Determining True North on Mars by Using a Sundial on InSight

In this work, we demonstrate the possibility to determine the true North direction on Mars by using a gnomon on the InSight mission. The Sun local coordinates are computed using the analytical solution VSOP87 for the planetary ephemeris. The Sun position in the sky induce specific gnomon shadow orientation and length that can be read on a target. By comparing the images of the shadow with the expected position of the Sun, we can determine the true North direction with an accuracy up to \(1^{\circ }\), well within the seismic requirement of \(5^{\circ }\), even if the lander position or the gnomon verticality are not accurate. To achieve this result, we determine the best periods of observation during the SEIS deployment phase in December 2018. A total period of 5 hours has been identified on mornings and evenings on Mars, during which the impact of errors on position and verticality on the true North direction are minimised.