# The (Alternative) Reason for the Seasons: Highly Eccentric Orbit

On Earth, we experience seasons because of our planet's axial tilt.

It is a common misconception that the seasons are instead caused by our planet's distance from the sun changing as it orbits. The Earth's distance from the sun does change throughout the year, but the change is far too small to have any effect on the planet's temperature. In fact, in the northern hemisphere, the Earth is actually furthest from the sun during summer.

Imagine an earthlike, inhabited planet with zero axial tilt (ignoring wobble) and a much higher orbital eccentricity than Earth (exact values aren't important at this point but feel free to calculate them for extra kudos). For this planet, "summer" is the time spent closest to the sun and "winter" is the time spent furthest from the sun.

# Is this setup a feasible explanation for the seasons of a fictional planet?

If so, how would the planet's seasonal cycle differ from Earth's (all other things being equal)?
Would there be any notion of an equinox?

• The solstice and equinox come from the varying length of day, your lack of axial tilt removes that. If your distance varies enough to effect seasonal changes, your seasons will not be equal in length. Summer will be shorter than winter, since the orbital velocity will be higher the closer to the star it is, and slower the farther away. Depending on how far away from the primary your planet is during the two seasons, it could be a fairly large difference. – Seeds Sep 16 '16 at 21:37
• That's right, the days would all be the same length. Hmm. Might be an interesting follow up question to see what sort of primitive astronomy might emerge on such a planet. – ApproachingDarknessFish Sep 16 '16 at 22:02
• @ApproachingDarknessFish Well, they might find astronomy a tiny bit more boring in the beginning, but quite as useful. – Karl Sep 17 '16 at 7:56
• This is a bit outside my expertise, but I think an additional scenario is also possible: a more elliptical orbit that is not eccentric. It would have two summers and two winters per year. – Dan Henderson Sep 17 '16 at 9:03
• @Dan Henderson I don't think that would work since the sun is always at one of the ellipse's foci (not in the center). – ApproachingDarknessFish Sep 17 '16 at 18:37