It isn't distance
Ask why summer is hot and you will often hear that the Earth gets closer to the Sun. In fact the Earth is closest to the Sun in early January and farthest in early July. At its closest it is about 147 million kilometres away and at its farthest about 152 million, a difference of only a little over 3 percent. Above all, if distance were the cause, both hemispheres would have summer at the same time, yet when Korea is in deep winter, Australia is in high summer. What really creates seasons is not distance but the tilt of the Earth's axis.
The axis is tilted about 23.4 degrees
The Earth spins once a day and orbits the Sun once a year, and its spin axis is not perpendicular to the orbital plane but tilted about 23.4 degrees. The crucial point is that the axis keeps pointing in almost the same direction in space throughout the orbit; its northern end aims near the Pole Star all year. So on one side of the orbit the Northern Hemisphere leans towards the Sun, and half a year later, on the other side, the Southern Hemisphere does. When the Northern Hemisphere leans sunward it is northern summer, and the south is in winter.
The angle of sunlight changes the heat
Shine a torch straight down at the floor and you get a small, bright circle; tilt it and the same light spreads out and dims. Sunlight works the same way. With the Sun high, more energy falls on each patch of ground; with the Sun low, the same energy spreads over a larger area. A low Sun's light also travels through more air and weakens. Seoul lies at about 37.5 degrees north, so the noon Sun climbs to about 76 degrees at the June solstice but only about 29 degrees in December. Noon in summer and noon in winter deliver very different sunlight.
Day length changes too
The tilt also changes the length of the day. When the Northern Hemisphere leans towards the Sun, it spends more of each day in sunlight, and less when it leans away. In Seoul the day lasts about 14 hours 45 minutes around the June solstice and about 9 hours 35 minutes around the December solstice, a difference of more than five hours. Stronger sunlight for longer means the ground and air take in far more heat in summer than in winter, with weaker sunlight for fewer hours. The difference grows with latitude: north of the Arctic Circle, at about 66.6 degrees, there are days in summer when the Sun never sets and days in winter when it never rises. Near the equator, by contrast, day and night stay close to 12 hours each all year, and the seasons differ little.
Solstices and equinoxes
The exact date can shift by a day from year to year, but the approximate timing is fixed. The June solstice is the day the Sun climbs highest and the day is longest in the Northern Hemisphere; the December solstice is the opposite. At the equinoxes the Sun rises due east and sets due west, and day and night are nearly equal everywhere on Earth. In the Southern Hemisphere the seasons of the two solstices are reversed.
- March equinox: around 20 March
- June solstice: around 21 June
- September equinox: around 23 September
- December solstice: around 22 December
Why the hottest day isn't the solstice
The day with the most sunlight is the June solstice, yet in Korea the hottest weeks are from late July to early August. The coldest period is not the December solstice but January. This lag of a month or more happens because land and sea store heat. For a while after the solstice, more heat still arrives by day than is lost by night, so heat keeps accumulating, and temperatures only start to fall after incoming and outgoing heat balance. The sea in particular takes a long time to warm and cool, so the lag is larger in coastal areas.
Common misconceptions
Most misconceptions about seasons come from not picturing the positions clearly. Take a ball and a torch, tilt the ball's axis to one side and carry it around the torch without changing the tilt: you will easily see that the side receiving most light changes even though the axis direction stays the same.
- Summer is hot because we're closer to the Sun: we're closest in early January
- The axis wobbles through the year to make seasons: its direction barely changes
- The solstice is the hottest day: land and sea need time to store heat
Everyday uses, and a caution
The changing height of the Sun shows up all over daily life. Eaves and awnings on a south-facing home block the high summer Sun but let the low winter Sun reach deep into the room. Shadows are long in winter and short in summer for the same reason. Ultraviolet light is also strongest at midday in summer when the Sun is high, so for long outdoor activities check the UV index published by your weather service and bring a hat and sunscreen. Sunrise and sunset times vary by location and season, so for exact times check an official astronomical source.
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