lunarsky

An extension to astropy, providing selenocentric and topocentric reference frames for the Moon and transformations of star positions from the ICRS system to these frames. This is to describe the sky as observed from the surface of the Moon.

Non-relativistic transformations are calculated using lunar orientation data from JPL SPICE kernel files, read with jplephem.

Dependencies

• numpy
• astropy>=6.0.0
• jplephem

Installation

lunarsky may be installed with pip:

pip install lunarsky

or from the repository:

git clone https://github.com/aelanman/lunarsky
python setup.py install

The first time lunarsky is used, it will automatically download the JPL DE430 ephemeris (~150 MB) into astropy's data cache (~/.astropy/cache/). Subsequent uses read from the cache. To pre-fetch the kernel (e.g. before going offline, or in CI), run:

from lunarsky import spice_utils
spice_utils.download_big_kernels()

Usage

Definition of the MCMF and lunar topocentric frames, from Fig. 2 of [Ye et al.][1]

lunarsky provides the following classes:

• MCMF – The "Moon-Centered-Moon-Fixed" frame, this is a cartesian reference frame that rotates with the moon. This is chosen to be the Mean Earth/Polar frame, with a Z axis defined by the mean rotation axis of the Moon and a prime meridian defined by the mean direction to the Earth's center (the X axis is through this meridian 90° from the Z axis, and Y is defined such that XYZ is a right-handed system). This is analogous to astropy.coordinates.builtin.ITRS.
• LunarTopo – A topocentric (East/North/Up) frame defined at a position on the Moon's surface. This is analogous to astropy.coordinates.builtin.AltAz.
• MoonLocation – Analogous to the astropy.coordinates.EarthLocation class, this describes positions on the Moon in either selenocentric (x, y, z) or selenodetic (lat, lon, height) coordinates. The cartesian axes of the selenocentric system are those of the MCMF frame. Selenodetic coordinates can be defined relative to a few different ellipsoid models, but the default is just a sphere of radius 1737.1 km.
• SkyCoord – A replacement for astropy.coordinates.SkyCoord, with modifications that ensure compatibility with the MoonLocation class.
• Time – A replacement for the astropy.time.Time class, which can accept a MoonLocation instance as its location parameter, allowing for the calculation of local sidereal times on the Moon. The LST of an observer on the Moon is defined as the right ascension of the zenith.

Credit

This package uses lunar orientation data from SPICE kernel files produced by the NASA Navigation and Ancillary Information Facility (NAIF) [3] [4]. The transformations are defined using data in kernel files pck/moon_pa_de421_1900-2050.bpc, moon_080317.tf, and moon_assoc_me.tf. These may be found at the NAIF website, and were produced by Nat Bachman (NAIF/JPL) in March 2008. Binary kernel files are read using jplephem [5]. Further information may be found in the comments in the kernel files in the data directory.

References

[1]: Ye, Hanlin, et al. "Looking Vector Direction Analysis for the Moon-Based Earth Observation Optical Sensor." IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, vol. 11, no. 11, Nov. 2018, pp. 4488–99. IEEE Xplore, doi:10.1109/JSTARS.2018.2870247.

[2]: Annex et al., (2020). SpiceyPy: a Pythonic Wrapper for the SPICE Toolkit. Journal of Open Source Software, 5(46), 2050, https://doi.org/10.21105/joss.02050

[3]: Acton, C.H.; "Ancillary Data Services of NASA's Navigation and Ancillary Information Facility;" Planetary and Space Science, Vol. 44, No. 1, pp. 65-70, 1996.

[4]: Charles Acton, Nathaniel Bachman, Boris Semenov, Edward Wright; A look toward the future in the handling of space science mission geometry; Planetary and Space Science (2017); DOI 10.1016/j.pss.2017.02.013; https://doi.org/10.1016/j.pss.2017.02.013

[5]: Rhodes, Brandon. jplephem: Python module for evaluating JPL ephemerides. https://github.com/brandon-rhodes/python-jplephem