According to the NASA Johnson video Space to Ground: A Star to Steer By: 08/10/2018:

A tool that once helped sailors cross vast oceans for centuries is now being put to the test in space. The crew has been testing out a sextant, a device with a small telescope-like sight that can take precise measurements between pairs of stars. These were actually used in the early days of human spaceflight on programs like Gemini and Apollo. And mission planners are once again looking at how they can be used for emergency navigation needs on deep-space exploration vehicles like Orion.

As for the title: And all I ask is a tall ship and a star to steer her by; 12 Teleplay by Dorothy Catherine Fontana, poem by John Masefield.

I show some screenshots below.

Question: What is the nature of the sextant tests on the ISS for Orion cis-lunar emergency navigation? I'm guessing that the idea is to use measurements of the separation between a star and a position on either the Earth or the Moon, rather than between two stars. Is there a description or a report?

In real practice, in an emergency, I'm guessing that you'd probably need an ephemeris printout, pen and paper, or a small handheld computer to do useful navigation using this technique assuming that you are beyond near Earth orbit and experiencing an emergency. It would be great to read about these procedures if there's anything written about it.

Shown in the NASA Johnson video using a sextant aboard the ISS are NASA Astronaut Serena M. Auñón-Chancellor M.D. (per comments and here) and German ESA astronaut Alexander Gerst (from here).


sextant tests on ISS

sextant tests on ISS

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    $\begingroup$ The other crewmember is Serena Aunon-Chancellor. Google for "space station highlights week of June 25 2018" on www.nasa.gov. $\endgroup$ Aug 12, 2018 at 3:51
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    $\begingroup$ Links about the Apollo sextant: 1. astronomy.com/news/2018/06/the-story-of-the-apollo-sextant 2. ion.org/museum/item_view.cfm?cid=6&scid=5&iid=293 $\endgroup$
    – Heopps
    Aug 12, 2018 at 7:06
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    $\begingroup$ @OrganicMarble thank you, and duly noted above. $\endgroup$
    – uhoh
    Aug 12, 2018 at 9:01
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    $\begingroup$ @Heopps those are great links, than you! $\endgroup$
    – uhoh
    Aug 12, 2018 at 9:01
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    $\begingroup$ A measurement of the angle between pairs of stars may be used to check your sextant as well as your measurement skills. The angular distance of pairs of stars many light years away is constant and does not depend on your position on Earth or in orbit. If you measured the tabulated angular disances of several pairs with good precision, your sextant and your usage of it should be OK. $\endgroup$
    – Uwe
    Nov 2, 2018 at 10:44

1 Answer 1


Reference: https://ntrs.nasa.gov/citations/20190001296


What is the nature of the sextant tests on the ISS for Orion cis-lunar emergency navigation?

ISS Astronauts performed "star / planet limb" sights as well as "planetary diameter" sights. The wording in the above document implies the "planet" is the moon.

I'm guessing that the idea is to use measurements of the separation between a star and a position on either the Earth or the Moon, rather than between two stars.

No, sights between a star and an Earth or Moon surface feature (as opposed to the limb) were not on the ISS agenda. However, this was done during Apollo missions. Sights were made from the Apollo CM between stars and lunar surface features from lunar orbit.

Instead, on ISS, sights measured the angle between a star and the Moon's limb (horizon)

Sights between two stars were also performed on ISS. These sights are not useful for acquiring position information since the measurement would only vary over interstellar distances, not during a voyage within the solar system. These two-star measurements may have been included to assess astronauts’ measurement errors or to check sextant calibration since the “right answer” is know to very high accuracy.

By comparison, sights involving Earth's limb have high inherent error rates due to atmospheric effects.

Measurements on ISS of moon diameter allow calculation of lunar distance.

Is there a description or a report?

YES: https://ntrs.nasa.gov/citations/20190001296

Two crew members with different skills, background, and ability were able to produce quality sights on-orbit through a spacecraft window with minimal training and practice. This supports the feasibility of a sextant for emergency cislunar navigation from the perspective of measurement accuracy. No extraordinary provision was made to minimize window refraction effects other than generally preferring sights near the center rather than edges of a window


Interesting historical note: one of the earliest marine celestial navigation systems (called Lunar Distance and published by Johannes Werner in 1524) used sextant angles between the moon and stars, just like the sights taken on board ISS.

The method was superseded by the “Jupiter’s Moons” method, and then eventually chronometer-based methods. The Lunar method relied on an ephemeris which published the moon’s position years in advance. Generating the ephemeris required solving a 3 body problem (Sun, Moon, Earth), years in advance, using Euler’s method.

Navigation accuracy “by Lunars” was routinely 15 nautical miles at the equator, sometimes half that.

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    $\begingroup$ Excellent answer! I ran these ops real time, but I hadn't seen the write up of the results. There was a last minute change to planned ISS attitude, and no one on my midnight shift knew if the ops products had been updated, as there were no handover notes in the log. I had to figure out where ISS would be on ground track during execution, then used Google Sky Map to figure out if the target stars were in the right orientation. It was that or wake up the payload developer at 2AM to ask. $\endgroup$
    – Doresoom
    Jan 26 at 1:09

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