Friday, June 21, at 1:04 a.m ., is the Summer Solstice for 2013. This is the moment when the Sun reaches the Tropic of Cancer, its highest point in the Northern Hemisphere. Though many people consider Memorial Day to be the first day of summer, the Solstice marks the official start of the season.

The term “solstice” comes from the Latin words “sol” (sun) and “sistere” (to stand still) because, during the solstice, the angle between the Sun’s rays and the plane of the Earth’s equator (called declination) appears to stand still.

So what does that mean? Essentially, our hours of daylight – the period of time each day between sunrise and sunset – have been growing slightly longer each day since the Winter Solstice in December, which is the shortest day of the year (at least in terms of light). Starting after June 21, the days will begin to grow shorter and will continue to do so until we reach the Winter Solstice on December 21, 2013, at 12:11 p.m., and begin the whole cycle anew!

While we celebrate the Summer Solstice, our friends in the Southern Hemisphere will be simultaneously marking the Winter Solstice. That’s because, while our half of the globe is incline toward the Sun, their half is inclined away. Starting this month, their days will begin to grow longer.