Register of solar disk and chromosphere drawings

anno 1871
[Angelo Secchi]
Pencil drawing
283 x 308 mm
Historical archives of INAF-Rome Astronomical Observatory, Monte Porzio Catone (Rome)

In 1871 Angelo Secchi made observations of the chromosphere in order to analyse the distribution of the protuberances around the solar disk and to investigate the relationship between spots, facolae and protuberances in the various solar regions. He uses the Merz telescope in combination with the spectroscope for his solar studies. It executes the observations by placing the spectroscope slit tangent to the edge of the solar disk image and then moving it parallel and along the solar rim to explore the low chromosphere; Another method often used for the analysis of the protuberance spectrum was to align the slit perpendicularly to the solar edge.
The drawing reproduced here shows an example of the panels produced during the observations. These panels contain drawings of the solar disk and the low chromosphere, with annotations. Note the various regions of the photosphere with spots, pores, facolae and polar regions (with attenuated granular structure) and of the chromosphere with protuberances observed beyond the sun’s edge. The annotations describe the time and duration of the observations, the nature of the observed regions and their characteristics. The drawings added in the lower parts of the panels (here showed below) summarize the data concerning the position, the shape and the dimensions of the various protuberances. In addition to the observed protuberances, the added sketch readily recognizes the small-scale and rapidly evolving plasma jets that are stretched out from the photosphere, now known as clots and fibrils.
Buckets noted that the protuberances are not evenly distributed in the two hemispheres and around the solar disk: they are present predominantly in the regions between 10 and 40 degrees of latitude and are scarce at the poles and at the equator. Moreover, from the evidences gathered, he inferred that the appearance of the crown during the eclipses “is closely connected with the protuberances and the Facolae”.
Several of these registers are kept at the Inaf-Rome Astronomical Observatory; They are of particular scientific interest for the current studies on solar activity, since they constitute a valuable archive of historical data on stains, facole and solar protuberances.

Ilaria Ermolli, INAF-Osservatorio Astronomico di Roma
Marco Ferrucci, INAF-Osservatorio Astronomico di Roma