Question

1. why is there one winter and one summer each year?

Explanation:

The Earth's axis is tilted at an angle of about 23.5 degrees relative to its orbital plane around the Sun. As the Earth orbits the Sun over the course of a year (approximately 365 days), different parts of the Earth receive varying amounts of solar radiation due to this tilt. When a hemisphere (Northern or Southern) is tilted towards the Sun, it experiences summer (more direct sunlight, longer days), and when tilted away, it experiences winter (less direct sunlight, shorter days). Since the Earth completes one orbit around the Sun per year, each hemisphere goes through one tilt towards and one tilt away from the Sun, resulting in one summer and one winter per year.

Answer:

The Earth has one winter and one summer each year because the Earth's axis is tilted (at about 23.5°) relative to its orbital plane around the Sun. As the Earth orbits the Sun once a year, each hemisphere (Northern/Southern) is tilted toward the Sun (causing summer, with more direct sunlight and longer days) and tilted away from the Sun (causing winter, with less direct sunlight and shorter days) once per orbital period.