To find your longitude at sea, you needed to know two things simultaneously:
Because the Earth rotates 360° in 24 hours, the globe is divided into time zones. Each time zone roughly covers 15° of longitude (360 degrees / 24 hours = 15 degrees per hour).
With the advent of space navigation, scientists realized that the Earth isn't a perfect sphere—or even a perfect smooth ellipsoid. Irregularities in the Earth's mass alter its gravitational pull, which skews traditional surface measurements. meridian longitude
Longitude is measured from 0° to 180° East (E) or West (W) of the Prime Meridian.
Originally based on the solar time at the Greenwich meridian. To find your longitude at sea, you needed
参考：： 1度 ≈ 111公里 × cos(纬度) 。
Example:
The Earth is divided into 24 time zones, each roughly spanning of longitude.