Mercury (planet) at a steep angle. Mercury is 10° above the horizon when the planet appears directly above the Sun (i.e. its orbit appears vertical) and is at maximum elongation from the Sun (28°) and also when the Sun is 18° below the horizon, so the sky is just completely dark. This angle is the maximum altitude at which Mercury is visible in a completely dark sky. At middle latitudes, Mercury is more often and easily visible from the Southern Hemisphere than from the Northern. This is because Mercury's maximum western elongation occurs only during early autumn in the Southern Hemisphere, whereas
Planet Nine and proposed a possible orbit for the planet. This hypothesis could also explain TNOs with orbits perpendicular to the inner planets and others with extreme inclinations, as well as the tilt of the Sun's axis. Planet Nine is hypothesized to follow a highly elliptical orbit around the Sun with a period lasting 10,000–20,000 years. The planet's semi-major axis is estimated to be , roughly 20 times the distance from Neptune to the Sun, and its inclination to the ecliptic, the plane of the Solar System, to be about . The high eccentricity of Planet Nine's orbit could bring it as