Equator one whose plane passes through the center of the globe. The plane of Earth's equator, when projected outwards to the celestial sphere, defines the celestial equator. In the cycle of Earth's seasons, the equatorial plane runs through the Sun twice per year: on the equinoxes in March and September. To a person on Earth, the Sun appears to travel above the Equator (or along the celestial equator) at these times. Light rays from the Sun's center are perpendicular to Earth's surface at the point of solar noon on the Equator. Locations on the Equator experience the shortest sunrises and sunsets
Mercator 1569 world map that OE=OD, here approximately 54°. This is a measure of the rhumb line distance between the points on the sphere corresponding to A and B on the spherical Earth. Since each degree on the equator corresponds to 60 nautical miles the sailing distance is 3240 nautical miles for this example. If B is in the second quadrant with respect to A then the upper rose is used and if B is west of A then the longitude separation is simply reversed. Mercator also gives a refined method which is useful for small azimuths. The above method is explained in Legend