Prime Meridian

The baseline of longitude on Equinox (0°). All longitudinal measurement west of the Prime Meridian is designated as “west longitude” and all measurements to the east of the Prime Meridian are designated as “east longitude”. The historical assignment of the baseline was establish by the Second Planetary Survey (SPS) team. The selection was arbitrary and coincided with a north-south baseline that intersected the base camp on Equinox.