# Seasons without an axial tilt?

Let's assume we have an otherwise Earth-like planet orbiting a star just like our own. Our planet has an axial tilt of 0 degrees, meaning its axis is perpendicular to its orbital plane.

Could this planet have seasons based solely on where it is in its orbit? My thought was to increase the eccentricity of the orbit, so that at aphelion it's significantly further away from its sun than at perihelion. But the question becomes how much eccentricity do we need to get seasonal variations similar to what we're used to on Earth? And is there even time for the planet to cool enough on its way to aphelion and winter?

• What variance do you want to match? Here the farther from the tropics the more (orbital mechanics based) seasonal variation there is, but for your world higher latitudes would just be strictly colder, and everyone would have the same relative effect of seasons. To get a polar variance would be more eccentricity than a temperate variance. Also orbits spend more time at perihelion than aphelion so you get longer winters than summers.
– user25818
Sep 12 '17 at 21:16
• Aiming for a variance that would match what we get in temperate zones, or more precisely what the Pacific Northwest of the US gets. Sep 12 '17 at 23:52
• The required eccentricity to match the Earth's change in mean solar flux varies with latitude. For 40 N (Denver, Baltimore, Beijing, Madrid, Rome, etc.) the change in solar flux from summer to winter is about 0.32 to 1:00. To match that with an eccentric orbit, requires an eccentricity of about 0.83, corresponding to a ratio of the semi-major axis to the semi-minor axis of 1.77 to 1:00. Note that this doesn't change the ratio of sunlight to dark hours. See: atmos.washington.edu/~caseyw8/Ch3_distribution_of_solar.pdf. Jan 15 '18 at 5:28
• – user3106
Jul 11 '19 at 7:29

Elliptical orbit can definitely be a replacement for the lack of axial tilt.

While all other climate impacting factors, like ocean currents and air systems are able to create significant variations, they all ultimately depend on sun's energy. If this energy remains constant, it is difficult to imagine regular temperature fluctuations like we observe here on Earth (outside of equatorial latitudes).

Earth's orbit has an eccentricity of 0.0167. While this means that it is almost a circle, still there is a difference between perihelion and aphelion of 5 million km (147.1 vs 152.1). This translates to solar irradiation variation of 7% between the two points.

Axial tilt contributes much more than 7% to seasonal variations, in fact, it is unnoticeable that summers in southern hemisphere (that's when the Earth is in perihelion) are supposed to be warmer. Northern landmasses more than compensating for the difference, because land warms up faster than sea.

How much variation in solar energy do we need to have real seasons? According to this article, at the Cairo's latitude (30N) variation between summer and winter is 2 times. At London's latitude (51N), it is about 5 times. If we want to have a higher eccentricity orbit with no axial tilt, effect will be uniform for all of the planet.

Let's say we want the solar energy difference of 3 times. This means that the orbit must have aphelion to perihelion rate of SQRT(3) = 1.732, which will lead to eccentricity of 0.268

It also has to be noted that high eccentricity orbits have bodies travel faster in their perihelion than in aphelion. This means that winters on such planets will be longer than summers.

• Exactly the stuff I was looking for, plus the raw work so I can crunch the numbers myself to get precisely the variation between summer and winter that I want -- thank you! Sep 13 '17 at 0:02
• @Alexander, I calculated the required ratio of the semi-major to semi-minor axes (for a latitude of 40N) and got a ratio similar to yours (3.13 to 1.00). However, I calculated an eccentricity of 0.825?? I used e = SQRT[1-SQ(1/1.77)]. Jan 15 '18 at 4:26
• The first value in () should have been: [SQRT(3.13) to 1.00]. (: Jan 15 '18 at 5:15
• I used periapsis to apoapsis formula from here: orbital eccentricity Jan 15 '18 at 17:17

Our tilted axis isnt the sole cause for seasons

First, let me thank you for not providing these restrictions:

-Did not say that seasons had to be globally uniform (even ours isn't)

In addition to tilt there are other factors that affect climate and thus our perception of "seasons".

• Pressure Systems: pressure systems can define the climate of region more so than tilt. For instance: the permanent pressure system over the Sahara has locked its climate into a desert where in similar latitudes you would see rainforest and occasionally even snowfall.

• Ocean Currents: Currents are huge circulating streams of water within the ocean (almost like a river). The gulf stream is a huge climate changing current. It delivers cold water to hot places and hot water to cold places and its effects are madness. Great Britain alone owes its warmer climate and rain to the gulf stream. At its latitude it should be much colder. Currents are also very stationary and can exist for many thousands even hundreds of thousands of years.

• Jet Streams: like the ocean currents there are similar constructs in the atmosphere with similar effects. Though not as dramatic as the gulf stream jet streams can drive plenty of in seasonal climates.

• The sun: this thankfully doesnt drive our climate diversity, our sun is very stable. However, it is theoretically possible that a stars output can be variable enough to drive climate change.