FirstLight Astronomy Club

33°29.6'N / 117°06.8'W / 1190 ft.

A Gathering of Wanderers

Some of the planets are currently playing a trick on us. This prestidigitation involves neither smoke nor mirrors, but like any good trick it does take advantage of illusion.

In the western skies this week, just after sunset, there will be three planets appearing to group together for a family portrait. It will end up being a tight shot, to be sure. Then after the weekend they will seem to go their separate ways. But are they really that close, nearly bumping?

It is this wandering characteristic that got the planets their name in the first place. Thousands of years ago the ancients noticed that the stars in the heavens were fixed, they didn’t move about. They predictably showed up in the same heavenly real estate year after reliable year.

But there were some stars that seemed to “travel.” Throughout the year anyone who kept an eye on them saw that they sort of… well, really wandered through the sky. So they were given the name “planets” from the greek work “planetes,” which means, to no one’s surprise, wanderer.

Of course we now know that the planets are not stars, but small rocky and gassy bodies orbiting the sun. They are not the incredibly distant actual stars that are so far away that most don’t appear to move at all in a person’s entire lifetime.
And since all the planets move on essentially the same plane, there are going to be times when they appear to get real close to each other.

Such is what is happening this week as Venus, Mercury, and Saturn group together for a nice portrait after sunset. But the grouping is just an illusion; they will be nowhere near each other. Let me tell you how they pull it off.

We are in the part of our orbit where we have passed distant Saturn months ago in our run around the sun, but are now being chased by Mercury and Venus on their inside tracks.

Imagine racing around a track with all runners in staying in their own lanes. Imagine looking back as you run around the turn, seeing a distant runner that you have already passed but, at the same time, spotting two runners who are fast catching up with you on the inside lanes.

Imagine looking over your shoulder at the precise moment when it appears that they – the runner you have passed and the two catching up – are coincidentally in the same line of sight. They appear to be grouped, but are actually nowhere near each other.

That’s what we’ll be seeing all this week about 45 minutes after sunset. We are actually looking back and seeing Mercury and Venus rounding the sun and ready to move by us, as poor distant Saturn lags way, way behind.

But it appears they are all gathering in one spot in the sky.

You can start watching this evening. Venus will be the brightest dot in the twilight skies. A good eye will pick out dim Mercury to its lower right at less than three degrees away. Saturn is a brighter dot on the opposite side of Venus and slightly farther.

Go outside about the same time in the next few days and see for yourself how it appears that Mercury is approaching Venus from below, Saturn from above. They are all “wandering” as they did for the ancients for thousands and thousands of years.

It all comes to a climax over this coming weekend when, over successive nights, the three - now so close they are called a “trio” in astronomy parlance - get together in the closest trio since the last century. The only thing missing from this sky scene is a beautiful crescent Moon as a backdrop.

Mind you, Saturn is actually 800 million miles farther away than our two inner neighbors, who themselves are more than 40 million miles apart. Remember their “grouping” is just part of the illusion.

After the weekend they will begin to part ways again. These get-togethers never last.
If you get a chance to see it, do. There will be another trio next year, but its proximity to the sun will make it very difficult to appreciate. After that you’ll be waiting decades for another great grouping of these wandering magicians.

Quick Quiz on the Gas Giants

We’ve recently passed two of the great gas giants, Jupiter and Saturn, in our orbit around the sun. Both of these planets are still up in the early evening skies, Jupiter overhead, Saturn more to the west.

Maybe now would be a fitting time to take a quick little quiz about the outer planets - Jupiter, Saturn, Uranus, and Neptune. The statements below are all true or false, but remember, as usual, you will not be graded - but you must have fun!

Statement 1: The gas giants are made of gas.

Asking people what the four gas giants are made of is like asking who’s buried in Grant’s Tomb. “Gas giants” pretty much spells it out – they are giant planets made of gas, of course.

Kinda, sorta… not.

If by gas one means hydrogen and helium and methane then yes, the four jovians, as they are often called, are all pretty gassy. But (cue dramatic music) those elements and compounds aren’t always gasses!

The outer layers of the planets are gassy, to be sure, with their cloud patterns and storms like that Great Red Spot on Jupiter. But when you sink below the cloudy upper deck and in through an outer layer of gaseous hydrogen, the atmospheric pressure starts to kick in pretty heavily, and things change dramatically.

Going further down, the pressure becomes so great that the hydrogen gradually becomes a liquid. Go further down into Jupiter and Saturn where the pressure reaches about 2 million times what it is here in our atmosphere and the liquid hydrogen becomes the rare metallic hydrogen.

And way, way, way down deep is a rocky core, probably as big as Earth, experiencing pressures greater than 50 million times our own atmospheric pressure.

Uranus and Neptune aren’t big enough to have the liquid hydrogen but neither are they completely gassy.

Below their upper cloud decks is a thick layer of gaseous hydrogen. But down below that is an entire sphere of icy water and ammonia and methane. And way down deep they, too have a rocky core.

So, yes the “gas giants” have lots of gas, but they are no means the great gas bags in space many people see them as.

Answer: false

Statement 2: The fact that they are all gigantic and have lots of gas gives us hints into how they were created.

When one sees four inner planets, grouped together and all earthlike, surrounded by distant gigantic planets all bathed in hydrogen and other gases, us nerdy science types get all excited. There’s a pattern here and the planets are trying to tell us something.

What we believe these giant outer four are telling us is that they were formed about the same time, far away from a burning sun.

Far away means cooler, and cooler means different materials can be used as building blocks for planets; like gases and water ice, for example. Try building a planet of gas near a star and it won’t work. The gases get blown away by the star. That’s why the inner planets are all rocky bodies.

But the Gas Giants are telling us that out there hydrogen and helium and methane and even frozen water can be used to build planets. And the huge masses of Jupiter and Saturn indicate that there was a lot of “stuff” to vacuum up out there in the early days of our solar system, but not so much way out where Uranus and Neptune were created.

Answer: true

Statement 3: Of the gas giants, only Saturn has rings.

Saturn’s rings were the first to be seen when Galileo spotted them nearly 400 years ago. But it wasn’t until the last decades of the 20th century that the other jovian planets revealed their own little secrets.

It turns out that all four have ring systems. Of course Saturn’s is most famous and arguably the most beautiful. But Jupiter has a faint ring system of its own, made of dust so small that it can’t reflect light back towards us. That’s why we can’t see it from here. But spacecraft flying behind it have seen its dispersed light and imaged it.

Uranus has its own unique and beautiful ring system that now, with powerful telescopes, can be seen from here.

And distant Neptune has a bizarre ring system, barely there, and made of barely visible carbon compounds making it very difficult to see even by the Voyager spacecraft that have flew by years ago.

Answer: false
Temecula Valley High School / Temecula, CA · Some images © Gemini Observatory/AURA Contact Me