Lunar Time

• Autores: Defraigne, P.; Meynadier, F.; Tavella, P.; Bourgoin, A.
• Año de publicación: 2025
• Idioma: inglés
• Tipo de recurso: documento de conferencia
• Materia: Ciencias Astronómicas; Lunar time scale; Relativistic timekeeping; Cislunar navigation
• Institución: Universidad Nacional de La Plata

These last years have seen a renewed interest for lunar missions. Would it be for scientific applications, navigation, or communication on the Lunar surface, a lunar standard reference time scale should be defined. Consider­ ing the difference of gravitational potential around the Moon or around the Earth, as well as their different motions in an inertial frame, the Einstein gen­ eral relativity theory predicts of course that the time measured on Earth and that measured on the Moon will not coincide. For example, an ideal clock on the Moon surface, when compared to an ideal clock on Earth surface, would exhibit a 58.7 μs/d drift plus some pseudo-periodic variations, which will be impossible to ignore for most practical purposes. It is therefore difficult to use UTC directly as the reference time scale in the cislunar environment, and space missions’ designers are asking for a practical time scale that would be used as the reference time scale in this environment. The IAU resolution II (2024) recommends constructing a Lunar Celestial Reference System (LCRS), with its time coordinate designated Lunar Coor­ dinate Time (TCL), using the same techniques as to construct the OCRS and TCG, and keeping the unit of measurement of TCL consistent with the SI second. In its resolution III (2024), the IAU also recommends that "the relationships between the possible versions of a lunar reference time scale and other time scales, in particular a lunar coordinate time and UTC, are pursued in a collaborative agreement among the relevant international organizations". One of these is the Consultative Committee for Time and Frequency (CCTF), a sub-committee of the International Committee for Weight and Measures (CIPM). CCTF experts and members of the national metrological institutes in close contact with space agencies started elaborating a strategy to work actively towards a consensus. In this presentation, we will provide a review of the relativistic time scales in the Solar System, and how a new reference scale should be considered for lunar applications considering the IAU recommendations. In particular, we will investigate the possibility of scaling TCL which would allow to remove secular deviation between the lunar time reference and UTC, or between the lunar time reference and the natural time of a clock at the lunar surface. We will analyse the theoretical and practical advantages and drawbacks of the different options for a lunar reference time scale. We will then propose a scheme for the realisation of a lunar reference time, for both the situation where only Earth clocks are available to provide this reference, and the situation where one or more clocks are on the Moon. And we will propose some practical implementation to maintain the traceability of a lunar reference time to UTC.
Facultad de Ciencias Astronómicas y Geofísicas