When I walked into work Monday, the first person I saw asked, “Are you going to talk about the planetary parade?” I asked what he knew about it. He said, “We’ll be able to see all the planets in a line Tuesday night.”
“Not so fast!” I replied.
The truth is, that the planetary parade is more hype than reality, but it certainly offers valuable lessons about our solar system.
What is true?
This week, every planet in the solar system is above the horizon after sunset because they’re all on the same side of the sun.
What is misleading?
- You probably won’t see Saturn and Mercury because they’re only a few degrees above the horizon, likely hidden behind trees or houses for most of us.
- Uranus and Neptune are too distant to see without a good telescope.
- The phrase ‘all planets are visible’ is misleading. Only a tiny fraction of people, with a sizable telescope and a clear western horizon, will actually see them all.
- Saying ‘all planets align’ isn’t accurate. The planets follow the ecliptic; the solar system’s plane. More on that below.
- Calling it ‘rare’ is a stretch. It’s not unusual to have multiple planets in the sky at once, but it is somewhat uncommon for the four brightest to be so well-placed for viewing.
Understanding the ecliptic:
The planets naturally appear in a line because they all orbit within a few degrees of the solar system’s plane, known as the ecliptic. Since Earth shares this same plane, we see the planets as bright beads strung along an invisible wire stretching from horizon to horizon. This is easier to visualize when you look at the solar system edge-on, as shown in the image below.
For all the hype about this “parade,” we actually saw a tighter gathering of planets back in June and July 2022, when all the planets—plus the Moon—spanned just 91° of the sky before dawn.
What made 2022 especially cool was that the brightest planets appeared in order of their distance from the sun, from Venus (farthest east) to Saturn (farthest west). No such symmetry exists in the current lineup.
Can the planets ever form a perfectly straight line?
The answer is no. Each planet orbits the sun at a slightly different inclination, ranging from .8° for Uranus to 7° for Mercury. Earth is a special case; its inclination is 0° because it defines the solar system’s plane. This means the planets, like the moon, slowly weave above and below the ecliptic, making a perfect bullseye alignment impossible.
But could all eight major planets at least cluster within 1° of each other? Sky & Telescope crunched the numbers and found that, yes, it can happen—but not for another 13.4 trillion years!
