Declination
Declination is a star's angular distance from the celestial equator, the same concept as latitude on Earth applied to the night sky. It ranges from -90° to +90°, with 0° at the celestial equator and ±90° at the celestial poles, and it's one of the two coordinates (alongside right ascension) astronomers use to fix a star's position independent of time or observer location.
For astrophotography specifically, declination determines how fast a given point in the sky appears to move — stars near 0° trail fastest, since they trace the widest possible circle around the pole in 24 hours; stars near ±90° trail slowest, since their circle shrinks toward a point. The full NPF rule includes a declination correction for exactly this reason, since the 500 rule alone assumes a single, worst-case-adjacent rate that isn't accurate for every target.
Frequently Asked Questions
What's the declination of the Milky Way's galactic core?
It sits relatively close to the celestial equator from most Northern Hemisphere shooting locations, which is why Milky Way exposure times are typically calculated closer to the faster-trailing end of the declination range.
What's the declination of Polaris?
Roughly 89.3° — close to, but not exactly, the celestial pole, which is why Polaris traces a small but real circle over 24 hours rather than staying perfectly fixed in the sky.