这句话怎么理解？

shuaiwhuUID: 2729769

As Kepler showed, the Earth’s orbit is an ellipse, with the sun at one of the two focal points. He also figured out the planet goes faster when it’s at perihelion, nearer the sun, than when it’s at aphelion, its farthest distance—which would explain why summer seems to zip by, except the seasons are a function of the tilt of the Earth’s axis, not its different distances from the sun.
"except...not..."看的有点蒙，这里该如何理解翻译？

苏梳眠UID: 2717864

which would explain why summer seems to zip by，except the seasons are a function of the tilt of the Earth’s axis, not its different distances from the sun.
the season为except之后主语，a function..和not its..为其宾语。
这就解释了，除去地轴的倾斜度会影响季节形成，不是离太阳的远近影响季节形成，夏季还会交替的原因。