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Ep. 302: Planetary Motion in the Sky

Ep. 302: Planetary Motion in the Sky

Even the ancient astronomers knew there was something different about the planets. Unlike the rest of the stars, the planets move across the sky, backwards and forwards, round and round. It wasn’t until Copernicus that we finally had a modern notion of what exactly is going on.

Transcript: Planetary Motion in the Sky


Fraser: Astronomy Cast episode 302 for Monday, April 15, 2013 – Planetary Motion in the Sky

Welcome to Astronomy Cast, our weekly facts based journey through the cosmos. Where we help you understand not only what we know, but how we know what we know.

My name is Fraser Cain, I’m the publisher of Universe Today. With me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville, and the director of Cosmoquest.

Hi Pamela, how are you doing?

Pamela: I’m doing well, almost got hit by a hurricane… it wasn’t a hurricane. Almost got hit by a tornado on Friday night… that was interesting. Things are better now.

Fraser: Yeah it sounds like it was a little scary there for a while.

Pamela: Yeah, don’t drive into a funnel cloud.

Fraser: Hey, that’s news you can use.

Pamela: (Laughs)

Fraser: So we have sort of an epic thing coming up.

Pamela: We do! On June 15th and 16th we’re going to take our funding into our own hands and take a page from the Jerry Lewis tel-a-thon playbook.

Fraser: A skeptics guide into the universe?

Pamela: Well them too. Host an epic 24 maybe even 36 hour hangout-a-thon using the Google hangout-on-air technologies. We are going to bring you scientists, science entertainers, communicators, Virtual Star Parties, hands-on demos, cooking planets, how to learn geology using Oreo cookies… It’s going to be science, science and more science. The goal is to try an raise the funds to keep CosmoQuest going throughout the current funding hold, stoppage and sequestration and everything else that is going on in science funding.

Fraser: Right, we’re not going to let a stupid sequestration keep us down.

Pamela: No, failure is not an option. We shall fund public engagement in science.

Fraser: So where can people find out what we’ve got planned?

Pamela: We have an event page set on up Google+ and we will be posting details on We will be doing that this evening on June 2nd.

Fraser: Great. We’ll be mentioning this again in the next few episodes as we record.

Fraser: Even the ancient astronomers knew there was something different about the planets. Unlike the rest of the stars, the planets move across the sky, backwards and forwards, round and round. It wasn’t until Copernicus that we finally had a modern notion of what exactly is going on. Now Pamela it’s been a really crazy week so I’m guessing that Mercury is in retrograde?

Pamela: Yeah I don’t care what Mercury is doing.

Fraser: (Laughs) Alright that was an astrology joke. It didn’t hit you as hard as I was hoping it would. You’re clearly numb to them now.

Pamela: (Laughs)

Fraser: Lets go back in historical time with the planets. There are what, five visible planets?

Pamela: Mercury, Venus, we’re standing on Earth, Mars, Jupiter and Saturn. So six, we just stand on one.

Fraser: (Laughs) Right we didn’t realize that one was also a planet. So the ancient astronomers knew, as far back in history as you can image, people could see these bright objects moving in the sky. They knew from night to night that these things were changing their positions; they knew something was going on.

Pamela: They called these objects “The little wanderers”; the word planets actually means “wondering objects”. What they noticed that stood out so much was that the planets seemed to move relative to the background stars which appeared to be fixed into the celestial sphere. They kept trying to figure out what was going on. For a while Venus and Mercury were two separate objects, one in the morning and one in the evening. They tried to figure out how to configure the solar system and even without understanding they did come up with special words for special positions in the sky for these different objects. They were very careful to notice and try to draw conclusions from things like Mars appearing to move backwards against the stars.

Fraser: So what was their explanation for why the planets moved in the sky?

Pamela: Well for the most part their explanation was, depending on when you look back in history, gods, glass spheres and things more and less illogical between those two extremes.

Fraser: In many cases they gave them the names of the gods, they had the chariots carrying them across the sky or various gods…

Pamela: Turtles

Fraser: (Laughs) Turtles? All the way down? Those turtles? That was the part was really interesting to me is that they used to think that Venus was two separate objects. You mentioned that sometimes you would see it in the morning and sometimes you’d see it at night.

Pamela: With the outer planets like Mars, Jupiter and Saturn you can watch them over time as they get closer and closer to the sun and then come out the other side. They move in predictable ways and because of this gradual motion relative to the background stars it was easy to see that this was continually the same object. With Venus and Mercury they would have these elliptical paths through the sky or snake-like paths through the sky as they got watched night after night. Because their period is so short it was hard to recognize that the morning star was becoming the evening star throughout the orbits. It was just two different snakes showing their path through the heavens.

Fraser: But they are never at the same time. It’s like Superman and Clark Kent.

Pamela: Exactly.

Fraser: You mentioned that they came up with their names. What are some of the names that they came up with?

Pamela: Well for instance you have, as you already teased, retrograde motion. Normally we talk about planets moving in prograde motion, this is where they appear to move in the same direction that the stars generally move. They come up in the east and move slowly towards the west and everything was creeping in the same direction. Occasionally as our orbit catches up and passes one of the outer planets, this difference in motion between the catching-up and the going-past will cause those outer planets to suddenly move backwards, to appear to move west to east relative to the background stars. What’s kind of fun is to grab an astrologer what is physically happening when Mercury is in retrograde. They don’t necessarily understand this physical motion and that it is just an appearance, not that Mercury stopped and reversed its actual direction in orbit. This is just an allusion of things moving past one another.

Fraser: You can imagine from our perspective from way back in the day, it really did feel like the entire universe was orbiting around the earth and that we were at the center of it. The stars kept going around and around and the planets made this journey, it really took a leap of logic for Copernicus to shift the thought process that we are just another planet and that we were going around the sun just like the rest of the planets.

Pamela: The real issue was one of understanding distance. We’re used to things near by, when you move your perspective, they seem to move relative to even further objects. For instance if you hold your thumb up and switch between looking at it with left eye and right eye, you’ll see the background appear to move relative to your thumb. There was an expectation that if the earth was going around the sun, how is it the stars wouldn’t appear to change in position radically, perhaps even, as the earth went from one side of the sun to the other. The expected change in position was something that people thought could be seen with the eye. Since we don’t see that expected change, well it must be that everything is going around the earth so you don’t have what we call a parallax effect. Well it turns out that there is parallax for the nearest stars but it requires telescopes to see it because the stars are so vastly far away that the Earth’s motion from one side of the sun to the other isn’t enough to see with the eye or to even see with most telescopes. That motion, in this case, against the background galaxies that we use to set our maps of the sky against.

Fraser: So which planets go through this retrograde motion?

Pamela: The typical retrograde that we think about in astronomy is where we see a planet appear to back up and then move forward. We only get that effect for the outer planets which are Mars, Jupiter and Saturn. We don’t get that from the inner planets because they cross going past us in one direction and then cross again going around the sun on the other side. They also have a type of retrograde motion but it’s not caused by the effect of us passing each other which lead to a forward and backward appearance, loop-de-loop in the sky. It’s instead caused by the planet being on a different half of its orbit where we see it going in the other direction.

Fraser: So how did the ancient astronomers try to explain what was going on with the retrograde motions?

Pamela: This is where we ended up with circles on circles on circles. It was Ptolemy who perhaps did the most work in trying to understand this. When he attached epicycles to his orbits so that as the planets went around the earth, the orbit was a circle and then attached to this orbit was another circle. It was through the combination of all of these circles that he could describe the motion. The crazy thing is, Ptolemy did so much work on this “circles all the way down” system that he was able to fairly accurately model the actual motions in the sky. His model was false, but it worked pretty well which is one of those frustrating things for those of us when lies work out to appear to be true. Copernicus on the other hand was determined that orbits had to be perfect circles. His system didn’t match the orbits of the planets in the sky and he also had to attach epicycles to his circular orbits that went around the sun. His circular orbits with epicycles going around the sun wasn’t better than Ptolemy’s in describing the motions and that is more than a little frustrating. It took Copernicus the realization that things can be ellipsis to get to actually get to model the real motions in the sky.

Fraser: So as an astronomer making observations night after night, how fast will you see the planets move in the sky?

Pamela: It depends on which planet you’re talking about. Jupiter is far out and Saturn is even further so they have very minor motions. Season to season they don’t vary too much; they will edge slowly from one constellation to another. Mercury and Venus, night after night, you can see the changes over the course of the evening. If you take a picture when they are first visible as the sun sets, or if you’re good, before the sun sets, you’ll be able to measure them to be in a different place in a photograph by the time they set a few hours later when they are at their highest point above the horizon.

Fraser: I always think it’s funny when people on Twitter will ask “What’s that really bright star beside the moon? I never noticed it before”

Pamela: It’s not a star.

Fraser: Right, and there is so much wrong in that comment. It’s not a star, it’s obviously a planet, it’s probably Venus or maybe Jupiter. The moon moves in the sky and Venus moves in the sky so the fact that you’re seeing this configuration of the moon and Venus in the sky at this time is something that happens every couple of months. The reason you never noticed it before is because it wasn’t there.

Pamela: Some of the really neat configurations are actually pretty rare. One of my favorite configurations is when you have the crescent moon sitting on the horizon so it looks like the mouth of a smiley face. You can occasionally, as in every few hundred years, get it lined up so that you have Venus and Jupiter appearing like two lop-sided eyes up above the smiling face. You can get all sorts of neat alignments. Every year it seems like we’ll end up with Venus and some planet near a crescent or a full moon and everyone drags their cameras out… you and I are guilty of that.

Fraser: Yes…

Pamela: These are awesome and they occur fairly regularly but it’s really amazing when all the planets are visible at once in a line, in the same direction in the sky. It’s those types of alignments that are more rare. It was all of these alignments that astrologers of old used to take care to take note of. They actually came up with a whole set of vocabulary words to describe the different positions that planets could acquire in the sky as measured against the sun.

Fraser: So what kinds of things can we see?

Pamela: The one that we’re most often excited about is when a planet is in opposition. This is when you can draw a straight line from the sun, through the earth, out toward that other planet be it Mars, Jupiter, Saturn, Uranus is one that you can only see with a telescope generally. When an object is at opposition it is highest in the sky at midnight which means you get the least light pollution and you’re also able to see it for the greatest portion of the night. It will often rise at sunset or a little before or after depending on what season of the year it is. You can follow it throughout the entire night taking image after image watching planetary rotation or watching the moon’s orbit so opposition is a pretty special thing.

Fraser: Right, that’s where with opposition you both get a minimal amount of atmospheric distortion and the planet is pretty much at its closest point from our perspective. It’s going to be its biggest, brightest, easiest to see and the best time to take photos.

Pamela: This is also when we’re closest to the planet. Several years ago we all had that horrific e-mail threat that “Mars is bigger than the moon and the…” No, no it wasn’t. Every once in a while, Mars orbit which is an ellipse and Earth’s orbit is an ellipse, will allow the Earth when it is at its furthest point from the sun to be in opposition with the sun and Mars at the same time as Mars is at its nearest point to the sun. That brings the two worlds just a little bit closer together and makes it appear to be just a little bit brighter in the sky and that is just fun to photograph.

Fraser: For everyone who hears “Mars Opposition” we use that shorthand in Universe Today and a lot of places,

Pamela: Virtual Star Party

Fraser: Yeah exactly. Opposition, opposition, that’s what you want. It’s not like it’s our enemy, it’s our friend and it’s lining up for a good picture.

Pamela: The one that is our enemy is the planet that is in conjunction with the sun. That’s when you draw a straight line from the Earth, through the planet then to the sun or from the Earth, through the sun then to the planet on the other side.

Fraser: Normally conjunctions are good things. They’re interesting because you get two objects close together in the sky but if you get a conjunction between the planet and the Sun, not good. Not good at all.

Pamela: Yeah, it’s a conjunction between the moon and a planet that we like; conjunction with the sun is the enemy for astrophotogrophers. That is unless of course you end up with a Venus transit. Most of the time when a planet is in conjunction with the Sun it’s either just a little bit above the Sun or just a little bit below the Sun on its orbit. That little bit above or below means that it’s not lined up so we that we see it directly in front of the Sun. As we experienced about this time last year, occasionally Venus and less rarely Mercury will cross directly in front of the Sun and we can see it moving like a migratory sun spot as it moves across the face of our nearest star.

Fraser: I guess that is a conjunction because they are in the same spot but in this case it’s in front of the object instead of behind the object.

Pamela: In front is an inferior conjunction and behind the sun is a superior conjunction.

Fraser: Right, I guess we can’t get that with Saturn and Mars, we only get opposition or conjunction with..

Pamela: It’s a superior planet in conjunction but it can never be in inferior conjunction unless something very bad has happened to our solar system.

Fraser: Any other positions in the solar system? What if it right angles to them?

Pamela: So right angles we call that being at elongation so you can have something where it’s: Sun, Planet , Earth and that is pretty much when the planet will appear to be highest in the sky relative to the Sun. Anything else and it will appear closer and closer to the Sun in the sky.

Fraser: You’ll see that as well in various calenders. It will say that Mars is in its greatest elongation.

Pamela: Depending on whether or not it’s a western elongation or an eastern elongation depends on whether it is furthest from the sun during daylight or at night. The goal is to have it so the sun sets first and then the planet sets second and we get that one for eastern elongation. The sun sets first and then the planet sets.

Fraser: As you said it’s high in the sky so is it good for observing?

Pamela: Right. The Sun gets down and then the planet is still high enough above the horizon, which is where all the mucky, nasty part of the atmosphere are, that you can still get some good pictures of it. Venus and Mercury like to resist being observed well in darkness so you wait for them to be in their greatest elongation and that’s when you try to capture their picture.

Fraser: Mercury’s elongation is never more than a few degrees above the horizon while Venus can get pretty high.

Pamela: Right and this is why we like to send spacecraft to them. At the end of the day this used to matter so much for observational astronomers. We used to wait with pent up anxiety for what we could see for Mars to be at opposition or for Venus and Mercury to be at their greatest elongation, but now we just send spacecraft and they can do so much better than we can ever do from earth. Now it’s more of an art form, it’s a way of monitoring the weather on Mars and to look for spots on Jupiter. If you want high resolution imagery, we are no longer trying to do that from the surface of our planet.

Fraser: Are there any other names for the way things can line up? I know there are transits.

Pamela: There is also what is called quadrature.

Fraser: Quadrature?

Pamela: When something is at greatest elongation the triangle is Earth, to the Planet, to the Sun. When something is at quadrature it’s the Planet, the Earth, the Sun is the right angle. With elongation the planet is the base of the right angle and then with quadrature the Earth is the base of that right angle.

Fraser: Got it. As I mentioned we can get transits which I guess are also conjunctions.
Pamela: Which are also inferior conjunctions.

Fraser: Right, well theoretically you could get Mars move in front of Jupiter, it would just take a few hundred thousand years for that to line up.

Pamela: Yes. Okay, let me rephrase that. You can get solar transits during inferior conjunctions. You can’t get solar transits during any other types of conjunction.

Fraser: There is a page on Wikipedia where they predict these really extreme conjunctions that you might be able to see.

Pamela: You can play with Ostillarium if you’re really diligent to find all of these different alignments and some of them you have to be in just the right place on the planet to see. Especially with some of the asteroid ones where we have the asteroid appearing to move in front of the planets.

Fraser: There is one coming in 2020 I think that is going to be Jupiter and Saturn so close in the sky that you will be able to see them in the same telescope view.

Pamela: Yeah that one is going to be awesome. It’s going to lead to some really great astrophotography and I’m hoping that the whole planet has good weather on top of the land masses. It can rain out at sea but we want good weather on land masses.

Fraser: We’re only gearing up the reporting team for this; it’s going to be amazing. How do the planetary motions impact our observations of the stars in the background. We see them passing in front of stars, is there any value in this?

Pamela: There can be in some cases, with Saturn in particular. There was a case several years ago of Omicron Ceti, also known as Myra, is a variable star and was seen to pass behind the rings of Saturn. Having this star behind the rings of Saturn allowed us to better map out the density of the material in those rings. That was a really neat event.

Fraser: Well I know it was used for Pluto too. They discovered Pluto’s atmosphere when it passed in front of a really dim star and they measured the light curve as Pluto was going in front of the star and they were able to detect that Pluto had a really thin, tenuous atmosphere.

Pamela: That’s been done with Mars as well; we use it to measure the atmospheres. One of the more interesting things is when asteroids pass in front of background stars. If we could get a group of people on a stripe across the planet Earth that can all see this event they will all have a different angle on the asteroid because they are in different places. From all of their different positions it’s possible to map out the shape of the asteroid based on the timing of when the asteroid blocks out the star. This is one way that we can measure the shape of potatoes from the surface of the earth.

Fraser: Or even if they have moons.

Pamela: Even if they have moons we can see that as well. Other interesting things that have been done was research that was done when Jupiter passed into the same field of view as several background quasars. Work was done to see if we could see how Jupiter’s gravity would bend the light from the quasars. The hope was that we would be able to match the speed of gravity and the results weren’t good enough to do that but it was still a really
neat experiment of being able to see directly that gravity does bend light in all of the different colors.

Fraser: What impact do the planetary motions have on the math for space scientists and for mission planet trying to send spacecraft to these planets? It has got to be mindbendingly complicated.

Pamela: It’s not mindbendingly complicated, folks like newton were able to do it by hand. That’s kind of awesome to think about.

Fraser: Yeah but Newton was very special.

Pamela: He was very special but he didn’t have a computer.

Fraser: Right

Pamela: And there were actually a whole lot of different people that worked to figure this stuff out by hand and that’s how people in the past were able to predict the position of Uranus based on some of the issues that they were seeing. We can see directly how the planets effect one another within our solar system. As mission planners work to calculate the optimal energy saving gravity assists orbits for spacecraft, they have to take into account all of the rocks and gas bodies that are going to get interacted with. I actually saw a science fair student do this without a computer so, yes, it’s mind numbing steps.

Fraser: Yeah it’s not mind boggling, it’s just mind numbing.

Pamela: Yeah that’s the direction I’m going to go. You can do it. Anyone can do it if you sit there long enough, it’s just tedious work.

Fraser: Or you just use the really cool solar system simulator from NASA.

Pamela: Stellarium, that also.

Fraser: There is a great simulator from NASA that lets you see any spacecraft from any location…

Pamela: Eyes on the solar system

Fraser: Eyes on the solar system, yeah, backwards and forwards in time.

Pamela: Doug Ellis’ work.

Fraser: Yeah it’s just fantastic. You can see it’s just running a full on simulation of every object in the solar system and all of their interactions. In the future it’s kind of difficult to map this out far in the future right? The chaos stars to build up.

Pamela: Yes, it’s one of those problems that you have to solve in steps. You figure out where everything is at a certain time interval then you figure out in another time interval where everything is and every one of these different calculations introduces a little bit of uncertainty. That uncertainty builds up over time. That’s why when we talk about the potential of asteroids hitting the earth we deal with probabilities and likelihoods but we don’t deal with certainties. There are errors in our calculations that we have to take into account. When I say error I don’t mean that we make mistakes, I mean that when we make measurements we’re not 100% sure of the shapes and colors of the asteroids and both the shape and the color affects how they’re going to orbit over time.

Fraser: Right and as you try to make these calculations further and further into the future the errors just pile up. The unknowns just pile up to where you have no idea where things are going to be in the future.

Pamela: There are crazy things like if you for instance have something nice and shiny get revealed when a comet melts off part of it’s darker charcoal colored surface, that nice bright place is going to interact with sunlight in a different way than darker surfaces and that’s going to change the orbit. There are so many things to take into consideration. It all just adds up through the millennia.

Fraser: Well thank you very much Pamela and we’ll talk to you next week.

Pamela: It’s been my pleasure, thank you.

This transcript is not an exact match to the audio file. It has been edited for clarity.

One Response to “Ep. 302: Planetary Motion in the Sky”

  1. I love what you guys do, but Pamela gave an erroneous description of planetary retrograde motion, saying it was motion opposite to the apparent diurnal motion of the stars. Prograde motion is actually west to east, and retrograde is east to west, relative to the stars and independent of diurnal motion, since it’s caused by relative orbital motion.

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