Know what is a 'Leap second' - Smaller sibling to Leap year.

"What are you guys doing tonight? Any plans?", I queried my ex-roommates. "I am completely free. Have nothing to do. Will be just watching some TV series over the laptop", replied Aman, in his inimitable style: eyes partially closed, hands open and a wide grin on face.

"Have you read my latest article on How does a painkiller works I published today? Itna time hai to bhai ka blog hi padh lo", I said with a pinch of anger. 

"You write about leap second and I will for sure read your blog." 

Quite curious myself to know about Leap second, I took a note of the request and hence I dedicate this blog to my dear friend and ex-roomie Aman. 

Leap year. A year that comes once in 4 years. Almost certainly, you all will be aware as to why it happens. Our earth revolves around the sun in 365 days and 6 hours. So after every 4 years, this extra time adds up to 24 hours or typically one full day, which is represented by the addition of February 29 in the leap year. But what is Leap second?

Before jumping the gun, let us first understand a few terms:

Fact 1: Ideal time taken by Earth to complete one rotation on its axis: 86,400 seconds (24 hours*60 minutes*60 seconds) 

Fact 2: Universal Time(UT1): UT1 is derived from the actual time taken by Earth to complete one rotation on its axis

Fact 3: International Atomic Time (TAI): TAI is a statistical 'time scale' measured based on the output of approx. 200 Atomic clocks worldwide. A second in the Atomic clock isn't defined as equal to 1/86400 of a solar day; rather is equal to the time taken by a Cesium-133 atom at the ground state to oscillate precisely 9,192,631,770 times. 

Thus, an atomic clock has its own parameters that define a second; it represents the 'ideal' scenario, much more precise than the one defined by our earth (or 1/86400 of a day).

This sounds crazy. But it is a fact. When I say it is more precise, I mean to say that earth actually is slowing down. As a result, it is taking more time to complete one rotation than before.

Fact 4: Coordinated Universal Time (UTC): UTC is a 'time standard' which defines the time of clocks worldwide. It is adjusted to be as close to Universal time (UT1) as possible, by the addition of a 'Leap second' whenever the difference between UTC and UT1 becomes 0.9 seconds. This ensures that the clocks across the world are aligned with the Solar day or with the actual time taken by earth for its rotation on its axis. 

Too much science? Let me try to put it in simpler terms. 

• Universal Time(UT1): 'Actual' time taken for rotation of earth
• International Atomic Time (TAI): Ideal time the earth 'should' take for one rotation
• Coordinated Universal Time (UTC): A standard to make sure earth's slowness is accounted for in the clocks worldwide

What is a Leap second?

Ohk, so we have come to the question that is the very raison d'être for this blog. What is Leap second? With the specific terms/facts mentioned above, hope the below explanation will make sense.

A leap second is an extra second which is added to UTC to sync it with UT1, i.e. to align our clocks with the actual time taken by the earth for one rotation. This is because earth's speed has slowed in comparison to the very consistent Atomic clocks. And hence, a leap second is added to a day to negate the 'laziness' of our planet Earth. And we get a precious extra second at our disposal. :) At the event of addition, the clocks read 60 seconds. For example, the time becomes: 23:59:60 (Usually it is 23:59:59 -> 00:00:00).

Since its inception, a leap second has been added 27 times, with the last addition happening on 31st December 2016 at 23:59:60 UTC. Thereby, the last day of the year 2016 had 86401 seconds instead of 86400. 

For us here in India, we had this addition on January 1 2017 at 5:29:60 (Since IST is 5 hours 30 minutes ahead of UTC).

Nerd notes:

1. The first leap second was added in 1972, when the UTC was 10 seconds behind TAI
2. The next Leap second addition is proposed on June 30, 2020
3. Collating the 27 leap seconds and the original 10 seconds difference (as on 1972), TAI is 37 seconds ahead of UTC today (27 July 2018).   

Thanks for reading. Keep smiling and stay fit. :) 

- Prakhar