How to watch this week’s rare planetary lineup!

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Look up! There’s stuff to see in the sky!

This week there’s a fairly rare event in the sky that, if you time things just right and are able, you can see for yourself. Get this: All the major planets in the solar system will be visible in the sky at the same time just after sunset!  That also includes Pluto, the Moon, and Vesta!

It’s quite the planetary cornucopia. But you’ll have to be on your toes to see it.

The planets all orbit the Sun pretty much in the same plane, called the ecliptic plane. The Earth does too, which means we see this plane from the inside, making it appear as a line in the sky (like a piece of paper held edge-on) called simply the ecliptic. The planets all stick close to this line.

They also orbit the Sun at different rates. Mercury takes just 88 days to circle the Sun once, but Saturn — the most distant planet easily visible to the naked eye — takes 29 years. So while they all stay near the ecliptic, they can be scattered 360° around it. To all be visible at the same time they have to be within a section of the ecliptic less than 180° long — from horizon to horizon. And that’s the technical definition; in reality they have to be crowded a bit closer together or else one will set in the west as another rises in the east.

The more distant planets like Uranus and Neptune move so slowly that it can be decades before they happen to be in the same section of the ecliptic as all the other planets. That makes this a relatively uncommon event. The last time it happened was in April 2002 and even then it was only the naked-eye planets visible in the evening; Uranus and Neptune were morning objects at that time.

You can see this lineup any day this week, but the best evening to view it is Wednesday, December 28th. The two most widely separated planets in the spread are Venus and Mars, with 136° between them. That’s room to spare, but you still have to be ready to see this. Why?

Right now Venus is on the other side of the Sun, swinging around toward us. But from our vantage point it’s only about 16° from the Sun. By the time the Sun sets and it starts to get dark enough to see planets, Venus will only be about 5 or 6° above the horizon: Less than the width of your hand held at arm’s length. It sets about an hour after the Sun does.

The good news is that Venus is extremely bright, so it’s easy to spot low to the horizon in the WSW after sunset. Binoculars can help, but you probably don’t need them.

Mercury is a bit tougher. Although it’s a little higher off the horizon (about a degree higher than Venus, roughly the width of your finger at arm’s length) it’s much fainter. You’ll likely need binoculars to spot it above Venus and to the right a bit (for northern hemisphere observers, above and to the left for southerners). See below for more about Venus and Mercury.

27° along the ecliptic (something more than two fist widths) toward the south from Mercury is Saturn, still brighter than the surrounding stars, and looking yellowish. It might be tough to spot in the twilight but is easy once it gets dark. On the 28th the next object in the lineup is the crescent Moon, which is obvious enough. Jupiter is some 8° to its upper left and really very obvious. It’s quite bright.

Then, almost 70° further along the ecliptic, ruddy brilliant Mars glows balefully in Taurus above Orion. The color makes it pretty easy to distinguish from the stars.

Those are the naked eye planets. Uranus, which takes binoculars but isn’t too hard to find, is between Jupiter and Mars. Neptune, which takes a telescope, is right next to the Moon. Pluto — which takes a telescope and a decent camera to spot — is just to the upper left of the Venus/Mercury pair. I’ll add that Vesta, the second largest asteroid (and more properly a protoplanet) is between Saturn and the Moon. It takes binoculars to see as well.

If you want more accurate positions, then I suggest using Stellarium, an online planetarium web app, and/or Sky Safari for your mobile device, which is what I use all the time. There are a ton of apps you can get for your mobile devices and most will show you where to look.

You don’t have to go out on the 28th to see all this — you could go out tonight and try! — but over the next week Venus pulls away from the Sun making it easier. After Dec. 31 Mercury starts divebombing the Sun as it laps us in its orbit, so after New Year’s it gets much harder to spot. A few days later it passes the Sun in the sky and becomes a morning object, ending this event.

And if it’s cloudy or you can’t see this for whatever reason, good news: Gianluca Masi of the Virtual Telescope Project will be streaming a live viewing of it all! That starts at 16:00 UTC.

I’ll note that if you do go out on the 28th, Venus and Mercury will be at their closest approach to each other. From our perspective — orbiting the Sun ourselves — Venus is heading away from the Sun while Mercury is heading toward it. Both are simply circling the Sun, of course, but we see them pass each other. It’s like standing on the side of a racetrack and watching one car on the near side of the track approaching you and moving to the right while the other is on the far side moving left. They seem to cross paths, even though one is much farther away.

On that evening they’ll be about a degree apart, twice the width of the full Moon on the sky. That’s pretty close, and should make a nice treat in binoculars or a low-power telescope. This sort of apparition is typically called a conjunction, though more accurately an appulse.

I took that photo above of them a week ago, and it was pretty easy with just a phonecam. Try for yourself! It’s amazing how good the photos you get can be.

Mercury is smaller and less reflective, so even though it’s much closer to us than Venus right now (122 million versus 240 million kilometers) it’s still a lot fainter. Venus is about 50 times brighter. It’s also low to the horizon, so you’ll want a clear view to the SSW to spot it. If you can see Venus, then Mercury should be visible too.

If you can’t see them this week, don’t despair! Venus is moving farther from the Sun every day, and by the end of January will be easy to spot after sunset. By then Mercury, orbiting the Sun much faster, will be on the other side of our star in the sky and easier to spot in the east rising before the Sun does in the morning. Note I said “easier”, not easy. It’s always a tough bugger to observe.

If you have a local observatory or astronomy club, you might want to contact them and see if they’re holding a public viewing (MASK UP PLEASE). Sky and Telescope has a page where you can see if any are nearby.

And think about this: A few hundred years ago we didn’t really know what the planets were, or if they orbited the Sun or the Earth. We only knew the ones we could see with our eyes! Now we know of two more, plus Pluto, millions of asteroids, and hundreds of moons. You can’t see all of that when you go out on the 28th and look, but that’s all there in the sky nevertheless.

A lot of the Universe is hidden from us, even our local patch of it. But what we can see is still beautiful and wondrous and absolutely worth going outside to gaze at.

P.S. And if you’ve heard some nonsense about this lineup causing any woe on Earth: NOPE. The planets are too far away to have any effect on us like earthquakes or the like. These conspiracy theories always crop up when an astronomical event like this happens. I debunked this back in 2000! So don’t let these knuckleheads drag you down, and instead enjoy this cool and rare event. 

Et alia

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