Final answer:
The Earth's tilt, or obliquity, is responsible for the different seasons we experience. If Earth's axis were straight up and down, there would be no seasonal variations, because sunlight would hit all parts of the Earth equally. This lack of tilt would result in constant day lengths and a very minor variation in temperature.
Step-by-step explanation:
The Earth's tilt on its axis, known as obliquity, significantly contributes to the changes in seasons that we experience. The Earth's axis has a tilt that ranges between 22.1 and 24.5 degrees, and this movement in tilt creates what we know as seasonality - the distinction between hot summers and cold winters, or mild seasons for lower tilt angles.
If the Earth's axis went straight up and down, meaning it had a 0-degree tilt, we would not have the traditional seasons. This is because the tilt of the Earth's axis causes sunlight to hit different parts of the Earth more directly during different times of the year. With a straight axis, every place on Earth would receive the same amount of sunlight every day, thus eliminating the variation in climate we experience as seasons.
Consequently, the day length throughout the year would remain constant, as well as the Sun's position in the sky. This would potentially result in a single, consistent season worldwide with minor temperature fluctuations due to the slight difference in Sun's heating caused by Earth's elliptical orbit.
Learn more about Earth's Tilt and Seasons