Question

Suppose the Earth's axis had no tilt. Would we still have seasons?

Answer 1

In a very minor way probably yes.

Explanation:

The axial tilt of the Earth is about `23^@`, resulting in a large difference in the amount of sunlight received in summer and winter.

Without an axial tilt there would still be some variation in sunlight received due to the eccentricity of the approximately elliptical orbit of the Earth around the Sun.

At perihelion (closest approach) the Earth is about `91` million miles from the Sun. This currently happens in early January.

At aphelion (furthest distance) the Earth is about `95` million miles from the Sun. This currently happens in early July.

As a result, the amount of sunlight received varies by about `6%`. This effect is much less significant than the one caused by the axial tilt.

So on average we would see some variation through the year. I think this might be too weak an effect to restrict growing times for plants etc. So in that sense perhaps you would say that we would not have seasons.