Circumpolar star trail photos — the ones with concentric arcs circling a fixed point — take a long exposure (or many stacked ones). This calculator simulates the pattern so you know what you're getting into before you commit hours to it.

Nothing new to learn here — it's the exact same Timer, just reached from a different pill. That consistency is by design: once you've learned one PhotoPills tool, you've basically learned three or four others too.

The Earth's rotation means any exposure long enough will start to blur stars into tiny trails. The Spot Stars calculator tells you the maximum exposure time you can use before that blur becomes noticeable — taking your exact camera's sensor and megapixel count into account.

Set your camera, focal length, and aperture, and the calculator gives you the maximum exposure time before stars start trailing — calculated two different ways.

The calculator's "Declination" setting affects the result — stars near the celestial equator trail faster than stars near the poles. Rather than guess, tap AR and point your phone at your actual composition. PhotoPills reads the declination of what you're framing and gives you an exact max exposure time for both landscape and portrait orientation.