1. The moment when the Sun is at its northernmost or southernmost point in the sky, signaling the first day of summer or winter: The summer solstice, when the Sun is farthest north, occurs around June 21 and the winter I>solstice takes place around December 21.

2. The point at which the Sun reaches its greatest positive and negative declinations: The referenced dates, summer solstices and winter solstices, are considered the longest and shortest days of the year, respectively.