On Tuesday June 30, 2015, the day will be a second longer than usual. Similar to how February gains an extra day every four years, the leap second will be added to accommodate a slight slowing of the Earth’s rotation. The mean solar day which is the average time it takes for the Earth to rotate around the sun is 86,400.002 seconds long. Earth’s rotation has been slowing due to a braking effect of the gravitational pull between the planet and the sun. Overtime, the daily difference in the speed of 2 milliseconds adds up to a second over the course of a year, necessitating the addition of the leap second.
Leap seconds were first implemented in 1972 and are typically added to the clock either on June 30th or December 31st. The rate of adding leap seconds has varied and only four leap seconds have been added since 2000. While the exact reason for why the frequency of leap seconds has slowed in recent years in unknown, scientists hypothesize that geologic events such as earthquakes and volcanic eruptions may play a role.
To learn more about leap seconds visit NASA Explains Why June 30 Will Get Extra Second.