Why does sunset come earlier day by day in autumn?

In Seoul, sunset is around 7:57 p.m. in late June. On October 8, it is about 6:05 p.m., almost two hours earlier after a little over three months. A 6 p.m. journey home is therefore bright in July, while in October the sun is just setting and streetlights are beginning to come on. Some light lingers in the sky for 20 to 30 minutes after sunset, but after that it gets dark quickly. Before an autumn hike or an evening walk, check that day's sunset time rather than relying on what you remember from last month.

Daylight does not shrink at the same rate every day. It changes very little near the summer and winter solstices, and shrinks fastest around the autumn equinox. At this time of year in Seoul, daylight shortens by a little over two minutes a day, while sunset moves about a minute and a half earlier. That adds up to more than ten minutes in a week, so it is natural to feel that evening is arriving especially quickly.

It is easy to assume days shorten in autumn because Earth is moving away from the sun. In fact, Earth comes closest to the sun in early January. The change in distance over the year is also only a little over 3%, not enough to create the seasons. The seasons come from Earth's tilted axis. After the autumn equinox, the sun rises south of due east and takes a lower, shorter path across the sky. Because it spends less time above the horizon, the days become shorter. At the same time of year in Australia, in the Southern Hemisphere, days are getting longer.

Korea last used daylight saving time in 1988, so the clocks do not move with the seasons. Only the sun's apparent movement changes. Interestingly, the earliest sunset is in early December, rather than on the winter solstice. In Seoul, the earliest is around 5:13 p.m. near December 7. On the winter solstice, sunset is actually a few minutes later. That is because in December, the time when the sun reaches its highest point in the southern sky gets a little later each day. For the same reason, sunrise keeps getting later until early January, even after the solstice.