[Jaeger]

you shouldnt define the conditions, when real navigation doenst work. you should define conditions, when it runs (sun must shining on day, stars must be visible at night) these conditions are not often available... and as i understand it right, on day you need sunrise, sunset or noon.
like vanjast descriobes it: dead reckoning is the normal use of navigation, celestial navigation fixes will be taken whenever its possible (not very often)