Earth from Orbit: Summer Solstice | NOAA June 21 marked the official start of astronomical summer in the Northern Hemisphere. The summer solstice—the longest day and shortest night of the year—occurred at 5:14 a.m. EDT. The seasons change at various times of the year due to the planet’s axial tilt of 23.5 degrees as it orbits the sun. This tilt means the two hemispheres are exposed to different sun angles and variable lengths of daylight throughout the year. The solstices mark when the sun reach