When was the last time all inner planets aligned in our solar system?

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Discussion Overview

The discussion revolves around the alignment of the inner planets in the solar system, specifically focusing on the conditions and timing of such alignments. Participants explore the statistical nature of planetary alignments, the significance of orbital periods, and the geometric considerations involved in defining alignment.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant suggests that determining the closest occurrence of the inner planets forming a radial alignment involves uncertainty in solar system models and computations.
  • Another participant argues that achieving a precise alignment of all eight planets within a degree or two may be practically impossible.
  • A different viewpoint proposes that the problem is more statistical, questioning the probability of all planets aligning in a semicircle and suggesting a probability of 2-7 for such an event.
  • One participant notes that all nine planets were on the same side of the Sun recently, but this does not meet the criteria for a narrow alignment.
  • Another participant emphasizes the importance of orbital periods in calculating alignment frequency, providing specific examples related to Mars and its opposition frequency.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility and definition of planetary alignment, indicating that multiple competing perspectives remain without a consensus on the specifics of alignment conditions.

Contextual Notes

Discussions include assumptions about orbital periods and the geometric definitions of alignment, which may affect the conclusions drawn about alignment frequency and conditions.

Who May Find This Useful

Individuals interested in astronomy, planetary dynamics, and statistical modeling of celestial events may find this discussion relevant.

Loren Booda
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Within the uncertainty of solar system models and their computation, what and when was the closest occurrence (in terms of solid angle) of the inner eight planets forming a radial alignment outward from the Sun?
 
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You'll need an accuracy to get a time. Ie, if you want all 8 within a degree or two then the answer is somewhere on the order of never.
 
russ,

I am thinking this to be more a statistical problem than one of astronomy in particular. For instance, if one required all these planets "align" in the same semicircle bisecting the orbits of the solar system, the probability of that occurring would be 2-7.

Would time constraints, such as orbital periods, be of secondary (if any at all) concern to the fundamental problem? If the time considered were on the order of 5 billion years, what would the minimum angular dispersion from alignment be?
 
Well, all 9 (former) planets were on the same side of the sun just a few years ago. That's an angle of <180 degrees. Probably you're looking for something a little more narrow...
 
It is a matter of time - you need orbital period to calculate alignment frequency. My geometry isn't good enough without putting some effort into it to calculate the frequency from the orbital periods, but for example, Mars has an orbital period of 1.9 years and an opposition frequency of 2.6 years. That means that once every 2.6 years, for 1.3 years, the Earth and Mars are in the same half of the solar system. And once every 2.6 years, for 2.6 days, they are within the same degree.
 

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