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Does the June 15 Total Lunar Eclipse Start at 19:22:30 UT or UTC?


According to the NASA Eclipse Web Site, the total lunar eclipse on June 15, 2011 will begin at at 19:22:30 Universal Time (UT). However, according to astronomia.org, the total eclipse will begin at 19:22.5 Coordinated Universal Time (UTC).

Universal Time or Coordinated Universal Time?

ClockSo which is the correct terminology when using a worldwide reference for time: UT or UTC? As my mother-in-law is fond of saying, "It's six of one and half a dozen of the other." In other words, it hardly matters, although - for the record - the U.S. Naval Observatory (as of May 15, 2011) predicts that UT will be 0.28234 seconds behind UTC on June 15, 2011.
Photo Credit: wonderferret
If we aren't concerned with fraction-of-the-second precision, we can, for our nontechnical purposes, regard UT and UTC as one and the same. There is never more than a difference of 0.9 seconds between the two systems of time. However, you must be able to convert UT (or UTC) to your local time to know the eclipse times at your time zone.
A very long total lunar eclipse on night of June 15
How do I translate Universal Time to my time?
If you live in North America, it's all academic, since you won't be able to view this lunar eclipse anyway. It is visible from Europe, Africa, Asia, and southern and eastern South America. To know when the eclipse will happen by the clock at your time zone, read the above EarthSky article, keeping in mind that the eclipse times are rounded off to the minute.

What's The Difference Between UT and UTC?

sundialUniversal Time (UT) is a rather generic term. As used in astronomical almanacs, UT is an abbreviation for UT1, which is pretty much the modern-day equivalent of old-fashioned Greenwich Meridian Time (GMT). Both UT1 and GMT are based upon the Earth's rotation, not the atomic clock. On the other hand, International Atomic Time (TAI) is based on the atom, not the Earth's rotation. UTC represents a compromise between time as kept by the Earth's rotation (UT1) and the atomic clock (TAI).
Photo Credit: Tim Green aka atoach
UT is used in astronomical almanacs and is a measure of mean solar time. UTC is the basis of most radio time signals and national and/or legal time systems.
Although Coordinated Universal Time (UTC) is based upon the atomic clock, UTC is never allowed to drift more than 0.9 seconds from UT (UT1), or Earth's rotation time. When the need arises, one second is added to or subtracted from UTC (usually on June 30 or December 31) to bring UTC into sync with UT. This is called a leap second. The last leap second was added to UTC on 2008 December 31, and the next one might be added at the end of 2011. Time will tell . . .

UTC Defers To UT

Of course, UTC defers to UT because day and night, and the rising and setting of the Sun, Moon, planets and extragalactic objects are subject to the vagaries of the Earth's rotation, not to the iron-clad dictates of atomic time. Like the sundial, the rotating Earth measures durations of earthly transformations by the daily passage of the heavenly bodies.

copyright 2011 by Bruce McClure

May 2011 Feature * July 2011 Feature