(§) for areas that do not have a legal standard time (the Greenland ice sheet and Antarctica). The United Kingdom specifies UTC−3 for the claimed British Antarctic Territory. Preference for GMT over UTC In maritime usage, GMT retains its historical meaning of UT1, the mean solar time at Greenwich, which is empirically adjusted to track unpredictable variations in the Earth's rotational period. UTC, atomic time at Greenwich, makes these adjustments on a coarser granularity than GMT. Establishing latitude by local observations of solar position requires determination of the latitude on Earth where the Sun is directly overhead at the time when the observation is taken. Thus the coarseness of UTC in determining solar time makes it inaccurate in establishing the reference latitude of solar meridian, differing by as much as 0.9 seconds from UT1, creating an error of of a minute of longitude at all latitudes and which is at the equator but less at higher latitudes, varying roughly by the cosine of the latitude. However, the time correction DUT1 can be added to UTC to correct it to within 50 milliseconds of UT1, reducing the error to only . Solar position can also be established by celestial observation of more distant stars taken at nighttime, but this also involves a calendrical correction due to the Earth's elliptical orbit around the Sun; establishing solar position by observations of bright solar objects, such as Venus (also with its own elliptical orbit), involves yet further complexity. Today In practice, nautical times are used only for radio communication, etc. Aboard the ship, e.g. for scheduling work and meal times, the ship may use a suitable time of its own choosing. The captain is permitted to change his or her clocks at a chosen time following the ship's entry into another time zone, typically at midnight. Ships on long-distance passages change time zone on board in this fashion. On short passages the captain may not adjust clocks at all, even if they pass through different time zones, for example between the UK and continental Europe. Passenger ships often use both nautical and on-board time zones on signs. When referring to time tables and when communicating with land, the land time zone must be employed. Application The nautical time zone system is analogous to the terrestrial time zone system for use on high seas. Under the system time changes are required for changes of longitude