ELi5: If seasons are due to the tilt of the earth and not the orbit around the sun, how do they occur on roughly the same days every year?

Question

Is it just coincidence that the 4 season begin and end at same point in the orbit around the sun?

In: Physics

Answer 1

On/about December 21st (varies a little with leap years) the North Pole is angles furthest from the sun. This causes the northern hemisphere to get the least hours of sunlight on that day, and the sunlight is weakest as it’s hitting the surface at an angle. This results in winter, going to zero sunlight above the arctic circle. There’s a bit of a lag between this and the temperatures hitting their winter low a few weeks later, so around this time is generally the start of winter.

On/about June 21st (leap years again) the opposite is true. The North Pole is angled as much as possible towards the sun. The northern hemisphere gets its maximum hours of sunlight, and places above the arctic circle experience zero hours of nighttime. In addition the sunlight is at its highest intensity as its hitting the earth straight on (as much as possible). This results in summer, with similar lag in warming so the hottest days are normally a few weeks later.