Question

What do you mean by parallelism of axis? How does it cause seasons?

Answer 1

Hint:The axis of the Earth is inclined at an angle of 66 1/2° from the orbital plane.
At different times of the year, different areas of Earth point toward or away from the Sun.

Complete answer:
The axis of the Earth is inclined at an angle of 66 1/2° from the orbital plane. If calculated from the perpendicular plane this would be 23 1/2°. Throughout the revolution of the earth, this tilt is maintained. This is referred to as the Parallelism of the earth’s axis.
What Causes Seasons on Earth?
Seasons happen because Earth's axis is tilted at an angle of about 23.5 degrees, which causes the amount of sunlight an area receives to vary. Due to which different parts of Earth receive more solar energy than others.
Because of Earth's axial tilt, our planet revolves around the Sun in a slant, hence different areas of Earth point toward or away from the Sun at different times of the year.
Around the June solstice, the North Pole is inclined toward the Sun hence the Northern Hemisphere gets more of the Sun's direct rays. This is why the summer months in the Northern Hemisphere are June, July and August. At the same time, the Southern Hemisphere points away from the Sun and hence experience winter.
December, January and February are the months during which the Southern Hemisphere experiences summer, when the South Pole is inclined toward the Sun and the Northern Hemisphere is tilted away.

Note: It is a common misunderstanding that seasons are caused because of Earth's elliptical orbit around the Sun, with winter occurring when Earth is farthest away from the Sun, and summer when it is closest to it.
However, the effect of our planet's distance from the Sun is very little on the onset of seasons. In fact, around the Northern Hemisphere's winter solstice, Earth is closest to the Sun, while it is farthest away from the Sun around the north's summer solstice.