Lunar ``West Pole'' Prime Meridian

Abstract

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 Current systems in use for the Moon (more than one are used) are awkward and out of date.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°S, 95°W); and, that longitude increase from 0° to 360° 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° to 195° are on the lunar nearside (includes libration) and 195° to 360° span the farside. Adding 5° to Earth’s angle from the eastern horizon gives longitude directly. The allpositive 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 naive 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. Background Designing the Planetary Grid. Humans impose coordinate systems on planetary surfaces to define locations, compute distances and areas mathematically, and provide a mapping control grid. The coordinate system is anchored to its parent body according to protocols. From time to time these protocols change, so at any given time different worlds may have different coordinate systems. There are only a few things to consider when anchoring a coordinate system to a planetary surface: axis of rotation, which defines the Poles, Equator, and parallels of latitude; definition of “north,” leading to or derived from the direction of rotation; location of the “Prime Meridian,” the starting point for marking longitude; and direction of increasing longitude, either prograde (in the direction of rotation) or * To whom correspondence should be addressed. † All authors: Lunar Base Research Team, The Oregon L5 Society, Inc., P.O. Box 86, Oregon City, OR 97045-0007; website http://www.OregonL5.org; email lbrt@OregonL5.org