• the regime where the **exposure is minutes to a year**, so the sky's motion *is* the subject:
• **star trails**: the Earth's rotation turns stars into arcs about the celestial pole — captured either as one very long exposure or (better, for noise/saturation) as a **stack of many short frames** combined with a *lighten* blend (cross-ref Multiple exposure)
• **solargraphy**: a **pinhole camera on photo paper** left for **days to a whole year** records the Sun's daily arcs marching across the sky with the seasons — *no development*, the latent image is scanned directly; the canonical "sun over a year" image
• **why you usually stack instead of one long exposure**: a single ultra-long digital exposure piles up **dark current, light pollution, and saturation**; many shorter **subs** that are aligned and averaged win on noise and dynamic range (→ DSO, below) — solargraphy survives only because photo paper is near-grainless and self-limiting
• ties to FUNDAMENTALS: long exposures expose the **thermal/dark-current** noise term and motivate **cooling**; the sky's motion motivates **equatorial tracking mounts** (and software **field de-rotation**)