What is the local time (LT) and different time zones?

                            

There are two important concepts, related to latitude and longitude (especially) are local time (LT) and universal time (UT).

Longitudes are measured from zero to 180 degree east and 180 degree west (or -180), and both 180 degree longitudes share the same line, in the middle of the pacific ocean.

As the earth rotates around its axis, at any moment one line of the longitude ‘the noon meridian’ faces the sun, and at that moment, it will be noon everywhere on it. After 24 hours the earth has undergone a full rotation with respect to the sun, and the same meridian again faces noon. Thus each hour the earth rotates by 360/24 = 15 degree.

• The date line and universal time (UT).

Longitude determines only the hour of the day, i.e. not the date, which is determined separately. The international date line has been established most of it following the 180^th meridian where by common agreement, whenever we cross it the date advances one day (going west) or goes back one day (going east).

That line passes the bering strait between Alaska and Siberia, which thus have different dates, but for most of its course it runs in mid ocean and does not inconvenience any local time keeping.

Astronomers, astronauts and people dealing with satellite data may need a time schedule which is the same everywhere, not tied to a locality or time zone. The Greenwich mean time, the astronomical time at Greenwich (average over the year) is generally used here. It is sometimes called universal time (UT).