Review of “Earth Observations from the Moon surface: dependence on lunar libration”

The manuscript entitled “Earth Observations from the Moon surface: dependence on lunar libration” provides innovative thoughts and visions on deploying DSCOVER/EPIC-Type camera on the moon for earth monitoring. Indeed, the Moon is a stable and longevous carrier for an Earth-observing sensor. It also exerts influences on precipitation, ice nuclei concentration, hurricanes, etc. For those Moon-related terrestrial phenomena, the lunar platform provides a unique perspective to understand the evolution of the phenomena.

Detailed analysis of the impact of the lunar libration on the visual position of the Earth on the Moon sky is presented, providing suggestions/insights on the possible field of view for the lunar-base earth observing sensors. This manuscript focuses on the impact of lunar libration on the sensor FOV. However, it would be more appealing if a more detailed discussion on the configuration of the potential lunar-base EPIC instruments.

Overall, I recommend the publication of this paper with minor revisions. The following are some minor points when I went through the paper in detail. They are only suggestions for the consideration of the authors when they revise the paper.

Overall, I recommend the publication of this paper with minor revisions. The following are some minor points when I went through the paper in detail. They are only suggestions for the consideration of the authors when they revise the paper.

Line 9, line 293, and line 363, it will be better to have consistent values for the angular diameter of the Earth in the Moon sky.

Line 92, Libration of the Moon. I like the detailed explanation of optical libration provided here. It looks like the physical libration is neglected in the discussion due to its small magnitude compared to the optical libration.

Line 165, what does Figure serve for? I cannot find any discussion in the main text.

Line 285, what is the scientific goal for the slit observation? It will be better to provide some potential applications there to enlighten the reader.

Line 298, does this WFOV camera have the same spectral configuration as the EPIC?

Line 292, is ‘wich’ a typo?

Line 307, what does Figure 11 serve for? It looks like only the polar regions of the moon are plausible areas for the long-term operation of a lunar sensor. I suggest combining Figures 10 to 12 into one figure and focusing on one period to avoid unnecessary confusion.

Line 342, it will be better to provide the percentage of the loss of the lit area as the decrease of the Earth phase to give readers a better concept of the spatial and temporal observational capability of a moon-base sensor. I guess the Earth phase is one of the disadvantages of the lunar-base sensor. But if there is a dark/nighttime component in the instrument, it will greatly fill the gaps and provide valuable information on the dark side of the Earth just like VIIRS DNBs do.

Line 363, What is the potential scan frequency of the WFOV camera and the hyperspectral sensor? Without this information, it is hard to follow the author’s claim ‘which will allow each slit to receive at least one scan of the entire Earth's surface in one pass. Please discuss more here if possible.

Although it might be beyond the scope of this paper, it will be better to also shortly discuss/mention the possible limitation of the lunar-base sensor, such as the impact of the lunar environment (lunar dust exosphere, high energy cosmic particle, meteoroid, etc.) to give readers a whole picture of the concept.

General comments

This is a useful study, quantifying the Earth movement in the Moon sky, which can then be used to optimise Earth observations from the Moon. Based on this analysis, it emphasis the possibility of using a multi-slit based spectrograph, each slit associated with a different spectral range, and with a different position of the Earth in the sky.

The authors should develop not only the advantage of a fixed observing platform on the Moon, but also the issues. For example, the libration is not always moving at the same speed, or following the same path (i.e. sometimes the Earth may not go through some of the slits; sometimes, it may stay on one slit, then return). Scanning different spectral ranges at different times may not be relevant to compare them.

The quality of the graphs in the draft is poor, raster at around 140 dpi. For the final paper please use vector based graphics or raster of at least 300 dpi!

Specific comments

31 – Forward direction is around 0^o, while backscatter direction is by definition around 180^o. Then, the phase angle cannot be between 2^o and 12^o. Also, why is starting at 2^o and not at 0^o? In addition, why is that much anyway if the size of the Earth as seen from L1 is < 1^o. Even if these affirmations come from other papers, they seem so strange that the reader may question if they are true. Please be proactive and give more info (for example, by answering my questions above) to convince the reader.

81 – Still concerning the Figure 2, I know you had this one for a long time in the manuscript, but I realise now that it is misleading to have the Epic image as an “insert” of smaller dimensions. It suggests you may have such a large change in angular dimension of the Earth between LRO and Epic. I suppose the Earth angular size should be similar in both. Then I would rather suggest to put those pictures side by side in Fig 2, with the same size of Earth.

167 - Figure 6 was never introduced or discussed in the text. The figure should be introduced after its first mention.

243 – Here you give a definition of the “draconic month” different from that of line 100. They may be equivalent, but that is not necessarily obvious for the reader. Please use the same definition, or explain why the two definitions are equivalent.

374 – I think you could remind in the conclusions more from your results that could be useful messages for the reader, like the statistics (density) of points, speed of libration etc. Otherwise, it looks as you didn’t find much.

Technical comments

1 – This comment is just an advice to avoid using “:” in the paper title “Earth Observations from the Moon surface: dependence 1 on lunar libration”. This is a special formatting character in Latex and it usually leads to errors when citing and referencing your paper.

9 – “1.9^o +/- 0.1^o” has to be in italic as well, if the entire Abstract is.

12 – Add: reaching “an amplitude of” 7.9^o.

13 – Add: center of the Earth “to” move

53 – The resolution of Fig 1 is 142 pixels/inch and the journal requires minimum 300 pixels/inch. If those pictures are not available at higher resolution, you may increase by resample it artificially to respect the rule.

81 – Same as for 53. I suspect your pdf maker was set to a lower resolution. Please check and, if true, increase its resolution rather than resampling the pictures.

84-85 – I think you mean “a linear (one-dimensional) array of 5064 elements.” instead of “the linear (one-dimensional) array from 5064 elements.”

114 – Concerning “between 0.0255÷0.0775”, I do not think the sign “÷” is correctly used. It is more for calling a “division”. You should better use “between 0.0255 and 0.0775”.

167 - Figure 6 is also of a too low resolution.

183 – Why do you have a break of line (row) here?

191 – Replace “over a given point” with “above a given point”.

266 – You forgot the verb “is” N = 2191.

283 – You have to be consistent when calling the Figures 10-11 (as everywhere else), not Figs 10-11.

336-338 – Use Figure 7 not Fig. 7.

340-343 – Use Figure 12 not Fig. 12.

378-379 – “We suggest that proposed lightweight EPIC-Moon instrument on a fixed platform will provide the proof of concept” is ill formulated. May be you want to say “We suggest a lightweight EPIC-Moon instrument on a fixed platform to serve as a proof of concept”, or something similar.