Obliquity is the most important factor of seasons. Without axial tilt, eccentricity causes small temperature variations. What would climate and weather on Earth look like, if Earth doesn't have any axial tilt, but also hasn't eccentricity?
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$\begingroup$ The eccentricity of Earth orbit is much too small to produce noticeable temperature variations. $e = \sqrt {1 - \frac {b^2}{a^2}} = 0.017$, therefore $\frac{b^2}{a^2} = 1 - 0.017^2 = 0.9997$, hence $b = 0.99986 a$; this means that the amount of solar energy reaching Earth varies by about 0.03% between perihelion and aphelion. For comparison, solar irradiance varies by about 0.1% during a solar cycle. $\endgroup$– AlexPCommented Mar 11, 2019 at 17:35
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It would eliminate seasonality. The sun would always track across the same part of the sky every day of the year. The sun would pass through a 180 degree arc from east to west directly overhead at the equator and would travel 360 degrees around the horizon at the poles every day.
The most powerful driver of weather on Earth is Earth’s rotation on its axis coupled with heat from the sun. With the poles stationary and the equator moving at close to 1000 mph the atmosphere is subjected to a lot of forces that drive weather systems.
Changing the Earth’s tilt and orbital eccentricity would not alter the net energy input from the sun nor the rotation of the planet. So temperate regions would remain temperate etc but each region would remain stuck in a single season which would be the average of the traditional 4 seasons in terms of temperature.
There would still be cloud, wind, rain and weather system variation but the seasonal temperature variation would be absent. Every single day would be a kind of average of 21st Match and 23rd September.
