century before the International Meridian Conference of 1884 established one for Earth. The definition of the Martian prime meridian has since been refined on the basis of spacecraft imagery as the center of the crater Airy-0 in Terra Meridiani. However, Mars does not have time zones defined at regular intervals from the prime meridian, as on Earth. Each lander so far has used an approximation of local solar time as its frame of reference, as cities did on Earth before the introduction of standard time in the 19th century. (The two Mars Exploration Rovers happen to be approximately 12 hours and one minute apart.) Since the late 1990s and arrival of Mars Global Surveyor at Mars, the most widely used system for specifying locations on Mars has been planetocentric coordinates, which measure longitude 0°–360° East and latitude angles from the center of Mars. An alternative system that was used before then is planetographic coordinates, which measure longitudes as 0°–360° West and determined latitudes as mapped onto the surface. However, planetographic coordinates remain in use, such as on the MAVEN orbiter project. Coordinated Mars Time Coordinated Mars Time (MTC) or Martian Coordinated Time is a proposed Mars analog to Universal Time (UT1) on Earth. It is defined as the mean solar time at Mars's prime meridian. The name "MTC" is intended to parallel the Terran Coordinated Universal Time (UTC), but this is somewhat misleading: what distinguishes UTC from other forms of UT is its leap seconds, but MTC does not use any such scheme. MTC is more closely analogous to UT1. Use of the term "Martian Coordinated Time" as a planetary standard time first appeared in a journal article in 2000. The abbreviation "MTC" was used in some versions of the related Mars24 sunclock coded by the NASA Goddard Institute for Space Studies. That application has also denoted the standard time as "Airy Mean Time" (AMT), in analogy of Greenwich Mean Time (GMT). In an astronomical context, "GMT" is a deprecated name for Universal Time, or sometimes more specifically for UT1. Neither AMT or MTC has yet been employed in mission timekeeping. This is partially attributable to uncertainty regarding the position of Airy-0 (relative to other longitudes), which meant that AMT could not be realized as accurately as local time at points being studied. At the start of the Mars Exploration Rover missions, the positional uncertainty of Airy-0 corresponded to roughly