Meteor Astronomy Terms & Definitions

IAU Commission F1 (Meteors, Meteorites, and Interplanetary Dust) officially approved some terms and definitions in meteor astronomy last year.  This is a revision of the terms and definitions that were approved in 1961.  Meteor astronomy knowledge has grown by leaps and bounds since then.

A solid natural object of a size roughly1 between 30 micrometers and 1 meter moving in, or coming from, interplanetary space
The light and associated physical phenomena (heat, shock, ionization) which results from the high speed entry of a solid object from space into a gaseous atmosphere
Any natural solid object that survived the meteor phase in a gaseous atmosphere without being completely vaporized

1“Roughly”, because the 1 meter size limit is not a physical boundary; it is set by agreement. There is a continuous population of bodies both smaller and larger than 1 meter. Bodies larger than 1 meter tend to be dominated by asteroidal debris, rather than debris from comets.  “Roughly”, also because the 30 micrometer size limit is not a physical boundary; it is set by agreement.  There is a continuous population of bodies both smaller and larger than 30 micrometers.  Bodies smaller than 30 micrometers, however, tend to radiate heat away well and not vaporize during an atmospheric entry.

“Small dust particles do not give rise to the meteor phenomenon when they enter planetary atmospheres.  Being heated below the melting point, they sediment to the ground more or less unaffected.”

“When collected in the atmosphere, they are called interplanetary dust particles (IDPs).  When in interplanetary space, they are simply called dust particles.  The term micrometeoroid is discouraged.”

Looking at the definition for meteorite above, what about meteoroids that reach the surface of a world with little or no atmosphere, such as the Moon?  The IAU Commission has a less-than-satisfying answer (to this writer, at least).

“Foreign objects on the surfaces of atmosphereless bodies are not called meteorites (i.e. there is no meteorite without a meteor).  They can be called impact debris.”

What’s the harm in calling any meteoroid that reaches the surface of a planetary body (planet, moon, asteroid, etc.) a meteorite?  To me, “impact debris” implies material pre-existing on the planetary body that is excavated by an impact event.

“In the context of meteor observations, any object causing a meteor can be termed a meteoroid, irrespective of size.”

“A meteoroid in the atmosphere becomes a meteorite after the ablation stops and the object continues on dark flight to the ground.”

“A meteorite smaller than 1 millimeter can be called a micrometeorite.  Micrometeorites do not have the typical structure of a fresh meteorite—unaffected interior and fusion crust.”

Meteor stream is a group of meteoroids which have similar orbits and a common origin.  Meteor shower is a group of meteors produced by meteoroids of the same meteoroid stream.”

Dust (interplanetary)
Finely divided solid matter, with particle sizes in general smaller than meteoroids, moving in, or coming from, interplanetary space.
“Dust in the solar system is observed e.g. as the zodiacal dust cloud, including zodiacal dust bands, and cometary dust tails.  In such contexts the term ‘dust’ is not reserved for solid matter smaller than about 30 micron; the zodiacal dust cloud and cometary dust trails contain larger particles that can also be called meteoroids.”
For consistency with the rest of the document, micron in the above paragraph should be micrometers.
Meteoric smoke
Solid matter that has condensed in a gaseous atmosphere from material vaporized during the meteor phase.

“The size of meteoric smoke particles (MSPs) is typically in the sub-100 nm range.”

“Meteors can occur on any planet or moon having a sufficiently dense atmosphere.”

“A meteor brighter than absolute visual magnitude (distance of 100 km) -4 is also termed a bolide or a fireball.”

The fireball definition makes sense, but it was always my understanding that a bolide is accompanied (later) by audible sound and is thus much rarer.

“A meteor brighter than absolute visual magnitude -17 is also called a superbolide.”

Meteor train is light or ionization left along the trajectory of the meteor after the meteor has passed.”

“Small (typically micron-size) non-vaporized remnants of ablating meteoroids can be called meteoritic dust.  They can be observed e.g. as dust trails in the atmosphere after the passage of a bolide.”

Again, for consistency with the rest of the document, micron-size in the above paragraph should be micrometer-size.

“The radiation phenomenon accompanying a direct meteoroid hit of the surface of a body without an atmosphere is not called a meteor but an impact flash.”

Koschny, D., & Borovička, J. 2017, WGN, The Journal of the IMO, 45,5

Illumination Levels: Then and Now

The following excerpts are from the 1911 and 1925 editions of A Text-Book of Physics by Louis Bevier Spinney, Professor of Physics and Illuminating Engineering at Iowa State College (now Iowa State University) in Ames, Iowa.

From the 1911 edition…


516. The intensity of illumination of any surface is defined as the ratio of the light received by the surface to the area of the surface upon which the light falls.  A unit of intensity which is oftentimes employed is known as the foot candle, and is defined as the intensity of illumination which would be present upon a screen placed at a distance of one foot from a standard candle.  The meter candle is a unit of intensity which is employed to some extent.

The table below gives a number of values of illumination such as are commonly observed, the intensity of illumination being expressed in foot candles.

Suitable for drafting table    .    .    .    .    .    5 to 10

Suitable for library table   .    .    .    .    .   .    3 to 4

Suitable for reading table   .    .    .    .    .   .  1 to 2

Required for street lighting   .    .    .    .    .  0.05 to 0.60

Moonlight (full moon)    .    .    .    .    .    .   .  0.025 to 0.03


And from the 1925 edition…


532.  The eye has a remarkable power of adaptation.  In strong light the pupil contracts and in weak light expands, so that we are able to use our eyes throughout a range of illumination which is really quite astonishing.  However, the continued use of the eyes under conditions of unfavorable illumination causes discomfort, fatigue, and even permanent injury.  Experiment and experience show that eye comfort, efficiency, and health considerations demand for each kind of eye work a certain minimum illumination.  Some of these illumination values taken from tables recently compiled are given below.


Streets    .    .    .    .    .    .    .    .    .     .    .    .    .    .    1/20 to 1/4

Living rooms; Halls and passageways    .    .    1 to 2

Auditoriums; Stairways and exits;
Machine shops, rough work    .     .    .    .    .    .  2 to 5

Classrooms; Laboratories; Offices;
Libraries; Machine shops, close work    .    .    5 to 10

Engraving; Fine repairing work; Drafting;
Sewing and weaving, dark goods  .    .    .    .     10 to 20


By comparing the 1911 and 1925 data with the illumination levels recommended today by IESNA, we can see that recommended light levels for streetlighting have increased anywhere from 40% to 380% since 1925.  A cynic might say that we need more light than our ancestors did to see well at night.  As you may have noticed, light levels have been steadily creeping upward, everywhere, over the last few decades.

Recommended Illumination Levels for Streetlighting

Year        Minimum    Average    Maximum

1911             0.05                ???               0.60

1925           0.05               0.25               ???

1996          0.07                1.20               ???

Have you ever noticed how well you can see at night when the full moon is lighting the ground?  The full moon provides surprisingly adequate non-glaring and uniform illumination at just 0.03 footcandles!  For inspiration, take a look at the following text from an Ames, Iowa city ordinance, dated July 8, 1895:

“The said grantees shall keep said lamps in good condition and repair, and have the same lighted every night in the year from dark until midnight, and from 5:00 a.m. until daylight, except such moonlight nights or fractions of the same as are not obscured by clouds, and as afford sufficient natural light to light the streets of said city.”

This was originally published as IDA Information Sheet 114 in November 1996, and authored by David Oesper.

Obsolete But Still Relevant

Under the direction of Friedrich Argelander (1799-1875), astronomers at the Bonn Observatory spent seven years (1852 to 1859) measuring the positions and magnitudes of roughly 324,000 stars, one star at a time.  This phenomenal work resulted in the Bonner Durchmusterung (BD) catalog and atlas, which included stars down to approximately magnitude 9.5 and is a tribute to the foresight of Argelander and the diligence of his small staff.  The Bonner Durchmusterung was the last star catalog to be produced without the benefit of photography, and it is certainly the most comprehensive of the pre-photographic atlases.

Back in 2007, Alan MacRobert stated (Sky & Telescope, July 2007, p. 59), “Someday machines will measure the brightness of every star in the sky to some amazingly deep magnitude many times a night, and blind software will compile and analyze light curves automatically.”  No doubt, he is correct, but he does add that this has not happened yet, despite years of pregnant expectations.

But we are getting closer to that day, with the Large Synoptic Survey Telescope (LSST) scheduled to come online in 2022 and many other similar survey instruments in the pipeline or already operational.  That is one reason as an amateur astronomer with limited resources (including time) I focus on observing the occultation of stars by asteroids and trans-Neptunian objects.  It is one of the few areas where an amateur observational astronomer can provide location-dependent observations.  You are either in the shadow path or you are not.  Though truth be told I would rather be studying exoplanets, we can only do what we have the resources to do—regardless of talent or potential.

History is full of examples of skills and techniques made obsolete almost overnight by new technologies (or a different point of view), but what is seldom recorded is the sense of desolation and indeed mortality experienced by those unfortunate enough to live to see that their highly-developed skills are no longer wanted or needed.  As my meteor-watching friend Paul Martsching has said, “It is good we don’t live forever: we are a product of our times.”  He realizes full well that someday automated systems will count every meteor above the horizon far better and more completely than any visual meteor observer can, but for many years he has carefully recorded meteor activity many nights a year.  The data he collects will always be relevant as part of the historical record, at least, and the sheer joy of being out under the stars and away from light pollution can never be replaced by a computer.  To us, astronomy is something much deeper than what can be delivered through a computer screen.

We are a product of our times, and as we approach the twilight (or autumn) of our lives we don’t necessarily feel compelled to embrace every new thing that comes along.  Peace.

From the standpoint of daily life, however, there is one thing we do know: that we are here for the sake of each other—above all for those upon whose smile and well-being our own happiness depends, and also for the countless unknown souls with whose fate we are connected by a bond of sympathy.  Many times a day I realize how much my own outer and inner life is built upon the labors of my fellow men, both living and dead, and how earnestly I must exert myself in order to give in return as much as I have received. – Albert Einstein (1879-1955)

What is a Vacuum?

A vacuum is not nothing.    It is only a region of three-dimensional space that is entirely devoid of matter, entirely devoid of particles.

The best laboratory vacuum contains about 25 particles (molecules, atoms) per cubic centimeter (cm3).

The atmosphere on the surface of the Moon (if you can call it that) contains a lot more particles than the best laboratory vacuum: about 40,000 particles per cm3.  This extremely tenuous lunar atmosphere is mostly made up of the “noble” gases argon, helium, and neon.

The vacuum of interplanetary space contains about 11 particles per cm3.

The vacuum of interstellar space contains about 1 particle per cm3.

The vacuum of intergalactic space contains about 10-6 particles per cm3.  That’s just 10 particles per cubic meter of space.

But what if we could remove all of the particles in a parcel of space?  And somehow shield that empty parcel of space from any external electromagnetic fields?  What would we have then?

It appears that even completely empty space has some inherent energy associated with it.  The vacuum is constantly “seething” with electromagnetic waves of all possible wavelengths, popping into and out of existence on unimaginably short time scales—allowed by Heisenberg’s energy-time uncertainly principle.  These “quantum flourishes” may be a intrinsic property of space—as is dark energy.  Dark matter, on the other hand, is some weird form of matter that exists within space, exerting gravitational influence but not interacting with normal matter or electromagnetic waves in any other way.

Is there any evidence of this vacuum energy, or is it all theoretical?  There are at least three phenomena that point to the intrinsic energy of empty space.  (1) The Casimir effect; (2) Spontaneous emission; and (3) The Lamb shift.

The Casimir effect
Take two uncharged conductive plates and put them very close to each other, just a few nanometers apart.  Only the shortest wavelengths will be able to exist between the plates, but all wavelengths will exist on the other side of the two plates.  Under normal circumstances, this will cause a net force or pressure that pushes the two plates towards one another.

Spontaneous emission
An example of spontaneous emission is an electron transitioning from an excited state to the ground state, emitting a photon.  What causes this transition to occur when it does?

The Lamb shift
The Lamb shift is a tiny shift in the energy levels of electrons in hydrogen and other atoms that can’t be explained without considering the interaction of the atom with “empty” space.

Reucroft, S. and Swain, J., “What is the Casimir effect?”, Scientific American,  Accessed 20 Feb 2018.

Koks,D. and Gibbs, P., “What is the Casimir effect?”,  Accessed 20 Feb 2018.


Brahms – Symphony No. 1

If I had to pick a favorite symphony—and that would be difficult to do as I love so many—then it would have to be Symphony No. 1 by Johannes Brahms.  Though he completed it in 1876 at the age of 43, he had been working on it for something like 21 years.  He was a consummate perfectionist, and it shows.

The Madison Symphony Orchestra performed this extraordinary work this past weekend as the second half of a really fine program featuring Alban Gerhardt  playing the Walton Cello Concerto, and Rossini’s Overture to Semiramide.  We are so very fortunate to have an orchestra of this caliber in southern Wisconsin, and music director John DeMain is a treasure.  I am a season subscriber, of course, and attend all the concerts except for the Christmas program in December.

Johannes Brahms in 1876

I cannot get through a performance of the Brahms First Symphony without being moved to tears, and Sunday’s excellent performance by the MSO was no exception.  The final section of the second movement (Andante sostenuto) features an incredibly beautiful violin solo, gorgeously played by concertmaster Naha Greenholtz.  The fourth and final movement (Adagio — Più andante — Allegro non troppo, ma con brio — Più allegro) is pure ecstasy.  Just when you think the symphony is drawing to a conclusion, it launches into another, even more remarkable, section.  And that happens more than once.  The modulating transition to the coda in measures 367-390 (about 15:42 to 16:24 into the movement, two minutes before the end) for me is one of the most exciting sections of the entire work.

I once asked my friend and accomplished horn player John Wunderlin—who is similarly deeply moved by orchestral music—how he keeps from choking up during the most moving passages he plays.  “Fear of messing up” he said, half jokingly and half serious.  Part of the discipline that any professional musician must have is maintaining composure  during even the most moving and beautiful sections.  I don’t think I could do it.  But I did once see a teary-eyed violinist in the orchestra at the conclusion of a work.  Want to know what that work was?  It was the Symphony No. 1 by Johannes Brahms.

Shostakovich – Symphony No. 4

The Fourth Symphony of Dmitri Shostakovich (1906-1975) was completed in May 1936, but had to be withdrawn before it was performed due to the withering criticism and scrutiny Shostakovich was at the time receiving from Joseph Stalin and his increasingly repressive government.  This symphony did not receive its first public performance until 1961.  To get a sense of the enormous difficulties Shostakovich had to endure under the Soviet regime—and the extraordinary music of one of the 20th century’s most gifted composers, and indeed the last great symphonist—I highly recommend Robert Greenberg’s eight-part video course, Great Masters: Shostakovich – His Life and Music.

Dmitri Dmitriyevich Shostakovich

The Fourth Symphony is certainly not one of Shostakovich’s more accessible works, but I want to draw your attention to the remarkable, ethereal conclusion of this symphony that few have ever heard.

My entire Shostakovich collection was lost in the Memorial Day weekend 2015 Houston flood, and I’m gradually trying to replace it.  I am currently listening to all fifteen Shostakovich symphonies in an excellent box set, conducted by Mstislav Rostropovich (1927-2007).  Rostropovich was a close friend of Shostakovich.

Here is the final 4m45s of the third and final movement (Largo — Allegro) of the Symphony No. 4 in C minor, op. 43, by Dmitri Shostakovich, performed by the National Symphony Orchestra conducted by Mstislav Rostropovich.  Turn up the volume—after the first couple of seconds, it is all very quiet.  Enjoy!

Scott of the Antarctic

I highly recommend the 1948 British film, Scott of the Antarctic.  It tells the story of Captain Robert Falcon Scott’s ill-fated attempt to lead the first team of explorers to the South Pole.  Once again, Amazon has bested Netflix in making fine historical movies like this one available.

The film score was written by the esteemed British composer Ralph Vaughan Williams (1872-1958).  This project served as a springboard for his remarkable and otherworldly Symphony No. 7, Sinfonia Antartica, completed in 1952.  It is a favorite of mine.

As I have written here before, it is good to see a film that communicates effectively without the need to resort to graphic violence, foul language, etc.  You can feel the dreadful cold viscerally watching this film.  Near the end of their journey, Scott and his team in March 1912 regularly experienced high temperatures no better than -30°F during the day and low temperatures around -47°F at night.  And then there was the wind.  It would have been horrible.

One question I had while watching the movie and thinking about the real-life expedition: how did they navigate across an endless terrain of snow and ice?  It appears they primarily relied upon a theodolite which was used to measure accurate horizontal and vertical positions of the Sun and Moon.  Knowing the position of the Sun or the Moon at a particular time allowed Scott and his fellow explorers to determine their geographic latitude and longitude by using a book of navigation tables.

Theodolite used by Lt. Edward Evans

Understanding Space and Time

Have you ever noticed how it is almost impossible to find documentaries made more than a few years ago?  I was doing some reading on the Casimir effect this evening and came across the name of Julian Schwinger (1918-1994), the American theoretical physicist who shared the 1965 Nobel Prize in Physics with Richard Feynman (1918-1988) and Shin’ichirō Tomonaga (1906-1979).  I remember, after all these years, that I had enjoyed watching a BBC documentary series that featured Schwinger (as well as George Abell) called Understanding Space and Time.  It was broadcast in 1979 or 1980 and featured thirteen 28-minute episodes.

  1. Ground control to Mr. Galileo
  2. As Surely as Columbus Saw America
  3. Pushed to the Limit
  4. Conflict Brought to Light
  5. Marking Time
  6. E = mc2
  7. An Isolated Fact
  8. The Royal Road
  9. At the Frontier
  10. Shades of Black
  11. Measuring Shadows: The Universe Today
  12. A Note of Uncertainty: The Universe Tomorrow
  13. Vanished Brilliance: The Universe Yesterday

Granted, some of this material is now dated, but much of it is still relevant and certainly of historical interest.  Why is it (and a host of other documentaries) not available on DVD or for downloading?

We really need a company to fill a different niche alongside The Great Courses, Curiosity Stream, and Netflix.  That niche would be to uncover and rerelease past documentaries of merit1, often hosted or presented by historically important individuals.  Documentaries such as Understanding Space and Time would be nice to own and watch again.

1One must certainly include many PBS documentaries and older episodes of documentary series—NOVA, for example—that are no longer available.

Dodgeville Street Project Proposals

As illustrated below, a lot of drivers in Dodgeville take a dubious “short cut” from King St. to Iowa/Bequette by way of W. Leffler instead of taking King St. all the way to Iowa/Bequette.  Most of the people taking this short cut are leaving Lands’ End and heading to their homes in the Madison metro area.  These folks are not Dodgeville / Iowa County taxpayers.  Here’s the problem.  W. Leffler has been beat all to hell and is badly in need of resurfacing.  All that Lands’ End traffic has contributed mightily to the degradation of W. Leffler.  Now, as a bicycle commuter trying to get from Lands’ End to most of the rest of Dodgeville (always a dangerous proposition), it makes sense to use W. Leffler to minimize the amount of time I have to ride my bike on busy King St. and very busy Iowa/Bequette.  But W. Leffler is so broken up that for safety reasons I need to ride near the middle of the road—but a steady stream of vehicles takes the short cut down W. Leffler instead of staying on King St. up to convenient entrance ramp to Iowa/Bequette.  It is a no-win situation for Dodgeville bicyclists.  One solution would be to have W. Leffler dead end at King St. with only a bike-path connector between King St. and W. Leffler, though I suspect that would be quite unpopular in our auto-centric community.  Another solution would be to resurface W. Leffler and never let it degrade this much again.  Is that too much to ask?  It is a short street, after all.

The Lands’ End Shortcut to the Madison Metro Area

I’m not a big fan of roundabouts, but if ever there was a case for one it would be at the intersections of Iowa/Bequette, N. Main, E. Spring, and W. Spring.  In my crude map overlay below, it looks like one building would probably have to be removed.  The roundabout would need to be designed to easily accommodate the comings and goings of fire trucks from the nearby fire station.  Presently, this “octopus” of an intersection is dangerous, and I completely avoid ever making a left turn there.  Why not prohibit all dangerous left turns at these intersections by installing a roundabout where every turn will be a right turn?

Where a roundabout is needed in Dodgeville

Dateline 2024: Total Solar Eclipse

In little more than six years, another total solar eclipse across the continental U.S. will pass as close as Southern Illinois and Indiana.  Like our recent eclipse of August 21, 2017, the next total solar eclipse will also take place on a Monday and, remarkably, just a few minutes earlier in the day.  Save the date: April 8, 2024.   Actually, not long to wait.  Think about what you were doing around December 7, 2011.  Can you remember?  No question about it, the next six years will go faster than the previous six did.  Seems that as we age our sense of time changes, and time seems to go faster and faster.

The point of maximum length of totality for the 2017 eclipse was 12 miles NW of the center of Hopkinsville, Kentucky, where totality lasted 2m40s and the path of totality was 71 miles wide.

The point of maximum length of totality for the 2024 eclipse will be near Nazas, Mexico (in the state of Durango), where totality will last 4m28s and the path of totality will be 123 miles wide.  Yes, this will be a longer eclipse!

Remarkably, there is a location in southern Illinois that is on the centerline of both the 2017 and 2024 eclipses!  That location is 37°38’32” N, 89°15’55” W, SW of Carbondale, Illinois, near Cedar Lake and the Midland Hills Country Club.

When did a total solar eclipse last grace Dodgeville, Wisconsin?  Nearly 639 years ago, on May 16, 1379.  The duration of totality was 3m48s.  Perhaps the Oneota people then living in this area witnessed the event.

The next total solar eclipse visible from Dodgeville won’t happen for another 654 years.  There’ll be annular eclipses in 2048, 2213, 2410, 2421, and 2614.  Then, finally, on June 17, 2672, the totally-eclipsed Sun will once again grace the skies of Dodgeville—weather permitting, of course.  The duration of the eclipse at Dodgeville will be 2m47s.  There will be another annular eclipse in 2678, followed by another total eclipse (duration 3m01s) on June 8, 2681.  Then, just two years later there’ll be another total eclipse at Dodgeville: on November 10, 2683 (0m49s).  That’s three total eclipses and one annular eclipse visible at Dodgeville in just 11 years!