New York City can feel brutally cold in January, with short days and low sun that never seems to warm the streets. Yet early in January is also when Earth is closest to the Sun in its yearly orbit, a point astronomers call perihelion. If closeness were the main driver of seasons, January in New York should be the time of easiest warmth, not the time of winter coats and icy sidewalks.
The puzzle sharpens when the calendar flips from New York to Sydney, Australia, because Sydney’s January is summer. The same Sun that hangs low over New York in January sits much higher over Sydney, and the days stretch later into the evening. Any explanation has to account for how one planet can deliver opposite seasonal experiences on the same date. Here is a simple picture of that January contrast: