Will there ever be a moment when all eight major planets are in a straight line on the same side of the Sun?

Jean Meeus addresses this in Mathematical Astronomy Morsels (Willmann-Bell, 1997). He points out that you have to start by defining the question precisely. Let’s reduce the problem to two dimensions and ask whether all the planets can have the same heliocentric longitude (they can never line up in three dimensions because their orbital planes are all slightly different). Then, to simplify the arithmetic, we’ll say that two longitudes count as “the same” if they’re within 1.8° of each other.

Mercury, the fastest-moving planet, laps Venus every 0.396 year, staying within the 3.6° arc centered on Venus for 0.004 year every time. On each pass, the chance that Earth will also be within this 3.6° arc is 1 in 100. So, on average, the three inner planets line up every 39.6 years. The chance that Mars, Jupiter, Saturn, Uranus, and Neptune will all be within this arc as well on any given pass is 1 in 100 raised to the 5th power, so on average the eight planets line up every 396 billion years. If you tighten the definition by requiring the planets to be within 1° of each other, the time increases to 13.4 trillion years. Either way, the Sun will become a red giant, shed much of its mass, engulf Mercury and Venus, and allow the other planets to drift into radically different orbits long before such a lineup takes place!

— Tony Flanders

feelie75

September 5, 2017 at 3:26 pm

Finally, a real answer! Every other article out there says they'll "kindof" align in 2492, which totally doesn't fit the definition of "align". I'm talking about how you define it, from "above", ignoring the offsets the planets have on the plane. Your answer has the math I was looking for and provides the answer I was looking for, so THANK YOU for answering this very simple question correctly unlike all those other articles 🙂

Julzee111

June 22, 2018 at 1:34 am

It absolutely CAN happen and WILL happen on July 9 2018

Larry McNish

December 26, 2019 at 3:35 pm

Uh... not even close!
Diagram of the planet locations on July 8, 2018:
https://grumpyoldastronomer.com/images/Planet_positions_2018-07-09.png

Larry McNish

December 26, 2019 at 3:41 pm

Oops - comment typo - July 9 not 8.
The diagram is correct for July 9.

Lindsay

October 15, 2022 at 8:22 am

Lmao you can just look at your diagram and see one after it will be almost the same. It’s now 2022 and it still has not happened. It will never happen and if it should possibly happen we will not be here for it