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The 500 rule caps untracked exposure at 500 divided by full-frame equivalent focal length - 20.8 seconds for a 24 mm lens. Stars near the celestial equator trail fastest, so dividing by cos(declination) extends the limit towards the poles: at 20 degrees declination it becomes 22.2 seconds. High-resolution sensors often need the stricter 300 rule.
500 rule
seconds = 500 / (focal length x crop factor)
Declination correction
seconds = 500 / (focal length x crop) / cos(declination)
Because stars near the pole trace smaller circles and move more slowly across the frame, so they tolerate longer exposures.
Because modern high-pixel-density sensors resolve trailing that the 500 rule allowed. Pixel-level tests are stricter still.