy. I have no idea what she's doing, really.

Then Ensign Now Lt JG Kel Bliss making sure the sextant is seaworthy. I have no idea what she’s doing, really.

Then Ensign Now Lt JG Kel Bliss adjusting the sextant.

Then Ensign Now Lt JG Kel Bliss adjusting the sextant.

I was lucky to have a pal suggest I bring a sextant, lucky to have another give me his, and three-times lucky to meet NOAA Corp Lt JG Kel Bliss in McMurdo. She has been on ships and used a sextant to do that say thing and figure out where she is. She checked out my sextant in early December and gave me a few lessons, making me an under-educated man with a tool and a will to use it. Some will to use it, anyway. Not enough to use it on the Solstice, which would have been cool, or maybe I’ll claim the weather was bad. Anyway, it’s been sitting around begging to be used before I go.

I don’t have any chance of seeing the North Star here for at least two reasons: There’s a much brighter star washing it out all the time, magnitude -26.78, our Mr Sun aka Sol; and there’s this pesky planet in the way, Earth. I wouldn’t wish ill on either of them, so I needed to work around these limits. The plan was to shoot the sun’s height, and use an ephemeris generated by NASA and the JPL and my tax dollars to find out where the sun should be at various times. I give thanks to another pal, my personal Astronomer Royal Elinor Gates up on Mount Hamilton. This shows that over the course of the day the sun would set about one-quarter of a degree — that’s about half of it’s diameter. To put that into perspective, one-sixtieth of a degree is one minute of arc, and if I made an error as big as that I’d be off by a nautical mile, or about 1.15 what-we-call miles. My sextant has a precision down to a fifth of an arc minute — about a fifth of a mile. But you’ll soon see how little that matters.

I am not at sea, except metaphorically, and so don’t have the same kind of horizon-line that I’d have on a ship. The horizon here is sometimes clouded, and not level, so the angle between it and the sun isn’t a good indicator of the suns path through the sky. If I knew the altitude at places along the horizon, I could correct for that, but I don’t. Foggy or otherwise obscured horizons were an issue for seafarers; as early as 1730 people were trying to hack around it. At sea there are bigger difficulties, but here on land we can use an artificial horizon. Mine was a plate with some water in it. Rather than using the sextant to align the images of the sun and the horizon, I decided to align the image of the sun with an image of the sun reflected in the water; half that angle would be the angle I was looking to find.

All of my sextanting gear. The gizmo, a plate of water for an artificial horizon, and my handy Android chronometer.

All of my sextanting gear. The gizmo, a plate of water for an artificial horizon, and my handy Android chronometer.


Equipped with a sextant, a chronometer, and a plate full of water, I actually did some measurements. At four times over 24 hours I did three readings. all readings were from the 2nd floor of the elevated station, so I was at the same elevation each time. When shooting the horizon, you have to correct for elevation. I have no idea if there should be a correction applied for my elevation, given that I was using an artificial horizon. I was shooting through several panes of glass, and I’m not sure if the refraction through the glass matters.

Here’s some Raw Data:

  • 2013-01-25  21:10 UTC
    • Expected 37 35.4 (twice 18 47 43.9)
    • 37 44.2
    • 37 41.8
    • 37 50.0
  • 2013-01-26  03:18 UTC
    • Expected 37 27.8 (twice 18 43 53.0)
    • 37 28.2
    • 37 35.8
    • 37 31.0
  • 2013-01-26  08:47 UTC
    • Expected 37 20.8 (twice 18 40 25.4)
    • 37 16.8
    • 37 14.0
    • 37 12.0
  • 2013-01-26  14:33 UTC
    • Expected 37 13.6 (twice 18 36 45.9)
    • 37 13.4
    • 37 11.0
    • 37 17.0

Something in the system is introducing errors: each set of three has between 4 and 6 arc-minutes of variation. That alone means I don’t know where I am within about 5 miles. I’m also shooting high sometimes and low other times. I’m willing to believe it’s me, I’m willing to believe it’s the sextant, and I’m willing to believe it’s something I don’t understand. I would have to do several days of this kind of thing to figure out if there was some kind of variance around the clocklike for some reason I’m always reading low at 3am and high at 3pm. That’s not going to happen. It might also be directional or the window I pick, or something.

I’m willing to believe, given these measurements, that I’m within ten miles of the South Pole. I have other indirect evidence as well, and at some point you have to take science on faith. So be it.

Here is a bit of history around a famous photo of the Amundsen party doing some surveying, and here is a deep analysis of the raw survey data from Scott and Amundsen done by Arthur R. Hinks in 1944. Man, that southpolestation.com has it all over.

Me, back in early December, getting a hang of this thing.

Me, back in early December, getting a hang of this thing.

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One response to “Sextant

  1. This morning I went up to a table of telescope beakers and reminded them that the sextant was theirs for the taking. They perked up like small eager puppies. It seems that just yesterday they broke their plastic sextant, and so this was a most welcome surprise for them. Chris, Pablo, the world thanks you.

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