Tightest optical planetary alignment

As seen from earth just before sunrise or just after sunset, what is the smallest possible region of sky that the five naked eye visible planets could encompass? A week earlier or later, how much larger would it be. What if it were just four of the planets? I am going to guess it would depend on how far off the ecliptic any given planet is at the time. Are there any fairly bright (apparent magnitude > +2) stars close enough to the ecliptic to join this event?

There’s no theoretical limit to how close they can appear. The only thing that might prevent that is an orbital resonance between two or more of them, but there are none. There have been some groupings from time to time. For instance, in May 2002.

I remember that one well, and another one in 1979 that was very impressive. Still, the outermost planets in these groupings were about 60-70 degrees apart, if I remember correctly.

There has to be some theoretical limit to how tight a grouping can be, since the orbits aren’t all in exactly the same plane, and there’s no line where all of those planes intersect.

But it would take more complicated data mining than I’m up for to find the tightest possible grouping.

Here’s a photo I took of a pretty tight grouping of the Moon, Venus and Jupiter from 2008.

Way cool.

Found this page, which includes:

The above page mentioned

Here is what that looks like (from my location) in Mobile Observatory.

Here is a list of quintuple groupings. Next one is in 2040 as mentioned. The tightest group over over 5000 years was 4.33 degrees.

Good point. I was thinking of only one dimension, right ascension. There’s nothing keeping the planets from all having the same value there. So the planets could be all in a vertical line, but some higher or lower.

I faintly recall seeing that. Living in the country and working the night shift doesn’t let much star stuff get by.
So the current leading answer is no theoretical minimum but over a 5000 year span its been below 10[sup]o[/sup] only a few times per the article cited.

There must be a theoretical minimum–just that none of us know it.

Could it be zero? Doesn’t every planet cross the ecliptic sooner of later in its orbit? Is it always the same for each planet? If not, couldn’t they all just coalesce at one point some time? Over 5-6 billion years?

The problem with predicting the true long-term dynamics of the Solar System (5-6 billion years qualifies!!) is chaos due to the gravitational interaction among the various planets. That said, consider, say the outer planets, whose orbital inclinations (relative to the solar system) remain small, while the inclination of the inner planets can get large, and indeed for Mercury never decreases below 4.57 degrees (value taken from here) or so.

Apparently may 6th 2462 all 8 planets (no Pluto) will be visable according to this website which say it is based on some modeling software, Stellariun, which is mentioned in the article.

Now it doesn’t answer the question as to what the tightest window is, but indicates it is possible that all are visable, and from the image at the bottom, it looks like they are all pretty close to the horizon.

Sorry 2492,
Also it might be as close as they get, I think Murcury will be dropping below the horizon as Jupiter is comming up, so that could be the closest grouping.


It does not even make Meeus’s list, which goes up to 25° for the five planets. He did neglect a couple of planets, but, for instance, Neptune looks like it would be pretty hard to spot under the conditions in that picture; how did they determine it will be visible?

Is it possible to dumb this down enough to explain why? No data mining required. Just generally; for the hoi polloi.

Is it because ‘non-coplanar’ orbits introduce extra degrees of freedom into any set of equations modeling the planets’ orbits?

The comment about inclination addresses the existence of a theoretical minimum for how close the planets can appear. Imagine, on the contrary, that the orbits were all in the same plane, so that the planets all whizzed about in their ellipses like hands on a clock. You could imagine that, if you only wait long enough, by chance they would all line up sometime relative to the Earth, say within a degree or two. Now, when two planets bunch together in the sky, there is a straight line in space connecting the Earth and the other two bodies. But since the orbits of the planets are in fact tilted in different directions, there is literally no line contained in the orbits of all six planets under consideration, so they can never get too close.

Thank you. Much obliged.

So it’s not a line, but a triangle (with an extra dimension needed, so to speak)?