Lunar ‘‘West Pole'' Prime Meridian

We impose coordinate systems on planetary surfaces to define locations, compute distances and areas mathematically, and give us a control grid for mapping. The poles, equator, and parallels of latitude are defined by the planet's intrinsic property of rotation, but placement of the Prime Meridian of longitude is arbitrary. Proper placement and use of the Prime Meridian can make the coordinate system easy and intuitive, or difficult and confusing. Current systems in use for the Moon (more than one are used) are awkward and out of date. We propose the Prime Meridian bisect a prominent feature close to the Moon's "West Pole": Mare Orientale (20 degrees S, 95 degrees W); and, that longitude increase from 0 degrees to 360 degrees in the direction of rotation. We call this the "Lunar West Pole Prime Meridian" system. Today's "Mean Earth / Polar Axis" system dates from 1775 when mariners used the Moon to find longitude at sea. The mean sub-Earth point, in the center of the nearside, defines the Prime Meridian. Meridians are referenced in degrees east and west, or + and –, from this point. No significant lunar feature marks this Prime Meridian. This system is still used, with one major change: "east" and "west" were switched by international agreement in 1961. Earth's Prime Meridian has changed several times. The lunar coordinate system should be convenient for those on the Moon and in space as well as those on Earth. It also should be referenced to an endogenous lunar feature, not another planet. The Lunar West Pole Prime Meridian system is an improvement over the present system for all users. Longitudes roughly from 0 degrees to 195 degrees are on the lunar nearside (includes libration) and 195 degrees to 360 degrees span the farside. Adding 5 degrees to Earth's angle from the eastern horizon gives longitude directly. The all-positive numbering system-makes computation of change or distance in longitude easier, and removes sources of error. This location of the Prime Meridian is clearly discernable from space: a naïve observer might easily pick Mare Orientale as a marker. The Lunar West Pole Prime Meridian system is useful, simple, elegant, intuitive, endogenous to the Moon, and conforms to modern standards.