The gnomon is a rudimentary piece of wood planted in the earth, widely used during Antiquity in Babylon. LThe idea was recovered to determine the geographic North of the planet Mars as part of NASA’s InSight mission.
A kind of gnomon in the InSight probe
Over 4,000 years ago, the Babylonians planted a stick vertically on level ground. They then observed its shadow in order to determine the time, the seasons, but also the geographical coordinates of different places. This apparently very basic instrument has in fact made it possible to determine the first constants of astronomy.
However, during the preparations for the InSight mission (NASA), engineers considered two solutions to determine the geographic North of the red planet. As the Paris Observatory explains in a publication of April 19, 2021, the first was to use the probe’s gyroscope. The second (the one that was chosen) proposed the use of shadow projection to deduce where the geographic North was. In addition, the experts wanted an accuracy if possible less than 5 °.
An excellent result considering the difficulty
The InSight probe did not embed a real gnomon, but the principle remains the same. A camera placed at the end of an articulated arm made it possible to observe the shadow projected by the gripping hook of the probe’s seismometer (SEIS) on a graduated staff. Thanks to this method, the precision to establish the geographic North of Mars was only +/- 3 °, which corresponded perfectly to the wish of NASA.
This is an excellent result as estimate exactly the position of the shadow is a difficult business.
In early April 2021, NASA’s InSight lander detected two new “Marsquakes” of magnitude 3.1 and 3.5. Operations are also underway to try to bury the cable connecting the seismometer to its lander to protect it from bad weather. Remember also that the main scientific objective of the mission is to have a better knowledge of the internal structure of Mars. The goal is to better understand its characteristics until today little known and to try to reconstitute its history.