Saturday, June 30, 2012

On July 30th: A Leap Second and Asteroids

Leap Second 

We have a leap second today. A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC) in order to keep its time of day close to the mean solar time. Today's leap second will be inserted at 23:59:60 UTC (29:59:60 EDT / 26:59:60 PDT).

The UTC time standard uses the international system (SI) definition of the second, based on atomic clocks. Like most time standards, UTC defines a grouping of seconds into minutes, hours, days, months, and years. However, the duration of one mean solar day is slightly longer than 24 hours (86400 SI seconds). Therefore, if the UTC day were defined as precisely 86400 SI seconds, the UTC time-of-day would slowly drift apart from that of solar-based standards, such as Greenwich Mean Time (GMT) and its successor UT1. The purpose of a leap seconds is to compensate for this drift, by scheduling days with 86401 or 86399 SI seconds.

Because the Earth's rotation speed varies in response to climatic and geological events, UTC leap seconds are irregularly spaced and unpredictable. The insertion of each UTC leap second is usually decided about six months in advance by the International Earth Rotation and Reference Systems Service (IERS), when needed to ensure that the difference between the UTC and UT1 readings will never exceed 0.9 second. Between their adoption in 1972 and June 2012, 25 leap seconds have been scheduled, all additions (no subtractions).

Close Approaches

Asteroid 17058 Rocknroll (1999 GA5) has its closest approach to Earth (1.789 AU). 17058 Rocknroll is a main-belt asteroid discovered on April 13th, 1999, from at Reedy Creek Observatory, Queensland, by Australian astronomer John Broughton.

Asteroid 7367 Giotto (3077 T-1) has its closest approach to Earth (2.128 AU). 7367 Giotto is a main-belt asteroid discovered on March 26th, 1971 from Palomar Observatory, San Diego, by Dutch astronomers Cornelis Johannes van Houten and Ingrid van Houten-Groeneveld, and American astronomer Tom Gehrels.

Asteroid 34901 Mauna Loa (2699 P-L) has its closest approach to Earth (2.864 AU). 34901 Mauna Loa is a main-belt asteroid discovered on September 24th, 1960 from Palomar Observatory, San Diego, by Dutch astronomers Cornelis Johannes van Houten and Ingrid van Houten-Groeneveld, and American astronomer Tom Gehrels.


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