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∘$1^{\circ }$, well within the seismic requirement of 5∘$5^{\circ }$, 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.

中文翻译：

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使用 InSight 上的日晷确定火星上的真北

在这项工作中，我们展示了在 InSight 任务中使用 gnomon 确定火星真正北向的可能性。使用行星星历的解析解 VSOP87 计算太阳本地坐标。太阳在天空中的位置会导致可以在目标上读取的特定 gnomon 阴影方向和长度。通过将阴影图像与太阳的预期位置进行比较，我们可以以高达 1∘$1^{\circ }$ 的精度确定真正的北向，完全符合地震要求 5∘$5^{\circ }$，即使着陆器位置或 gnomon 垂直度不准确。为实现这一结果，我们确定了 2018 年 12 月 SEIS 部署阶段的最佳观测时间。 在火星上的早上和晚上总共确定了 5 小时，