Star Struck

Pluto Is Still Not a Planet

Sorry, Pluto lovers. A recently published paper by Jean-Luc Margot of UCLA (as a Star Trek Next Generation fan, I had to give the author’s full name) proposes a mathematically rigorous way to define a planet. Pluto, for all its undeniably fascinating appeal—it just doesn’t make the cut.

The official body tasked with naming and defining astronomical objects is the International Astronomical Union (IAU), of which most people had never heard until 2006. That was when the IAU gave official sanction to what astronomers had known for years, that Pluto was qualitatively distinct from what we now think of as “classical” planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The decision prompted millions of people to mostly good-natured outrage. You mean my fourth-grade teacher lied to me? Why can’t those scientists get their story straight? It didn’t help that the proposed definition was both vague and confusing. Here is the original IAU definition.

“A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.”

Well, that first criterion is simple enough and explains why the moon (which orbits Earth) is not considered a planet. It does exclude any possible planets that orbit other stars, but we’ll get back to that.

The second criterion is hard to judge. How “nearly” round does the planet have to be? No one would argue against calling Jupiter a planet, but its fluid composition and rapid rotation flatten it at the poles and bulge it at the equator. It is certainly not perfectly round! The dashed line in the image below is a perfect circle.

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Posted in Exoplanets, Planets, Solar System Tagged with:

What’s Up?

Two short-lived sky phenomena are on tap in the near future, one taking place only tens of miles above us, the other millions of miles away.

Geminid meteor shower:

Each December we are treated to one of the better meteor showers of the year. The Geminids (so named because they appear to originate from the constellation of Gemini) promise rates as high as one meteor per minute at their peak. They are well situated for early evening viewing, and they come at a time of year when sunset comes very early. Rising at 3 am is not necessary! And this year there will be no moon in the sky to spoil the view. The peak of the shower will be the evening of Sunday, December 13, but there are meteors visible for a week before and after this date.
A few basic reminders are in order. Although meteors belonging to a single shower appear to originate from a specific point called the radiant, you can see them all over the sky. Their apparent paths are a matter of perspective; their paths appear to converge for the same reason parallel railroad tracks appear to converge in the distance.


Although the stuff of meteors (bits of cometary dust and particles generally no bigger than a grain of sand) comes from far away, the incandescent streak that marks their demise is generally between 50 and 75 miles above us. Find a site as far away from artificial lights as you can, lie on a blanket or a reclining lawn chair, and look up. No need for binoculars or a telescope–they are useless for viewing meteors. If you want to look in any particular direction, the radiant will rise above the northeast horizon around 7 pm and gradually move across the southern sky. It reaches its highest point around 2 am, but meteors will be visible all night long.

Happy viewing!

Comet Catalina:

Comets come in two somewhat arbitrary categories: short period ones that complete a single orbit of the sun in less than 200 years, and long period ones that take, well, longer. Comet Catalina falls into the latter category. It comes from a vast region of icy objects far from the sun called the Oort Cloud. Inferred by tracing back the paths of these first-time visitors to the inner solar system but never actually directly observed, the Oort Cloud surrounds the sun in a spherical distribution, with its members not confined to the flat plane occupied by the planets.

Oort-cloud diagram

Occasionally some gravitational perturbation will start one of these iceballs on a long, slow drop into the sun’s gravity well. With orbital periods in the tens of thousands of years, these are one-time visitors for all practical human purposes. They are pristine in the sense that the volatile materials that are frozen solid in the Oort Cloud have never—or at least seldom–been vaporized by a close passage to the sun, and represent well the primordial composition of these 4.5 billion year old relics.

Comet Catalina (its formal designation is C/2013 US10) is typical of a long period comet in that its orbit is highly inclined to the ecliptic plane in which planets orbit, and retrograde—clockwise from above the north pole as opposed to the counter clockwise orbits of all the planets.


Catalina will require binoculars to see it. Right now it is in the early morning sky close to Venus (the brightest object in the east before sunrise). Here is a finder chart for you.


And here is the comet in all its glory.


The ion tail (blue) points directly away from the sun; the dust tail (yellow) trails behind as its heavier and slower-moving components are pushed away from the comet’s nucleus by the solar wind.

Get those binoculars out!

Posted in Sky Phenomena, Solar System Tagged with: ,

Planet Lineup

These beautifully clear fall mornings we’ve been experiencing have offered a rare opportunity to see four of the five naked-eye planets all lined up for our viewing pleasure. This morning at 6:45 am EDT, this was the view on Lynchburg’s eastern horizon. The almost vertical blue line is the ecliptic, the plane of the Earth’s orbit around the sun. Since all of the planets orbit the sun in very nearly the same plane, all of them will appear near this line in the sky.

October 18 2015

So where is Saturn, the only one missing? Currently it is on the other side of the sun as we view it, rising about 10:30 this morning, invisible without a telescope in the daytime sky, and best viewed shortly after sunset in the western sky.

The ancients spoke of seven planets: the sun, the moon, Mercury, Venus, Mars, Jupiter, and Saturn. Today we don’t consider the sun and the moon to be in the same category as the other five, but the word planet actually means wanderer. These seven objects did not stay put in the sky! Unlike the well-behaved fixed stars, these celestial objects moved across that stellar background. We know now that this is because they are so much closer to us than the stars. The stars do in fact move, but their great distance makes that motion difficult to detect over a human lifetime.

Have you ever wondered why we have seven days in a week–why this arbitrary number and not some other? Seven planets, seven days. Some of our day names reveal their origin in English: Sunday, Monday, Saturday. Others are more apparent in other languages such as French: Mardi (Tuesday), Mercredi (Wednesday), Jeudi (Thursday), and Vendredi (Friday). We can all be grateful that Uranus was not discovered until the era of the telescope, thereby sparing us from decades of middle school jokes.

Posted in Sky Phenomena, Solar System

Space Station Pass in Lynchburg Area

It looks as though Lynchburg area sky watchers will actually have clear skies for a celestial event! The International Space Station will be visible tonight in an especially bright and high-in-the-sky pass. It will appear low in the southwest at 7:13:33 pm EDT, reach its highest altitude of 72° in the northwest at 7:16:47, and disappear above the northeast horizon at 7:20:02. For those of us who still remember a time when the only satellite of Earth was the moon, this is an event we never take for granted.

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Total Lunar Eclipse

It seems as though Lynchburg’s record of clouding over for interesting celestial events is going to hold true for this weekend’s total lunar eclipse. But just in case all the forecasters are wrong, and for those of you who live where clear skies reign, here is the relevant information.

First, just the basic information concerning timing, then more details for those of us who like that sort of thing. You may have seen information giving the date of the eclipse as September 28, but for observers in North America it will occur late in the evening of Sunday, September 27. All times given are EDT.

  • Moon enters Earth’s umbral shadow; you will begin to see a dark shadow creeping across the moons face: 9:07 pm
  • Totality begins; the moon is fully within Earth’s umbral shadow: 10:11 pm
  • End of totality; moon begins to exit umbral shadow: 11:23 pm
  • Moon is completely out of umbral shadow: 12:27 am on September 28.

Or more succinctly, the total eclipse lasts from 10:11 pm EDT to 11:23 EDT on Sunday night, September 27.

So what’s up with this umbral shadow thing? Remember first of all how a lunar eclipse occurs: Earth comes between the sun and the moon and casts its shadow on the moon.

03_Lunar Eclipse

Now imagine yourself standing on the moon. If the moon and you are within the umbra, Earth completely blocks the sun. Within the penumbra however, only some of the sun’s disk is obscured by Earth. So while the Sun’s light is completely obscured within the umbra, it is only partially obscured within the penumbra. Totality occurs only when the moon is completely within the umbral shadow.

So the moon ought to be completely dark during totality, right? As anyone who has seen a lunar eclipse can tell you, it isn’t. This is more like it.

If the earth were an 8000-mile diameter ball with no atmosphere, the moon would indeed be dark. But our atmosphere bends (or refracts) the sunlight passing through it, and the light that is bent least is long wavelength red or orange light.

The darkest eclipses occur when the moon passes through the exact center of Earth’s umbral shadow, which is seldom the case. For this eclipse, it passes nearer the edge.

September 27 lunar eclipse

And the parts of the Earth’s surface where it will be visible?

September 27 lunar eclipse visiblity

The next total lunar eclipse with this optimal visibility for Americans will not be until 2019. But well before then, we will have a total solar eclipse in August 2017 that will reach its peak in mid-America where Missouri, Kentucky, and Tennessee come together.

You will not want to miss this—experiencing a total solar eclipse is something that should be on everyone’s bucket list. Witness the excitement in this video taken aboard the cruise ship where my wife and I witnessed the solar eclipse of July 2009. It is an amazing natural phenomenon!  And yes, I was whooping and shouting along with everyone else.

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New Horizons Update

If you’ve been wondering why you haven’t seen any more images from the Pluto flyby lately, here is the reason:


But there is exciting news from the New Horizons team! They have selected a tentative target among Kuiper Belt Objects (KBOs, distant and icy solar system bodies of which Pluto is only one of the larger representatives) for a future flyby. The candidate for a close up look has the prosaic name of 2014 MU69, a name which reveals its order of discovery among similar objects. It’s enough to know that it was discovered only last year.
This KBO is especially intriguing because it is believed to have formed in situ, where it is orbiting now, meaning that it should be essentially unaltered since its creation 4.5 billion years ago.
It is also more easily accessible than other possibilities. The image below shows the relative positions of the outer planets (Jupiter, Saturn, Uranus, and Neptune), Pluto, and 2014 MU69, both on the date of the Pluto flyby in July 2015 and on the date of the anticipated 2014 MU69 flyby in January 2019.

New Horizons

You may notice that in the 3 ½ year interval between these two flybys, only Jupiter and Saturn have noticeably changed their positions. Pluto and 2014 MU69 have barely moved at all! Objects orbiting this far from the sun move much more slowly than do, for example, the inner planets. While Earth’s average orbital speed around the sun is 30 kilometers per second (19 miles per second), that of Pluto is only 6.1 km/s (4.7 mi/s).
Keep in mind that this rendezvous date is only approximate. As the spacecraft draws nearer, its path may be refined as the target is better resolved, and the time may shift slightly. Indeed, one of the attractions of this target is that very little fuel expenditure should be required to come very near it. When you are more than 6 billion km (4 billion miles) and 6 hours in radio transmission time away from home, you have to conserve your resources!

Posted in Solar System, Spacecraft Tagged with: ,


Over the next 16 months, the data collected during the New Horizons spacecraft’s mid-July flyby of Pluto will find its way back to Earth, a few bits at a time. Already there are surprises in the data so far received. How could there not be? We are seeing a whole new world revealed in detail for the first time in human history. The analyses and debates will surely only have begun once all the data are in.
The image below on the left is from the Hubble Space Telescope, and until July 2015 it was the best we had. The true-color image on the right is from the New Horizons flyby.
Pluto before and afterI thought it might be useful to write on what we have learned about the small icy worlds of the solar system from our previous explorations. Perhaps a more appropriately humble description would be that this is what we think we know! Each new world forces us to re-examine our assumptions, each new question begets ten more. Such is the nature of science, and the source of its endless fascination for so many of us.
Worlds of Rock, Gas, and Ice
Leaving out a lot of detail, we can think of solar system bodies as being composed of rock, gas, ice, or some combination thereof. The four innermost planets (Mercury, Venus, Earth, and Mars) are relatively small and rocky, and the four outermost (Jupiter, Saturn, Uranus, and Neptune) are mostly gaseous, with ice becoming more prominent as we reach the more distant environs of Uranus and Neptune.
800px-Size_planets_comparisonAmong the inner planets, Mars has two tiny rocky moons that may be captured asteroids, and the Earth has a rocky moon that is likely the result of a massive collision early in its history. Neither Mercury nor Venus has a moon.
The inner solar system has numerous smaller bodies collectively called asteroids that are mostly rocky. The greatest numbers of these lie between Mars and Jupiter, or co-orbiting ahead of and behind Jupiter in its orbit. Icy bodies wander into the inner solar system from farther out, spewing gas and dust tails as they are warmed by the sun. These are the comets that periodically grace our skies.

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Posted in Planets, Solar System, Spacecraft Tagged with: ,

The Great Age of Planetary Exploration

How do you measure the milestones in your life? We all have events that help us mark the passage of years: school and jobs, marriage and family, births and deaths. These are of course primary in all our lives, but the recent encounter of the New Horizons spacecraft with the Pluto system has reminded me of the very special period of time that my own life has spanned. In the last 65 years, we have slipped the bonds of gravity that held us to the Earth and completed a survey of our solar system. Nothing comparable has happened since the great age of Earth’s exploration begun 500 years ago; nothing comparable is likely to happen for decades or centuries to come.

I was barely three weeks old when the first rocket was launched from Cape Canaveral, Florida in July 1950.

July 1950 Cape Canaveral
I was fifteen years old and completely obsessed with space when Mariner 4 flew by Mars in July 1965 and sent back the very first pictures of another planet.
Mars had no canals! Instead, it had moon-like craters and a disappointingly desolate appearance. As it turned out, Mariner 4’s flyby path took it over some of the least interesting of Mars’ topography. It fell to Mariner 9, which settled into orbit around the planet late in 1971 and patiently waited while global dust storms abated, to discover the solar system’s largest (extinct) volcano and a chasm that could swallow Earth’s Grand Canyon in one gulp.

But perhaps the greatest planetary explorers of all time were the twin Voyager spacecraft launched in 1977. Both Voyager 1 and Voyager 2 explored Jupiter and Saturn and their moons, while Voyager 2 was directed on to Uranus and Neptune. The images that the latter craft took of these giant planets are the only close-ups we are likely to see in our lifetimes.

And now, 50 years to the day after those first grainy black-and-white images from Mars that so astounded us, we have our first images of Pluto, taking four and a half hours to travel over 3 billion miles, and providing our first clues to new mysteries of planetary formation that await us.

It is a remarkable time to be alive. I can’t help but wonder what it must have been like for people greeting Captain Cook and Joseph Banks on the English docks in 1771 as they returned with stories of exotic Pacific Islands and hundreds of miles long coral reefs. How their world must have been enlarged by this new knowledge!

The world is much larger than our own dust mote of a planet, larger even than the system of planets that circles our life-giving star. But we have completed an important first round of exploration of the vastness that surrounds us.

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Nine Years to the Ninth Planet

Yes, you read that right. In honor of Pluto’s imminent debut on computer screens worldwide, I’ll award it planetary status for the next few months. After all, when the New Horizons spacecraft nearing Pluto now was launched in January 2006, the (in)famous demotion to dwarf planet had not yet occurred.

If you are an aficionado of rocket launches, you will note that the New Horizons booster rocket fairly leaped off the launch pad. In fact, this spacecraft left the Earth faster than any other in history, passing the moon’s orbit in nine hours and reaching its encounter with Jupiter in less than fourteen months.

The Jupiter fly-by not only redirected New Horizons’ trajectory, it sped the spacecraft up by “stealing” a little energy from the massive planet in what is sometimes called a gravity assist maneuver. The mass differential between the spacecraft and the planet means that Jupiter will never miss that stolen energy. Without this maneuver, it would have taken even longer to reach this outer region of our solar system.
So why was every third-grader’s favorite planet demoted, anyway? Rather than the last planetary outpost, Pluto is better regarded as the threshold of a different region, the Kuiper Belt. These are icy remnants of the primordial solar system, so distant and small that their very existence was not confirmed until 1992. As more and more of them were discovered, the realization dawned that Pluto was just the nearest and one of the largest of these objects. Their remoteness makes knowledge of their detailed nature not much beyond speculation. But Pluto is about to help us better understand what they are about. As unaltered specimens dating to the very earliest days of the solar system, they should help us better understand how it evolved over its 4.5 billion year history.
Closest approach to Pluto is at 7:50 am EDT on Tuesday, July 14th. It will take some time to transmit all the data collected back to Earth. Pluto is so distant that it takes the radio signal four and a half hours to reach us. The spacecraft will be busy as it whizzes by at almost nine miles a second.

The time given are UTC (Universal Coordinated Time) on July 14th; Eastern Daylight Time is four hours earlier.
And finally, here are the latest (as of July 2nd) pictures of Pluto and its largest moon Charon, each showing a different hemisphere of Pluto. These will quickly be outdated as an entire new world is unveiled for the first time in all human history.

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Venus, Jupiter…and Earth

Those of us of a certain age will remember these song lyrics:

When the moon is in the seventh house
And Jupiter aligns with Mars…

Jupiter is about to align not with Mars, but with Venus, the ancient goddess of love. We can only hope that peace will guide the planets as a result!

The commonly used term for this is a conjunction, when two celestial objects appear very near each other in the sky. Strictly speaking, this is incorrect; the astronomical term applies to two objects that share only one of the two dimensions used to map objects onto the sky. Imagine Memphis and New Orleans. They share the same longitude (roughly 90 degrees west), but Memphis lies about 5 degrees of latitude north of the Big Easy. We won’t split hairs here, however.
Here is a series of screen shots showing how the two planets will move ever closer to each other over the next few nights. Each of these shows the view from Lynchburg, Virginia at 9:00 PM EDT.

Venus Jupiter Conjunction
The exact moment of closest approach is at 10:55 PM EDT on June 30.

Their proximity is only apparent, however, not real. I can line up my finger with a distant mountain peak without having climbed the mountain! This view from above the plane of the solar system illustrates the point.

Venus Jupiter Conjunction Overhead
Enjoy the view! If you forget which of the two planets is which, Venus is by far the brighter of the two.


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