At this point you may see a link between the geographic coordinate system on Earth—that is longitude and latitude, and that of the celestial sphere—Greenwich hour angle and declination. And there is a connection. For every coordinate on Earth, there is exactly one corresponding point in the sky that is directly overhead. The point directly overhead is called our zenith. Conversely, for every star in the sky, there is exactly one location on Earth directly under the star. We call this the star's geographic position, abbreviated “GP”.
It is worth noting that in this module, we often use the word "star" for any navigational object in the sky. In reality, we can (and do) use the Sun, Moon, and bright planets as navigational objects. The concepts are the same so we will simply use the word ``star" as a general term for all of these objects.
So how are celestial objects used in navigation? If we look up and there is a star at our zenith (that is, directly overhead), then we are at the star’s geographic position.
If we know the star's Declination and Greenwich Hour Angle, or GHA, we know our latitude and longitude. Our latitude is simply the star's Declination. And our longitude, measured westerly, is the star's GHA.
You will recall that the declination of Arcturus was 19 degrees north. When Arcturus is at zenith over Hawaii, its geographic position is located at a precise coordinate on the island, with latitude and longitude corresponding to the declination and GHA of Arcturus. Thus, Hawaii’s latitude is 19 degrees north. We can easily verify this on a map.
Here is an important concept to remember. A star's geographic position, that is, its (west longitude, latitude), is its (GHA, Declination).
This is the basic idea of how a star’s position can be used to find our location on Earth.