Leap second what is

Astronomical time UT1 , or mean solar time, is based on the rotation of Earth, which is irregular. There are two main reasons that cause leap seconds to occur. The first is that the duration of the atomic second was measured and defined by comparing cesium clocks to the Ephemeris Time ET scale, an obsolete time scale that defined the second as a fraction of the tropical year.

The duration of the ephemeris second was slightly shorter than the mean solar second and this characteristic was passed along to the atomic second. If the atomic second had been defined with respect to the mean solar second, it is likely that leap seconds would have been required much less frequently. The second reason for leap seconds is that the speed of the Earth's rotation is not constant.

It sometimes speeds up, and sometimes slows down, but when averaged over long intervals the trend indicates that it is gradually slowing. This gradual decrease in the rotational rate is causing the duration of the mean solar second to gradually increase with respect to the atomic second. Leap seconds are added in order to keep the difference between UTC and astronomical time UT1 to less than 0. At the time the corrections started in , a necessary correction of ten seconds was made to UTC, and there have been leap seconds about every year and a half on average.

Leap seconds have always occurred at the end of December or the end of June, on the last second of the UTC day. The designation of the sequence of seconds is:. So far, there have only been positive leap seconds. However, there is a provision for negative leap seconds if it becomes necessary due to changes in Earth's rotation.

The designation of the sequence to remove a second would be:. If there were no longer a correction to UTC for leap seconds, then adjustments would have to be made to time stamps and legacy equipment and software which synchronize to UTC for astronomical observations. In practice, the discrepancy is usually considerably smaller than 0. It is generally kept between 0. Star tracks caused by the Earth's rotation, made visible in this long-exposure shot. Since then, the Earth has slowed down an additional 27 seconds, and a total of 27 leap seconds have been added see table.

Leap seconds can be positive 1 second added to the schedule or negative 1 second omitted. So far, all leap seconds have been positive.

However, if the Earth's rotation keeps accelerating, as it has done in , the IERS may decide to announce the world's first negative leap second at some point in the future.

Not only do days become longer, but the rate at which day lengths increase also grows over time—but only by about two-thousandths of a second per century, according to Dr. At the moment, days are 0. Topics: Timekeeping , Clocks , History , Earth. Sign in. Home Time Zones Leap Second. You might think the leap second is just like a leap day. Well, it's the same in that it adds time to the value of a yearbut the reasons are slightly different.

The leap day is an extra day to account for the non-integer number of Earth rotations as it orbits the Sun. But what about the leap second? Let's take a look. The source of the problem behind the leap second is that there are two definitions of the unit of time. From the beginning, humans measured time with the Sun. At some point, someone decided that the time it takes for the Sun to go from the highest point in the sky noon to the next time it's at the highest point would be a day.

Once you have a day, you can divide this into 24 hours. Each hour is divided into 60 minutes and each minute is divided into 60 seconds. Fast forward to modern times. We now have a better definition of a second atomic clocks.

There are two reasons to use an atom for a clock instead of the Earth's motion around the Sun. First, the time scale is smaller.