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Time and Frequency from A to Z, L

Laser Cooling

A technique that uses laser beams to slow down the motion of atoms and cool them to temperatures a few millionths of a degree above absolute zero. This technique is used to improve the performance of cesium fountain clocks and other standards, since it increases the interrogation and observation time of the atoms.

Leap Day

The extra day added to a year to make it have 366 days. Leap days are added on February 29th during leap years.

Leap Second

A second added to Coordinated Universal Time (UTC) to make it agree with astronomical time to within 0.9 second. UTC is an atomic time scale, based on the performance of atomic clocks. Astronomical time is based on the rotational rate of the Earth. Since atomic clocks are more stable than the rate at which the Earth rotates, leap seconds are needed to keep the two time scales in agreement.

The first leap second was added on June 30, 1972. Since then, leap seconds have occurred at an average rate of less than one per year.  Leap seconds are announced at least several months in advance and are implemented on either June 30th or December 31st. Although it is possible to have a negative leap second (a second removed from UTC), so far all leap seconds have been positive (a second has been added to UTC).  Based on what we know about the Earth's rotation, it is unlikely that we will ever have a negative leap second.

Leap Year

Leap years are years with 366 days, instead of the usual 365. Leap years are necessary because the actual length of a year is about 365.242 days, not 365 days, as commonly stated. Basically, leap years occur every 4 years, and years that are evenly divisible by 4 (2004, for example) have 366 days. This extra day is added to the calendar on February 29th.

However, there is one exception to the leap year rule involving century years, such as the year 1900. Since the year is slightly less than 365.25 days long, adding an extra day every 4 years results in about 3 extra days being added over a period of 400 years. For this reason, only 1 out of every 4 century years is considered as a leap year. Century years are considered as leap years only if they are evenly divisible by 400. Therefore, 1700, 1800, 1900 were not leap years, and 2100 will not be a leap year.  However, 1600 and 2000 were leap years, because those year numbers are evenly divisible by 400. 

Line Width

Another name for resonance width. The term line width is generally used to refer to the resonance width of an atomic oscillator.

Long-Term Stability

The stability of a time or frequency signal over a long measurement interval, usually of at least 100 seconds. In most cases, long-term stability is used to refer to measurement intervals of more than one day.


A ground based radionavigation system that operates in the LF radio spectrum at a carrier frequency of 100 kHz, with a bandwidth from 90 to 110 kHz. LORAN-C broadcasts are referenced to cesium oscillators and are used as a standard for time and frequency calibrations. LORAN-C stations are operated in several regions of the northern hemisphere outside of the United States. However, in accordance with the Department of Homeland Security (DHS) Appropriations Act, the U.S. Coast Guard terminated the transmission of all United States LORAN-C signals on February 8, 2010.

Low Frequency (LF)

The part of the radio spectrum ranging from 30 to 300 kHz. A number of standard time and frequency signals are broadcast in this region, including the 60 kHz signal from NIST Radio Station WWVB.  These signals are most commonly used as time-of-day references for radio controlled clocks.

Created May 12, 2010, Updated March 1, 2023