Latitude and colatitude sum up to 90°.
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The colatitude is most useful in astronomy because it refers to the zenith distance of the celestial poles. For example, at latitude 42°N, Polaris (approximately on the North celestial pole) has an altitude of 42°, so the distance from the zenith (overhead point) to Polaris is 90 - 42 = 48°.
Adding the declination of a star to the observer's colatitude gives the maximum latitude of that star (its angle from the horizon at culmination or upper transit). For example, if Alpha Centauri is seen with a latitude of 72° north (108° south) and its declination is known (60°S), then it can be determined that the observer's colatitude is 108 - 60 = 48 (i.e. their latitude is 90 - 48 = 42°S).
Stars whose declinations exceed the observer's colatitude are called circumpolar because they will never set as seen from that latitude. If an object's declination is further south on the celestial sphere than the value of the colatitude, then it will never be seen from that location. For example, Alpha Centauri will always be visible at night from Perth, Western Australia because the colatitude is 90 - 32 = 58, and 60 is greater than 58; on the other hand, the star will never rise in Juneau because its declination of -60° is less than -32° (the negation of Juneau's colatitude).
- Weisstein, Eric W. "Colatitude." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/Colatitude.html
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