Solargraphy at Paris Meudon observatory

Jean-Marie Malherbe

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Jean-Marie Malherbe. Solargraphy at Paris Meudon observatory. observatoire paris. 2025. hal-04964291

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# Solargraphy at Paris Meudon observatory

Jean-Marie Malherbe (emeritus astronomer)

Observatoire de Paris, PSL Research University, LIRA, France

Email: [Jean-Marie.Malherbe@obspm.fr](mailto:Jean-Marie.Malherbe@obspm.fr); ORCID: [https://orcid.org/0000-0002-4180-3729](https://orcid.org/0000-0002-4180-3729)

24 February 2025

## ABSTRACT

Solargraphies are long exposure images of the Sun's apparent trajectory got during several months that can be simply recorded with a black and white photographic paper inserted into a cylindric can through a pinhole. We present in this short paper simulations and results of solargraphies obtained at Meudon observatory, together with prospects for the future.

## KEYWORDS

Solargraphy, pinhole camera, Sun apparent motion

## INTRODUCTION

Solargraphies are images of the Sun's apparent trajectory got during several months. They result from long exposure pictures that can be recorded with a black and white photographic paper inserted into a cylindric can through a pinhole. This is a low cost device (20 euros), also called solargraph, which can operate everywhere but needs to stay permanently in a fixed position for several months (preferably for 6 months between two solstices). The photographic paper recovers the cylindrical surface of the inner scan and light penetrates trough a pinhole to form images of the Sun's apparent motion, as a moving spot. The paper is naturally slowly impressed by the direct light of the Sun and does not need to be developed after the long exposure. The removal from the can does not require obscurity, but the paper must not be exposed any more to solar light; a colour computer scan (despite the fact that it is a black and white paper) is sufficient to reveal the image. The history and descriptions of this process were previously written by Fosbury & Trygg (2010), Trygg (2012) and Joglar (2017) and many results got world wide are presented by the Solargraphy project coordinated by Tarja Trygg (Aalto University, Helsinky, Finland) at: [http://www.solargraphy.com/](http://www.solargraphy.com/)

Prior to observations with solargraphs, it is of interest to simulate the images that can be expected according to the latitude and longitude of the observing site. Our computer program is an IDL procedure based on the NASA astronomy library (IDL-ASTRO). The result strongly depends on the latitude, as shown by figure 1 (Meudon, 48° latitude), figure 2 (polar circles at 66.5°) and figure 3 (tropics at 23.5°). The height of the image (z direction), for a vertical scan of radius R = 1 cm, varies much, from 2 cm at the polar circle to infinity at the tropics (because the angular height of the Sun can reach there 90° at the solstice); at middle range latitudes, such as the one of Paris, the height of the solargraphy is typically of the order of 5 cm. The size is proportional to the radius R of the cylinder both in x and z directions. We used at Meudon black plastic cans of old 35 mm films, with double height for some of them.
<figure><img src="image_2.png" /></figure>

**Figure 1:** Computation of the cylindrical projection (left) and of the angular height of the Sun versus time (right) for Meudon observatory from 21 June to 21 December (6 months). The (x, z) cylindrical projection (left) was done for a can radius R = 1 cm; x and y are proportional to R.

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

**Figure 2:** Computation of the cylindrical projection (left) and of the angular height of the Sun versus time (right) for the 66.5° latitude (polar circles) from 21 June to 21 December.

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

**Figure 3:** Computation of the cylindrical projection (left) and of the angular height of the Sun versus time (right) for the 23.5° latitude (tropics), from 21 June to 21 December.

<figure><img src="image_7.png" /></figure>
## SOLARGRAPHIES AT MEUDON

We put some solargraphs at the top of Meudon solar tower, 35 m above the ground (Figures 4, 5 & 6) and at the “Grand Sidérostat”, just a few metres above the ground, near the spectroheliograph building (Figures 7, 8 & 9). The scans were performed by Tarja Trygg in Finland.

<figure><img src="image_8.png" /><figcaption>Figure 4 : Meudon observatory, top of solar tower, 2012, double height can</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Figure 5 : Meudon observatory, top of solar tower, 2012, single height can</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 6 :** Meudon observatory, top of solar tower, 2012, double height can</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 7 :** Meudon observatory, Grand Sidérostat, 2013, single height can</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure 8 :** Meudon observatory, Grand Sidérostat, 2021, two processings of the same image</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 9 :** Meudon observatory, Grand Sidérostat, 2022, double height can</figcaption></figure>
The main difficulty we often met was the entry of water inside the cans; as they are exposed during 6 months to wind and rain (these are usual conditions in the north of France), one has to be careful with humidity which alters the photographic paper inside. Many of our cans were not waterproof and, in some of them, the paper was destroyed. We mainly used cylinders made of double height plastic cans; the simulation shows that, at high latitudes (60°), single height suffice, but it was not the case at Meudon (48°). We started solargraphy in 2012 and we continue today. The next step will concern analemmas.

## FUTURE PROPECTS: ANALEMMAS

Analemmas are solargraphies which use a pinhole not permanently open, but only for a short time (a few minutes) at specific hours of the day. The aim is to evidence the properties of the terrestrial orbit around the Sun, in particular the elliptic motion which implies speed variations. For instance, if one observe with the solargraph at 12:00 UT during the full year, the trajectory of the Sun is not a thin line, but has the shape of the “8” number. In order to prepare such observations, we first produced the computer simulations of figures 10 & 11.

Figure 10 shows what we could get on the solargraph photographic paper at Meudon with the pinhole open during 3 minutes at 9, 10, 11, 12, 13, 14 and 15 H UT. As it is of interest to investigate other latitudes, we computed, using IDL-ASTRO, analemmas for latitudes of 30° and 60° (figure 11). The code (.pro procedure) is available upon request.

<figure><img src="image_14.png" /><figcaption>**Figure 10** : Analemma computation for Meudon observatory at various times, 9:00, 10:00, 11:00 UT (left); 12:00 UT (centre); 13:00, 14:00, 15:00 UT (right). The small sun traces correspond to 3 minutes of exposure time, while the time interval between traces is 5 days.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Figure 11** : Analemma computation for latitude 30° (left) and 60° (right) at various times: every 30 min around noon at left, every hour around noon at right. The sun traces correspond to 3 minutes exposure time, while the time step between traces is 5 days.</figcaption></figure>

## CONCLUSION

Solargraphy is a pedagogic tool to discover the apparent motion of the Sun and discuss (with anal emmas) the characteristics of the elliptic orbit of the earth. As images vary with the latitude of the observing site, it is of great interest to have images produced at different places around the world, in the spirit of the project initiated by Tarja Trygg.

## REFERENCES

* Fosbury, R., Trygg, T., 2010, “Solargraphs of ESO”, *The messenger*, 141, 43-45

* Joglar, J., 2017, “Solargraphy”, *Revista de la Societat Catalana de Química*, 16, 38-44

* Trygg, T., 2012, “Solargraphie, rencontre avec l’astronomie”, *l’Astronomie*, September 2012, 42-45