Date: 7 Nissan 5573
From: Rabbi Moshe Sofer
To: Unknown recipient
The Chasam Sofer is asked whether the Birkas HaChamah (blessing of the sun, recited every 28 years) may be said when clouds completely obscure the sun. He rules emphatically that actual visual sighting is required, though the blessing may be recited if the sun's outline is visible through clouds.
Source: Shu"t Chasam Sofer OC §56