365daysDate: September 11, 2009

Title: Pluto, Stand Thou Still!


Podcaster: Larry Sessions

Organization: None

Description: Pluto, the only object ever to be demoted from full planet status, is stationary today, even though it currently is traveling at about 5 kilometers per second! It never stops, so how can it be “stationary?” Motion – or lack thereof – can be real or illusory, depending on what you are using as a reference. Pluto is so far from Earth and so dim that it was unknown in ancient times, but early astronomers could see when closer planets such as Mars and Jupiter became “stationary.” It confused them, and led to a “revolution” in science.

Bio: Larry Sessions is a former director and staff astronomer at Denver’s Gates and Fort Worth’s Noble planetariums, and now is an instructor for Metropolitan State College and the Community College of Aurora, Colorado. He also is the webmaster and editor for the Southwestern Association of Planetariums, as well as his own website, North American Skies, and a contributor to both and A NASA/JPL “Solar System Ambassador,” he has every copy of the Royal Astronomical Society’s annual handbook since 1971.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Drew Roman.


Pluto, Stand Thou Still!

Welcome to this edition of the 365 Days of Astronomy podcast. I’m Larry Sessions, former planetarium director and now an astronomy instructor in Denver. I also write a little, occasionally lecture at Gates Planetarium, produce the website North American Skies, tweet frequently at NASkies, blog on the EarthSky website, and serve as a NASA/JPL Solar System Ambassador. When I can, I sleep.

On September 11, in this International Year of Astronomy, the dwarf planet Pluto is said to be “stationary.” Now this is an odd use of the term, as in fact Pluto is never truly motionless, so I thought I would try to clarify.

There are lots of astronomical oddities in ancient literature, some requiring the intervention of a Deity, others just misinterpretation of natural events, and others pure fiction. For the Sun and Moon to stand still, as Joshua commanded in the Bible, may have been a divine miracle, but from an astronomical standpoint, it is, of course, impossible. Not only is it impossible, but the phrase can’t in any way mean what it was intended to mean. It betrays a false belief that daily motions of the Sun and Moon are the result of their orbiting of our planet. Certainly this was the prominent belief in Joshua’s time – common sense in fact. But the modern understanding – formulated and fortified by Copernicus, Galileo, and Kepler – is that the daily motion of heavenly bodies is due not to their own motion, but to the Earth’s rotation. This is no longer theory or opinion, but demonstrated and documented physical fact.

Yes, of course the Moon does orbit the Earth, but it is the Earth’s own spinning that accounts for most of the Moon’s apparent daily motion. And even if the Earth did not rotate, the Sun would slowly creep across the sky, but again not due to the Sun’s own motion through space, but due to Earth’s orbital motion.

In order for the Sun and Moon literally to “stand still” as Joshua commanded, the Earth would have had to stop suddenly in its rotation, and the other object would have to stop as well. If indeed the Earth had stopped rotating in the matter of a few seconds, it would have caused mass destruction, evidence of which would last to this day.

“Nevertheless, it moves” as Galileo so famously muttered as he was led away to house arrest. The Earth moves and it certainly is not in the power of humans to stop it. The Sun, Moon, planets, and every other astronomical object moves. They never stop, and yet even astronomy itself persists, oddly, in referring to the Sun standing still, or a planet becoming “stationary.” As mentioned before, on September 11, 2009, the dwarf planet Pluto is “stationary.” Admittedly, in modern astronomy, this is a fairly unimportant event, and not at all how it sounds. Astrologers, casting their predictions and mystical prognostications, seem to be more interested in such events.

There are a number of other odd or archaic usages of words in astronomy. As of about a hundred years ago, astronomers had come to realize that the vast bulk of the Sun was composed of hydrogen and helium, the first two and lightest elements. In fact helium, which comes from a Greek word meaning “Sun,” was discovered on the Sun before it was ever found on Earth. Helium is important in a number of industrial processes, and of course is the main component in making party balloons float and voices squawk like Donald Duck. In fact it is so important these days that it has its own monument at the Don Harrington Discovery Center in Amarillo, where a former volunteer assistant of mine, Chip Lindsey, is assistant director. (Yea, Chip!)

Although hydrogen and helium were known to comprise almost 99 percent of the Sun, there are many other elements in the Sun, such as nitrogen and oxygen, the principle components of Earth’s atmosphere. We are accustomed to thinking that gases in our atmosphere are “light” elements, but oddly, astronomers decided to call any element more complicated and massive than helium, a “heavy element” or a “metal.” So not only are nitrogen and oxygen called “metals,” but so are other so-called gaseous elements such as Fluorine, Chlorine, and Argon. Even solids such as sodium, calcium, sulfur, and carbon are “metals” in astronomy.

Maybe that’s not quite so strange after all, because scientists know that hydrogen, when greatly compressed, does indeed take on the electrical properties of a metal. But it doesn’t stop there, because astronomy is a science where the terms “red giants” and “old degenerate white dwarfs” don’t represent freak show attractions and are not as derogatory as they might seem to the non-astronomically inclined.

Then there are other word usages that might not mean what you first think. Think of a “red hot” deal or “cool blue” ice. In ordinary usage, we think of red as signifying “hot” and blue as signifying “cold.” A fireplace poker or electric burner turns read when it is hot, but that’s just because we don’t normally heat them up very far. And ice does sometimes tend to have a blue cast, especially in polar regions. But in physics and astronomy, it is just the opposite. Red stars are the “coolest” (if you can call several thousand degrees “cool”) and blue stars are the hottest, with temperatures in the range of 50,000 degrees F.

There are other examples of differences between our ordinary use of words and their meanings in astronomy and other sciences. But I’ve gotten off track here. Let’s get back to the question at hand. So, what exactly do I mean when I say an object in space is “stationary?”

There are a couple of meanings. As mentioned before, we sometimes consider the Sun to have reached a “stationary” point, which is called a “solstice,” from Greek meaning “Sun standing still.” This signifies the beginning of Summer or Winter. Because the Earth is tilted on its axis, during each orbit we are alternately tilted toward or away from the Sun, affecting where the Sun appears in the sky. From the first day of Winter to the first day of Summer, the Sun progressively appears higher and higher in the mid-day sky. But on the first of Summer, it has reached its highest point and thereafter gets lower and lower in the mid-day sky each day. At the point at which it starts getting lower instead of higher each day, the Sun is said to be “stationary” and we refer to this as a “solstice.” This is a mathematical instant in time, and says nothing about the Sun’s actual motion. More precisely, it defines a specific point in the Earth’s orbit. The same happens in reverse as we go from the first of Summer to the first of Winter, and the lowest point is said to be the solstice.

But what about dwarf planet Pluto – or any other planet? We could define it in the same way, but rather than giving it the name “solstice” we would refer to the object as having it’s “greatest northern or southern declination.” The term “stationary” has a special, if still confusing, meaning in planetary astronomy.

Think of the solar system as a roughly circular racetrack. Planetary orbits are elliptical, not circular, but it is OK to consider them circular for now. The planets that are closer to the Sun move faster than those that are farther away. Thus the inner planets have a shorter distance to go and in fact move faster than the outer planets. As an example, it takes the Earth one year to orbit the Sun, but Jupiter, 5 times farther away, takes about 12 years. Pluto, much farther still, takes about 248 years.

Back to our racetrack analogy. The Earth, moving faster, laps each of the outer planets and dwarf planets much like a car in a faster inner lane can lap slower cars in the outer lane. The geometry is such that shortly before the Earth catches up to and passes the outer object, that object as viewed from Earth appears to slow down, stop and then for a short while, reverse its apparent direction in the sky. This geometric illusion is known as “retrograde motion.”

Ancients believed that the Earth was the center of all motion and that the Sun, Moon, and planets all orbited Earth. However, this “retrograde,” or backwards, motion was difficult to explain if the Earth is the center of everything. The ancients at first thought it was a real motion and then realized that it was likely a problem in spacial geometry. The real solution didn’t come until astronomers proved that the other planets do not orbit the Earth, but rather the Sun.

After we notice an object is in “retrograde motion,” the Earth moves on in its orbit, and the situation eventually returns to normal as the planet appears again to slow down, stop, and then resume its “normal,” or forward, motion. The two points at which the planet or other object appears to reverse direction are called “stationary” points. Once again, it is an illusion as nothing really stops.

In 2009, Pluto became “stationary” on April 4 and subsequently entered retrograde motion. It is stationary again on September 11, after which it returns to its normal eastward motion among the stars. Mars, Jupiter, Saturn, Uranus, and Neptune also go through stationary points and retrograde motion, and also have the advantage of being considerably easier to see!

In a planetarium we can turn a dial and make Pluto, or any other celestial object, stand still. But in real life, as in the planetarium, it’s just an illusion. OK, class dismissed for today!

Thanks for listening.

End of podcast:

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at or email us at Until tomorrow…goodbye.

Subscribe To Our Newsletter

Subscribe To Our Newsletter

Join our mailing list to receive the latest news, show schedules, and updates from our team.

You have Successfully Subscribed!