There are four main time standards based on the earth’s rotation of which each has had it’s own day in the sun so to speak. All of these time telling methods use celestial bodies to calculate the earth’s rotational position within an approximated day.
The first time standard utilized was True Solar Time. True Solar Time or Apparent Solar Time began as soon as early humans began relying on the motion of the sun through the sky as a way of telling the estimated time of day. Other methods evolved such as early sun dials called Gnomons (a simple vertical pole casting a shadow from sunlight) and then around the first millennium BC water clocks or Clepsydras. These water clocks became advantageous as the first clocks to be used at night because of a method of estimating the mean of time of day based on the sun’s movement throughout the day.
This was to be followed by the next time standard, Sidereal Time. Invented by the Babylonians, Sidereal Time utilizes the earths rotation in relation to the stars. Because stars are so far away their position in the sky is much more consistent and this creates a highly accurate way of calculating the time of day. Less of a time standard than a method of adjusting True Solar time, Sidereal Time is still used in calculating the current most widely used Universal Time.
Universal Time succeeded the most well known time standard GMT or Greenwich Mean Time as the most excepted Time standard. GMT and Universal Time are often mistaken for one another. GMT is the mean solar time at the Royal Observatory in Greenwich, England. It became the standard time of Britain in the mid 1850s and was used by the vast British Navy as maritime which is the main reason for it’s spreading popularity as the standard time of many nations within the British empire.
GMT has become a common term used to refer to a countries standard time even though many have their own separately named standard time. It is also specifically considered Winter Time in the United Kingdom as each country has it’s own Summer Time ie. BST (British Summer Time) and IST (Irish Summer Time). On January 1 1972 GMT was replaced by Universal Time as the international time reference.
Universal Time or Standard Time is an updated version of Greenwich Mean Time it is ambiguous in that there are many versions of UT. The most widely used versions are UTC and UT1. All types of UT times use the Sidereal method along with the most modern calculated adjustments to reach the closest match to Solar Time. Universal time originated as Standard Time in 1879 to coordinate with the steam engine as travel became faster the time had to be adjusted to match with destination’s Solar Time.
Standard Time divided the world into 24 time zones, each 15 degrees longitude wide. Greenwich, England was chosen as the universal standard from which the time zones were attached. This along with maritime usage is what popularised Greenwich Mean Time. In 1928 Universal Time was accepted internationally as a more accurate term for Standard Time.
Currently more and more accurate versions of Universal time are being made using distant stars called quasars to the point that International Atomic Time is within microseconds of perfect accuracy.