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Lunar Season Calculator

The Lunar Season Calculator was built to aid mission planning for the next decade of robotic and human exploration by providing easily accessible information on the timing of the lunar seasons. Seasons on the Moon, like on Earth, can be measured by subsolar latitude, which describes the degree of tilt toward or away from the Sun in the northern and southern hemispheres. This value oscillates between ±1.59° due to the inclination of the spin axis of the Moon with respect to the ecliptic [Figure; Refs. 1, 2]. When subsolar latitude is positive, the sun at local noon is directly overhead at a point in the northern hemisphere; this is referred to as Northern Summer or Southern Winter and the inverse is true when subsolar latitude is negative. Although lunar subsolar latitude variation is small compared to that of Earth, it has significant effects on the illumination conditions on the surface, especially at the poles, and therefore has important implications for proper timing of exploration activities.

Lunar Season Calculator

As lunar seasons do not correspond to consistent calendar dates, this Excel workbook tool was developed to allow users to efficiently explore expected future seasonal cycles using one of two methods. The first is a direct lookup of subsolar latitude and hemispheric seasonal state for a user-defined date. The second allows the user to specify a subsolar latitude range and temporal search window and will return all date ranges for which the subsolar latitude criterion is met. Both options provide plots that place the results in context.

Subsolar latitude values were calculated for 00:00:00 UTC on each day between 1 Jan 2020 and 31 Dec 2029 using the SpiceyPy subslr module and the following kernels: naif0012.tls (LSK), de438.bsp (SPK), moon_080317.tf (FK), pck00010.tpc (PCK), and moon_pa_de421_1900-2050.bpc (PCK) [3, 4]. Reported values have about ±0.04° uncertainty due to the maximum progression of the true value over the course of 24 hours; this is assumed to be higher than the degree of precision achieved with SPICE kernels over the decade the calculations cover.

The Lunar Season Calculator was developed by Sam Cartwright and Jordan Bretzfelder. We thank Dr. David Kring and LPI for their interest in hosting the tool. We are also greatly indebted to Andrew Annex and the other contributors to the SpiceyPy project. For questions or comments, please contact [email protected].

[1] Vasavada A. R. et al. (2012) JGR, 117, E00H18, https://doi.org/10.1029/2011JE003987
[2] Williams J.-P. et al. (2017) Icarus, 283, 300–325, https://doi.org/10.1016/j.icarus.2016.08.012
[3] Acton C. et al. (2017) Planetary & Space Science, 150, 9–12, https://doi.org/10.1016/j.pss.2017.02.013
[4] Code was based on the “Binary PCK Hands-On Lesson” available at https://spiceypy.readthedocs.io/en/master/binary_pck.html

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