<p>Understanding how climate has changed in the past has contributed to the development of sustainability. Solar radiation plays a fundamental role in the global energy balance, agricultural productivity, and the hydrological cycle. Here, we applied a method for estimating solar radiation—originally developed using modern daily weather descriptions from the Japan Meteorological Agency—to historical weather records. This approach aimed to reconstruct a long-term variation in solar radiation and explore its implications for past climate and society in Japan, including famine events in the latter part of the Little Ice Age. We first reconstructed solar radiation from 1720 to 1912 using daily weather records from historical diaries in Tokyo, Japan and evaluated these reconstructions against solar radiation estimates derived from the Japan Meteorological Agency’s sunshine duration records. Finally, we combined the reconstructed solar radiation with sunshine-duration-based estimates from 1896 to 1985 and direct observations from 1961 to 2022. The three-level classification effectively captured year-round fluctuations and substantiated significant low-insolation episodes coinciding with cool summers and famines in the eighteenth and nineteenth centuries. Compared to other methods or proxy data, the proposed method effectively reconstructed solar radiation for all seasons and provided higher temporal resolution data for historical reconstruction. This study demonstrates the feasibility of translating qualitative diary descriptions into quantitative solar radiation, providing a framework for future spatiotemporal expansions and deeper investigations into the linkages between climate variation and social change.</p>