For us earthlings, the concept of "year" came from the observation that the Sun reaches a minimum and a maximum span in the rise-set path in the sky.

On a planet experiencing extreme perihelion precession (about 30 degrees) is there any recurring astronomical event which can mark the end of a year for the inhabitants of the planet? (picture is meant to illustrate the extremity of the precession, not the eccentricity of the orbit)

enter image description here

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    $\begingroup$ Umm... is that illustration in any way representative of the system you have in mind? Because it looks like you've got an orbit with apastron about nine times as distant from the star as periastron. With such an extremely elliptic orbit, I suspect life would be unable to develop, and certainly complex life would face enormous challenges. $\endgroup$ – a CVn Dec 27 '17 at 9:55
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    $\begingroup$ Not to mention that there is no reason why "the end of the year" necessarily needs to be tied to anything astronomical. New year in the Gregorian calendar occurs close to, but is not identical to, Earth's perihelion (which coincides with winter in the northern hemisphere due to Earth's axial tilt); in 2017, winter solstice apparently occured on Dec 21 16:28 UT, and it typically occurs on either Dec 21 or Dec 22. $\endgroup$ – a CVn Dec 27 '17 at 9:56
  • $\begingroup$ @MichaelKjörling, I edited my question. I hope this explains better. $\endgroup$ – L.Dutch - Reinstate Monica Dec 27 '17 at 10:19
  • $\begingroup$ What's a "year"? $\endgroup$ – Mołot Dec 27 '17 at 10:23
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    $\begingroup$ Adding to my first comment, to give an idea of what a 9x orbital radius change over the course of an orbit means: If the near point (periastron) is at Earth's orbit around the Sun (1 AU orbital radius), then the far point (apastron) is just inside Saturn's orbit around the Sun (a little less than 10 AU orbital radius). If periastron is approximately at Mercury's orbit around the Sun (a little under half an AU), then apastron is near Jupiter's orbit around the Sun (a little over 5 AU). $\endgroup$ – a CVn Dec 27 '17 at 10:30

I would propose "The day of the burning Sun", or "The day of the dim Sun". Basically, either the perihelion or the apohelion could mark a periodic event defining the end of a year and the start of a new one. Your inhabitants would see the Sun going away and going back peridically, which could hold quite an interesting mythology.

The cool thing is, they'll notice that the stars aren't at the same place each of these special days. If your precession is of 30° exactly, this implies that the stars will be at the same spot relative to the day every 12 years, which could lead to a special event in your inhabitants cultures. If it is almost 30°, you can have almost a 12 years periodic event (and keeping the big party), plus a real coincidence with a previous orbit (exact same stars position) every few centuries or so (and you can throw an even greater party!).

(Of course, as @Michael Kjörling stated, life would have a hard time developing on such a planet. But let's forget that.)


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