This timing device is based on a yitterbium lattice that combines two atomic watches, giving it the opportunity to stabilize much faster than other watches. Credit: N. Phillips/Nist/Wikimedia Commons
Since astronomers have divided the globe into 24 time fuses, the use of a clock time all over the world is not very practical. That’s why the times of astronomical events are often administered in universal time (UT). This practice began in 1928, when the international astronomical union recommended that title for the time used in the compilation of astronomical almani. Ut is the time kept on the 0 ° longitude meridian, the one that crosses the old Greenwich Observatory in England. This meridian is five hours ahead of the Eastern standard time of the United States. The times indicated in UT are shown in terms of 24 -hour clock. So 14:42 is 14:02 and 21:47 is 9:17 pm
Different meanings of the time
When a precision of a second or better is needed, however, astronomers must be more specific on the meaning of UT. For this purpose, several UT designations have been adopted. In astronomical and navigation use, UT often refers to a specific time called UT1, which is a measure of the rotation corner of the earth as observed astronomically. It is influenced by small variations in the way our planet turns and can differ slightly from time on Greenwich’s meridian. The times labeled UT in astronomical almanacs such as those produced by the United States Naval Observatory are UT1.
In common use, UT refers to a temporal scale called universal coordinated time (abbreviated UTC), which is the basis for the time system used all over the world. This time scale is determined using highly precise atomic watches, which are accurate to about a nanosecond (billion second) per day.
UTC is the time distributed by the standard radio stations that transmit time, such as WWV and WWVH. It can also be obtained promptly from the satellites of the global positioning system (GPS). UTC is the basis for the time of civil standard in the United States and its territories. Standard time within the US time spindles is an integral number of offset hours from UTC.
Greenwich average time
The average time of Greenwich (GMT) is a temporal scale based on the apparent movement of the “medium” sun compared to the meridian through the old Greenwich observatory (longitude 0 °). The middle sun is used because the time based on the apparent movement of the sun does not “appear” at a constant rhythm.
The terrestrial orbit is slightly eccentric and its plan is inclined with respect to the equator of about 23½ °, therefore in different periods of the year the sun seems to move faster or slower in the sky. That’s why a meridiana can be wrong (if you suppose it is about average time) up to 16 minutes. So, if the middle (i.e. correct) sun is directly on the meridian through Greenwich, it is exactly noon GMT. The average time on meridians selected 15 ° away is generally known as standard time. For example, oriental standard time (est) is the average sunburnt time of the meridian at 75 ° to the west.
If you are an amateur astronomer and keep only a personal register of your observations, the use of the local time is fine. But if you are someone who, for example, shares images on the internet, consider the use of universal time. It will make your data a little more friendly for others in our spherical world.
