This manuscript presents a tour of measurements realised over Iberian Peninsula, with a focus on Royal Observatory of Madrid. The information are interesting and somehow unique. Nevertheless, I am in the inevitability to recommend a major revision of this manuscript. It has to be re-worked to attain an acceptable level for publication in HGSS. 

Specific comments

• The paper structure needs to be revisited, with a clear introduction, a section dedicated to ROB where all information about it are gathered, a section on other Spanish observatories, a result session to show different comparisons between observatories and with models, before concluding.
• This manuscript invokes to provide an historical evolution of the geomagnetic declination at the Royal Observatory of Madrid. In describing the history it is needed to tell more about the first instruments (e.g. lines 74-77 – which kind of magnetometers of inclinometers, etc). Figure 5 shows an inclinometer, maybe some others photos can be provided?
• Many information are difficult to read. For example, in the section “Observatory data selection” I suggest the authors to draw a map with the used observatories. It would be nice to provide a table gathering the main information (observatory name, iaga code, coordinates, starting time - closing time, instruments, data availability - minutes, hourly, monthly, annual, etc).
• Some figures need to be revisited. E.g.
  1. In Figure 4 the comparison with the gufm1 model is presented, but this model is not built to describe variations on daily basis. Moreover, the difference is not du only to crustal field, but maybe to other factors, as unmodelled contributions or errors in measurements. To compute the crustal field (which can be considered constant in time) I suggest you to adapt methods proposed in the literature.
  2. Figures indicating series and comporting many plots may be view over the same time-span (min _ max of the series). This refers mainly to figures 7, 11.

• Coimbra observatory has a long series of observation. A comparison with the series published in 2021 (and not cited), can bring interesting information and discussions.
• The IGRF model is invoked (line 369), without citations. This choice is not very clear, and I wonder why not to use the gufm1 and COV-OBS models, only.
• The last session needs to be revisited, to get into some real discussions/ conclusions. The section on ROM observatories has to include the Table 1 and the Figure 16. At the end of the paper we would expected some interpretation of the observed secular variation curves.

Finally, efforts are needed to improve the English. Some parts are difficult to read: e.g. abbreviation used and not defined or defined after some pages, as IGN; “Declination data is translated from SPT to ROM » - data are not translated, they are adjusted; some expressions are not clear (“Note that the solar activity is not recorded before 1700 due to the scarce number of declination data”). I encourage the authors to work on the manuscript to provide a more fluid reading.

Some references:

Mandea, M. and Langlais, B. (2002) Observatory Crustal Magnetic Biases during MAGSAT and Oersted Satellite Missions. Geophysical Research Letters, 29.

Morozova, A. L., Ribeiro, P., and Pais, M. A.: Homogenization of the historical series from the Coimbra Magnetic Observatory, Portugal, Earth Syst. Sci. Data, 13, 809–825, https://doi.org/10.5194/essd-13-809-2021, 2021

The contour line of zero geomagnetic declination always exists somewhere on the Earth's surface. The fact that it passes through a certain point at a certain time has no special physical meaning. On the other hand, it is meaningful to know when the geomagnetic declination becomes zero at a specific observation point that has played a historical role in measuring the geomagnetic field, because it adds new data to the geomagnetic record accumulated at that observation point.

The main objective of this paper is to determine the date and time of occurrence of zero declination at the Royal Madrid Observatory（RMO）, the first observatory in Spain to begin routine geomagnetic observations, and in relation to this, to summarize the distribution and temporal changes of declination in the Iberian Peninsula and its surrounding areas, as well as the history of the scattered observatories.

The structure of the paper is reasonable but there is room for improvement to make it easier for the casual reader to understand.

1. The font on the diagrams is too small.
2. The full names should be included in parentheses when their abbreviations appear at first.
3. Locations of the observatories referred should be shown on the map. This could be included in Figure 8.
4. Figure 4: The time variation is unclear. The vertical scale should be changed so that the variation is clearer.
5. lines 24 and 31：What "Supplementary Material" do refer to?
6. Lines 25-26: "that this observatory does not have a great tradition on geomagnetism"): What does this mean? Is RMO not important?
7. Figure 2 : Zero declination horizontal line should be indicated.
8. A brief explanation of the geomagnetic database (HISTMAG, gufmq1model), is necessary
9. Is it possible to know the date and location of the first self-recording magnetometer observations?