We’ve all had days that feel longer than others. Tomorrow, it actually will. A ‘leap’ second is officially being added to June 30.

To us, one day lasts 86,400 seconds. To scientists, the average length of a day is 86,400.002 seconds. That pesky .002 seconds? Scientists point the finger at Earth’s rotation.

Our planet’s rotation is slowing down ever so slightly thanks to the gravitational interactions between Earth, the moon and the sun. Add that .002 seconds together over the span of a year and you get nearly one second a year. Sounds simple, right? Far from it.

The length of each day on Earth can vary. I’ll let NASA explain:

The length of day is influenced by many factors, mainly the atmosphere over periods less than a year. Our seasonal and daily weather variations can affect the length of day by a few milliseconds over a year. Other contributors to this variation include dynamics of the Earth’s inner core (over long time periods), variations in the atmosphere and oceans, groundwater, and ice storage (over time periods of months to decades), and oceanic and atmospheric tides. Atmospheric variations due to El Niño can cause Earth’s rotation to slow down, increasing the length of day by as much as 1 millisecond, or a thousandth of a second.

Well, that’s a lot to take in. How does NASA combine all of this data to figure out when to add the leap second? They use a technique called Very Long Baseline Interferometry (VLBI). In English? Astronomers use radio telescopes around the world to measure Earth’s orientation. Using quasars, you can determine how much the Earth is moving by observing the time differences between the radio waves reaching the different radio telescopes.

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Here’s a fun NASA video explaining VLBI.

Neil deGrasse Tyson tweeted a picture of what the last leap second looked like on a clock.

Not everyone is a fan of leap seconds. Some want to do away with it because computers can’t plan for leap seconds.

Leap second detractors have a point. They do not occur on a regular basis. When leap seconds were first added in 1972, an extra second was added at around one per year. That changed starting in 2000. Tomorrow’s leap second will be just the fourth one in the past 15 years.

Why the slowdown in additional leap seconds? Scientists aren’t sure. Earthquakes and other seismic activity could be influencing the Earth’s rotation.

Today’s VLBI measurements are accurate to at least 3 microseconds. But, NASA wants it better.

A new system is being developed by NASA’s Space Geodesy Project to increase accuracy to 0.5 microseconds. It won’t make a difference for people like us, but is “designed to meet the needs of the most demanding scientific applications now and in the near future,” says Stephen Merkowitz, NASA’s Space Geodesy Project manager.

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