• 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**)

• **deep-sky objects (DSO)** — faint nebulae/galaxies: track the sky, shoot many long **subs**, calibrate and **average** → see [[Slopendium/09 Multiple exposure imaging|Denoising by averaging]]

• **planetary / lunar** — bright but blurred by **atmospheric seeing**: shoot thousands of fast frames and keep only the sharpest — **lucky imaging** → see [[Slopendium/10 Big data|Big data]]