During summer, the North Pole is leaning towards the Sun and the South Pole leans towards the Sun in winter. Fall sees the planets axis directly in line with the Sun. The Sun is directly over the Equator during the autumnal equinox (September) and heating the northern and southern hemispheres equally.

When the winter solstice happens in the Northern Hemisphere, the North Pole is tilted about 23.4° (23°27′) away from the Sun. On this day of the winter solstice in the Southern Hemisphere, the Sun’s vertical overhead rays progress to their northernmost position, the Tropic of Cancer (23°27′ N).

The seasons in the Northern Hemisphere are the opposite of those in the Southern Hemisphere. This means that in Argentina and Australia, winter begins in June. Seasons occur because Earth is tilted on its axis relative to the orbital plane, the invisible, flat disc where most objects in the solar system orbit the sun.

Fall or autumn in the Northern Hemisphere begins September 22 or 23. Remember that the tilt and direction of Earth’s axis of rotation is the same as Earth revolves around the sun. Winter in the Northern Hemisphere begins on December 21 or 22, when the axis of rotation is tilted a full 23.5° away from the sun.

At the December solstice, Earth is positioned in its orbit so that the sun stays below the North Pole horizon. As seen from 23 1/2 degrees south of the equator, at the imaginary line encircling the globe known as the Tropic of Capricorn, the sun shines directly overhead at noon.

Suppose Earth’s axis were tilted at 35° instead of 23.5°. How would this affect the seasons in North America? Winter would be cooler, and summer would be warmer.

If there were a 40 degree tilt, we would hit a 90 degree sun angle here at 26 N twice during summer (likely about a month on each side of the solstice) but we would also go down to a 24 degree sun angle in winter, which is similar to about 42 N in the current scenario.

If Earth’s orbit was a perfect circle, the Sun would cross the meridian at noon every day (ignoring daylight savings time). But our orbit is slightly oval-shaped. In July, we are at our furthest point from the Sun, and Earth moves slower than average along its path.

Without tilt, there would be no summer season and no winter season. The equator would always be hot and the poles would always be cold. If you start at the equator and move north, the further north you go the colder it would get. The equator would always be hot and the poles would always be cold.

With no sunlight, photosynthesis would stop, but that would only kill some of the plants—there are some larger trees that can survive for decades without it. Within a few days, however, the temperatures would begin to drop, and any humans left on the planet’s surface would die soon after.