If the eatorial plane of earth is NOT inclined to its orbit, then

The Earth's axis of rotation is currently tilted at approximately 23.5° relative to the normal of its orbital plane (the ecliptic) [3]. This axial tilt, or obliquity, is the primary driver of seasonal variations because it causes different hemispheres to receive varying amounts of solar radiation throughout the year as the Earth orbits the Sun [1]. If the equatorial plane were not inclined to its orbit (meaning the axial tilt was zero), the rotational axis would be perfectly perpendicular to the orbital plane [1]. In such a scenario, the Sun would always be directly overhead at the equator, and every location on Earth would experience nearly constant day lengths and solar intensity year-round [2]. Consequently, the distinct cycle of spring, summer, autumn, and winter would cease to exist, leading to a lack of seasonal change.

Sources

1. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 12: Earth, Moon, and the Sun > 12.2.2 Seasons on the Earth > p. 177
2. [3] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 19: The Motions of The Earth and Their Effects > 19.1. Rotation of Earth > p. 251
3. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 19: The Motions of The Earth and Their Effects > Lunar Nodal Cycle > p. 266