Want to get some extra sleep or need some more time to get some things accomplished Tuesday? Well no problem, you'll have an extra second to do so. Tuesday will officially be longer than normal because an extra second or "leap" second will be added. Because of the tug of war played out in the cosmos amongst the Earth's relationship with the Moon and the Sun, the Earth's rotation is actually slowing over time.
The average length of a day based solely on how long it takes the Earth to rotate is actually 24 hours and .002 seconds. An innately small amount of extra time on any given day, but added up over the years and it actually can accumulate. By the time a few years go by, our clocks are actually a second off from the actual rotation of the Earth. So to compensate, we add an extra second to a planned day, in this case, Tuesday June 30th.
While this may seem like a radical idea, it has happened roughly every 3 or 4 years since 2000, and far more often in the 20th century, but most of us probably never noticed. It didn't really become an "event" until the computer age because computer systems, the internet, and about anything else electronic, doesn't like the added second. This has cause computer and telephone system outages in the past, but none are currently anticipated.
How do you add an extra second? Well, the global UTC (Universal Time Coordinated) clock set to the Prime Meridian will read 23:59:59, then 23:59:60, then transition to the next day to 00:00:00. So when the clock strikes midnight at the Prime Meridian on Wednesday, which is in London, the clock will read 7pm in Minnesota. So for us, that extra second comes at 6:59:60pm.
So why doesn't this become an annual 4 year event like leap year where an extra day is added to the calendar to account for the uneven revolving of the Earth around the Sun? Well, the revolution of the Earth is currently stable at 365.25 days to make one full trip around the sun, but dozens of factors play into how quickly the Earth spins. Weather patterns, ground water distribution, ocean tides, climate events like El Nino, polar ice amounts, tsunamis, and earthquakes are just a few of the factors the can shift Earth's rotation one way or another. These factors are all calculated over time, and then scientists can calculate just how far off our current time is from actual astronomical rotation. With no way to know how many of these will affect the rotation in the future, there is no way to plan for what the rotation will be like in the years to come, so we have to make small adjustments when we can.