An Introduction to the Kodaikanal Digital Archive

Dr. Dipankar Banerjee visited NAOC on April 18, 2016 and presented a lecture about efforts to digitize over 100 years of solar image data that have been acquired at the Kodaikanal Observatory in India. Dr. Banerjee is from the Indian Institute of Astrophysics in Bangalore and is the principal investigator for the Kodaikanal Digitization program. He has visited Beijing several times before, for example, he also delivered a lecture at NAOC in the spring of 2015 about new results related to solar astronomy and the audience this time appreciated the similarly animated, enthusiastic style of his presentation. 

Dr. Banerjee started his talk by saying that the Kodaikanal Observatory has the oldest continuous set of solar observations in the world, which started in 1904. Originally, these images were taken on glass plates, but later they transitioned to records taken with photographic film and a CCD. He continued by saying that his group has been calibrating the historical data set and it is now ready for scientific analysis. He then showed the audience the website where these data can be searched and downloaded (, and discussed various attributes associated with the data, including white light images and Halpha spectroheliograms. He also described the various types of scientific studies that can be performed with this data set, including analysis of how historical solar activity might be connected to Earth's climate. 

He continued his talk by comparing early results of the Kodaikanal data with the Greenwich data set, another historical data set of solar activity that spans most of the 20th century. Dr. Banerjee mentioned that on the whole, results from these two data sets agree very well, and there is a correlation coefficient of 0.94 between them. However, because they were acquired from different geographic locations and under different weather conditions, these two data sets do show some differences in terms of sunspot numbers at various times, especially during maxima in the solar cycle. The addition of this newly compiled Kodaikanal data set that is ready to be released to the scientific community will help advance efforts to understand solar cycles and related science.