<p>It is a common knowledge that leap years occur once every four years, adding an extra day to February, which otherwise has only twenty eight days. Not so well known is the advent of so called leap second where one second is added to make that particular year longer. For instance, on December 31, 2008, one second was added at midnight. The New Year celebration was one second longer that year! <br /><br /></p>.<p>During the past 40 odd years, more than 25 such leap seconds have been added. What is the motivation behind this and the underlying reasons for advancing time in this manner? To understand this, let us recall why leap years were introduced in the first place. <br /><br />Calendars to record events evolved early with meticulous book-keeping (in the temporal sense) of religious festivals, harvests, sowing of crops etc. The year (associated with the earth’s orbital period) was divided into twelve months (lunar orbit period) and months into weeks and days (for the diurnal rotation of the earth). <br /><br />Necessities<br />The need to measure time over the passage of a day led to invention of accurate pendulum clocks, improvising earlier devices like sundials, hourglass sand clocks etc. The year was divided into 365 days, a round number which was convenient as all the months added up to that many days. <br /><br />However, more precise astronomical observations (even in antiquity) revealed that the year actually has 365.25 days, so to accommodate the extra quarter day, the leap year was invoked, adding one day every four years to February. <br /><br />However, it turns out that even the extra quarter day is approximate and a year is actually 365.243 days long. After a few centuries, one could clearly see the effect; the vernal equinox was ‘off’ by several days. By about 1582, Pope Gregory realised (or was made to realise) that the error in the calendar is now about two weeks, and had to be corrected for celebration of various events among other things. This led to the Gregorian calendar, which is universally used at present. <br /><br />Suddenly 12 days were removed from the calendar in 1582 to compensate for the error (accumulating over a millennium and a half). <br /><br />Reason for inclusion<br />So why the leap second? The advent of atomic clocks and the more recent nuclear thorium clock has enabled a second to be defined to an accuracy of one part in a billion-billion. <br /><br />The caesium atomic clock has already been in existence for quite some time. There are now more accurate mercury and strontium ion clocks which lose one second in a hundred million years or so. We can accurately measure the slowdown in the earth’s rotation rate; the day gets longer by about two milliseconds every century. <br /><br />This slowdown is just what we expect from the tidal action of the moon on the earth. The tidal friction has been continuously slowing down the earth (the day would have been twenty hours or less a few hundred million years ago). Atomic clocks have enabled GPS systems to achieve unprecedented accuracy in measurement of times and distances. So to match astronomical time and atomic time, leap seconds have to be added now and then to compensate for the earth’s slowdown. So enjoy the longer day today with an extra second, while mulling over all this.<br /><br /></p>
<p>It is a common knowledge that leap years occur once every four years, adding an extra day to February, which otherwise has only twenty eight days. Not so well known is the advent of so called leap second where one second is added to make that particular year longer. For instance, on December 31, 2008, one second was added at midnight. The New Year celebration was one second longer that year! <br /><br /></p>.<p>During the past 40 odd years, more than 25 such leap seconds have been added. What is the motivation behind this and the underlying reasons for advancing time in this manner? To understand this, let us recall why leap years were introduced in the first place. <br /><br />Calendars to record events evolved early with meticulous book-keeping (in the temporal sense) of religious festivals, harvests, sowing of crops etc. The year (associated with the earth’s orbital period) was divided into twelve months (lunar orbit period) and months into weeks and days (for the diurnal rotation of the earth). <br /><br />Necessities<br />The need to measure time over the passage of a day led to invention of accurate pendulum clocks, improvising earlier devices like sundials, hourglass sand clocks etc. The year was divided into 365 days, a round number which was convenient as all the months added up to that many days. <br /><br />However, more precise astronomical observations (even in antiquity) revealed that the year actually has 365.25 days, so to accommodate the extra quarter day, the leap year was invoked, adding one day every four years to February. <br /><br />However, it turns out that even the extra quarter day is approximate and a year is actually 365.243 days long. After a few centuries, one could clearly see the effect; the vernal equinox was ‘off’ by several days. By about 1582, Pope Gregory realised (or was made to realise) that the error in the calendar is now about two weeks, and had to be corrected for celebration of various events among other things. This led to the Gregorian calendar, which is universally used at present. <br /><br />Suddenly 12 days were removed from the calendar in 1582 to compensate for the error (accumulating over a millennium and a half). <br /><br />Reason for inclusion<br />So why the leap second? The advent of atomic clocks and the more recent nuclear thorium clock has enabled a second to be defined to an accuracy of one part in a billion-billion. <br /><br />The caesium atomic clock has already been in existence for quite some time. There are now more accurate mercury and strontium ion clocks which lose one second in a hundred million years or so. We can accurately measure the slowdown in the earth’s rotation rate; the day gets longer by about two milliseconds every century. <br /><br />This slowdown is just what we expect from the tidal action of the moon on the earth. The tidal friction has been continuously slowing down the earth (the day would have been twenty hours or less a few hundred million years ago). Atomic clocks have enabled GPS systems to achieve unprecedented accuracy in measurement of times and distances. So to match astronomical time and atomic time, leap seconds have to be added now and then to compensate for the earth’s slowdown. So enjoy the longer day today with an extra second, while mulling over all this.<br /><br /></p>
