Star Trail Calculator

How it works

frames = ⌊total time ÷ (exposure + gap)⌋ · sky rotation = 15.041° per hour (sidereal rate) · trail arc = 15.041° × time covered in hours · trail length on sensor = focal length × arc (radians) at the celestial equator, × cos(declination) elsewhere · storage = frames × file size

The sky turns once every sidereal day (23 h 56 min), about 15° per hour, so star trails are arcs centred on the celestial pole. Shooting many 20–30 second frames and stacking them with a "lighten" blend gives long trails with less noise and fewer satellite or plane problems than one long exposure, and keeps the gaps short — set a 1 second interval so the trails look continuous. Trails are longest near the celestial equator (looking east, west or south from mid-northern latitudes) and form tight circles around Polaris or the south celestial pole.

Worked example

One hour of 30 s frames with a 1 s gap gives 3600 ÷ 31 = 116 frames covering 59.9 minutes. The sky turns 15.0°, which at 16 mm is a 4.19 mm trail on the sensor for stars on the celestial equator, and 116 × 30 MB = 3.48 GB of RAW files.

Sources: Wikipedia — Star trail · Wikipedia — Sidereal time

Frequently asked questions