What Is Universal Time?
The times of various events, particularly astronomical and weather phenomena, are often given in “Universal Time” (abbreviated UT) which is sometimes referred to, now colloquially, as “Greenwich Mean Time” (abbreviated GMT). The two terms are often used loosely to refer to time kept on the Greenwich meridian (longitude zero), five hours ahead of Eastern Standard Time. Times given in UT are almost always given in terms of a 24-hour clock. Thus, 14:42 (often written simply 1442) is 2:42 p.m., and 21:17 (2117) is 9:17 p.m. Sometimes a Z is appended to a time to indicate UT, as in 0935Z. When a precision of one second or better is needed, however, it is necessary to be more specific about the exact meaning of UT. For that purpose different designations of Universal Time have been adopted. In astronomical and navigational usage, UT often refers to a specific time called UT1, which is a measure of the rotation angle of the Earth as observed astronomically.
Universal Time, UT, is essentially what many of us learned as Greenwich Mean Time. Universal Time is based on the prime Meridian. Your local time and the difference between your local time and UT is based on how many degrees longitude you are from the prime meridian, 0 degrees. West longitude is earlier so you would subtract 1 hour from UT for every time zone west until you reach the International Date Line at 180 degrees West. East longitude is later so you would add 1 hour for each time zone you are east of the prime meridian until you reach the International Date Line. For example I live in Lee’s Summit Missouri at 94 degrees west longitude. Each time zone is 15 degrees wide and represents approximately 1 hour of Earth rotation. By dividing 94 by 15 I find that I am 6 time zones, or 6 hours of Earth rotation, away from the prime meridian. So for my time zone I would subtract six hours from whatever the UT time is difference.
bar The times of various events, particularly astronomical and weather phenomena, are often given in “Universal Time” (abbreviated UT) which is sometimes referred to, now colloquially, as “Greenwich Mean Time” (abbreviated GMT). The two terms are often used loosely to refer to time kept on the Greenwich meridian (longitude zero), five hours ahead of Eastern Standard Time. Times given in UT are almost always given in terms of a 24-hour clock. Thus, 14:42 (often written simply 1442) is 2:42 p.m., and 21:17 (2117) is 9:17 p.m. Sometimes a Z is appended to a time to indicate UT, as in 0935Z. When a precision of one second or better is needed, however, it is necessary to be more specific about the exact meaning of UT. For that purpose different designations of Universal Time have been adopted. In astronomical and navigational usage, UT often refers to a specific time called UT1, which is a measure of the rotation angle of the Earth as observed astronomically.
The times of various events, particularly astronomical and weather phenomena, are often given in “Universal Time” (abbreviated UT) which is sometimes referred to, now colloquially, as “Greenwich Mean Time” (abbreviated GMT). The two terms are often used loosely to refer to time kept on the Greenwich meridian (longitude zero). Times given in UT are almost always given in terms of a 24-hour clock. Thus, 14:42 (often written simply 1442) is 2:42 p.m., and 21:17 (2117) is 9:17 p.m. Sometimes a Z is appended to a time to indicate UT, as in 0935Z. When a precision of one second or better is needed, however, it is necessary to be more specific about the exact meaning of UT. For that purpose different designations of Universal Time have been adopted. In astronomical and navigational usage, UT often refers to a specific time called UT1, which is a measure of the rotation angle of the Earth as observed astronomically.
