either of the two times of the year when the sun is at its greatest distance from the celestial equator
"After years of tracking the stars, she finally calculated the exact moment of the summer solstice to mark her garden's longest day."
One of the two points in the ecliptic at which the sun is furthest from the celestial equator. This corresponds to one of two days in the year when the day is either longest or shortest.
"The astronomer explained that during the summer solstice, the sun reaches its highest point in the sky, marking the day with the longest period of daylight for the Northern Hemisphere."