How do you calculate Co-Altitude?

To calculate Co-Altitude, you need to determine the angular distance from the observer's zenith (directly overhead) to the celestial body being observed, which is typically measured in degrees. Co-Altitude is calculated by taking the complement of the object's altitude above the horizon.

When using the formula of (90 degrees - Altitude), you are effectively calculating how much further you need to look upward from the horizon to reach the zenith. This result is then converted to nautical miles. The conversion of degrees to nautical miles is based on the fact that each degree of latitude corresponds to approximately 60 nautical miles. Therefore, the correct approach ensures that you are determining the angular distance in a way that corresponds to real distances on the Earth's surface.

The other options incorrectly apply operations to the altitude calculation or confuse the relationship between altitude and distance. They do not properly consider how to measure the angular distance from the horizon to the zenith in a way that can be quantified into nautical miles for navigation purposes.