Because Niépce's exposure of Le Gras took about eight hours, during which the sun moved from east to west, the finished image appears illuminated on both sides of the building.
Notes on verification
The dual-illumination effect from a long exposure is well corroborated across multiple independent sources (Wikipedia, PetaPixel, Harry Ransom Center). However, the specific 'eight hours' duration is described as a traditional estimate, disputed by at least one modern recreation study suggesting several days instead. [cascade flags: wikipedia_dropped_other_sources_exist | tier=silver indep_score=0.833 clusters=3 claim_tier=notable]
Sources
- History of photography | History, Inventions, Artists, & Facts - Britannica (seed:geography_and_travel)
- https://www.historyofinformation.com/detail.php?id=3665 (corroboration)
- https://petapixel.com/the-first-photo/ (corroboration)
- https://www.hrc.utexas.edu/niepce-heliograph/ (corroboration)