500 or 600 rule

[Astromontufar]

Hi all,

I understand the 500 rule or 600 rule perfectly fine, however a question came up while talking about the rule, does any knows why is 500 or 600, I mean where did that number came from?

Thank you,

Sergio

[Chris Peterson]

I understand the 500 rule or 600 rule perfectly fine, however a question came up while talking about the rule, does any knows why is 500 or 600, I mean where did that number came from?

It's just an approximation based on a more rigorous calculation for the distance stars move during a long exposure at the image plane of a camera or telescope of some given focal length. If you want to know exactly what the trail length will be, you need to do the full calculation, and also take into consideration the declination range of the sky you're imaging.