Glossary · Thiên văn học
Solstice
The two moments in the year when Earth's axis is tilted furthest toward or away from the Sun: the summer and winter solstices. For every place north of the Tropic of Cancer, the summer solstice is the day the Sun is highest in the sky and the longest day of the year.
Earth's axis is tilted about 23.5°, and the tilt's orientation in space does not change through the year, so the Northern Hemisphere leans toward the Sun in June and away from it in December; this tilt is what causes the seasons. On the Northern Hemisphere's summer solstice the Sun is directly over the Tropic of Cancer (23.5° N), and its noon position changes very little for several days before and after. The winter solstice marks the shortest day and longest night of the year; in the Northern Hemisphere it occurs when the Sun is directly over the Tropic of Capricorn (23.5° S).
The two moments in between are the equinoxes: when the axis is tilted neither toward nor away from the Sun, day and night are nearly equal at all latitudes. The longest day is not the warmest either: for most mid- and high-latitude locations, the highest average temperatures arrive after the solstice, because ground and water take time to heat up.