During NASA’s preparations for the InSight mission, the problem of determining Martian geographic north quickly arose. Two solutions were chosen: the first was to use InSight’s gyroscope, which worked during the lander’s descent into the Martian atmosphere, to land the probe in the right direction. But once on the ground, the instrument stopped working.

The second solution was then to use a projected shadow and deduce, with an accuracy of less than 5° if possible, where the geographic North, i.e. the direction of the planet’s rotation axis, was located. This principle of using the shadow of a gnomon, i.e. a vertical stick planted on horizontal ground, dates back to the Babylonian astronomers who, more than 4000 years ago, began to determine the first constants of astronomy. It must have been noticed very early on that the shadow of a gnomon, infinite at sunrise, shortened in the morning to be as short as possible at solar noon when the Sun peaked: the shadow then indicated either North or South depending on the location. Then the shadow lengthens in the afternoon and becomes infinite again at sunset.

On Mars, the same principle was used, but not limited to the moment of culmination: knowing the Martian geographical coordinates of the seismometer and having developed a theory that gives the position of the Sun on the planet as a function of time, a succession of images of the shadow was taken during the day thanks to the camera at the end of the articulated arm. As it was not possible to fit SEIS with a real gnomon for reasons of space, the seismometer’s gripping hook was used, whose circular shadow was projected onto a graduated sight, which was far from ideal because of the ovoid shape of the shadow.

At the top of the RWEB heat shield surrounding the SEIS seismometer, the grip hook also serves as a gnomon.

Finally, a dozen images were used to determine where the geographic North was located, and the comparison with the orientation obtained by the gyroscope showed a deviation of about ± 3°. This result can be considered excellent, given the difficulties of estimating the exact position of the shadow.
No doubt the Babylonians would have been astonished that such a seemingly rudimentary instrument was ever used to find North on another planet!

For more information:
Researcher : Denis Savoie

The link to the scientific paper "Finding SEIS North on Mars: Comparisons Between SEIS Sundial, Inertial and Imaging Measurements and Consequences for Seismic Analysis" in the journal Advancing Earth and Space Science ->https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020EA001286]

A short video "Is Gnomon Time Back?" :
https://www.youtube.com/watch?v=MIMI-Ee6c48
and the link to the related lecture presented at TimeWorld

Highlights @ INSU
Highlights @ Observatoire de Paris

Credits:
Image: © IPGP/David Ducros.
Thumbnail: © NASA/JPL