Scientists estimate sunlight in 18th and 19th century Tokyo using historical diaries‎

The amount of sunlight, or solar radiation, that a location receives makes a big impact on weather conditions, crop success, rainfall and overall climate trends. Today, instruments called pyrheliometers are used to carefully calculate how much sunlight occurs at a particular location, but these instruments weren't available to quantify sunlight until 1838, and automated pyrheliometers weren't invented until the early 20th century.