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.

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.

The Good Old Days of Astronomy…

Those of you who grew up in the 1950s and 1960s as I did will especially delight in reading the July 7, 2007 entry of Uncle Rod’s Astro Blog, courtesy of Alabama astronomer Rod Mollise.  What a hoot!

And here’s a note from Phil Harrington’s website about Celestron’s ads in the 1990s: “It must be good to be an amateur astronomer in California, judging by the ads run by Celestron over the years…Yup, just another typical club star party, right?”  Photo montage by Rod Mollise.

What Is and What Might Have Been

We continue our series of excerpts (and discussion) from the outstanding survey paper by George F. R. Ellis, Issues in the Philosophy of Cosmology.

Thesis E2: We cannot take the nature of the laws of physics for granted.
One cannot take the existence and nature of the laws of physics (and hence of chemistry) as unquestionable in cosmology—which seems to be the usual habit in biological discussions on the origin and evolution of life.  This is in stark contrast to the rest of science, where we are content to take the existence and nature of the laws describing the fundamental behaviour of matter as given and unchangeable.  Cosmological investigation is interested in the properties of hypothetical universes with different physical behaviour.  Consideration of ‘what might have been’ is a useful cosmological speculation that may help throw light on what actually is; this is a statement of the usefulness of ‘Gedanken experiments‘ in cosmology.

Practical science, engineering, and technology are prescriptive.  If we do a, we know from experience that b will occur.  Using the laws of physics, we can predict the location of the Moon as a function of time, put a spacecraft in orbit around Saturn, or build a light bulb that will illuminate.  Though we may be curious, we are not required to know why or how these laws exist—or how they might have been different—only that they do work, time and time again.

Cosmology, though firmly rooted in science, is different.  We are passive observers in a very large and very old universe, and there is no absolute guarantee that the laws of physics that work for us so well in the here and now apply to all places and at all times.  We must attempt to understand the laws of physics in a larger context that does involve some well-reasoned and reasonable speculation.

“Not only does God … play dice, but He sometimes confuses us by throwing them where they can’t be seen.” – Stephen Hawking

“Sometimes attaining the deepest familiarity with a question is our best substitute for actually having the answer.” – Brian Greene

In politics, governance, sociology, and philosophy, too, I would submit to you that consideration of “what might have been” is useful in helping us to understand what actually is.  Such reflection, en masse, might even lead to substantive change.

“Why is it that here in the United States we have such difficulty even imagining a different sort of society from the one whose dysfunctions and inequalities trouble us so?  We appear to have lost the capacity to question the present, much less offer alternatives to it.  Why is it so beyond us to conceive of a different set of arrangements to our common advantage?” – Tony Judt

Getting back to cosmology, however, for the moment…

Indeed if one wants to investigate issues such as why life exists in the universe, consideration of this larger framework—in essence, a hypothetical ensemble of universes with many varied properties—is essential (this is of course not the same as assuming an ensemble of such universes actually exists).  However, we need to be very cautious about using any claimed statistics of universes in such a hypothetical ensemble of all possible or all conceivable universes.  This is usually not well defined, and in any case is only relevant to physical processes if either the ensemble actually exists, rather than being a hypothetical one, or if it is the outcome of processes that produce well-defined probabilities—an untestable proposal.  We can learn from such considerations the nature of possible alternatives, but not necessarily the probability with which they might occur (if that concept has any real meaning).

It is easy to imagine a universe without life.  But we obviously do not live in such a universe.  There may be other universes devoid of life.

For the more thoughtful among us, it is easy to imagine a civilization without war, guns, violence, extrinsic suffering1 caused by others, or deprivation.  Obviously, we do not live in such a society.  But how can we say it is impossible, or even improbable?  It would be easy to find many millions of people in the world even today that would never fight in a war, would never own or use a gun, who would never resort to violence, who would never cause others to suffer, and who would make eliminating deprivation and poverty a top priority.  The question for the scientists is: what is wrong with the rest of us?

1Extrinsic suffering is suffering caused by others or circumstances completely outside of one’s control.  Intrinsic suffering, on the other hand, is self-inflicted—through our own failings, poor judgement, or mistakes that we make.

Growing Older

As we grow older,
That which is older grows upon us.
Time accelerates,
And the world seems a smaller place.

The years go by like months,
The months go by like weeks,
The weeks go by like days,
The days go by like hours,
And the hours go by like minutes.

And our world which in our youth was all that we knew
Slowly reveals itself to be a surprisingly alien place,
Full of centuries of hard work, unlikely events, and compromise:
The world could be a very different (and better) place,
Even within the confines of human nature.

Taken to its natural conclusion
Were we each to live for millennia, perhaps longer
We would find eternity in an instant
And infinity at the door.

David Oesper

Ellis, G. F. R. 2006, Issues in the Philosophy of Cosmology, Philosophy of Physics (Handbook of the Philosophy of Science), Ed. J. Butterfield and J. Earman (Elsevier, 2006), 1183-1285.

Einstein, Brahms, and Exoplanets

What do Albert Einstein, Johannes Brahms, and exoplanets have in common?  They are all great courses provided by The Great Courses.

Call me old fashioned, but I love a great lecture presented by an expert in the field.  What a wonderful way to get introduced to a new subject, or refamiliarize yourself with an old subject, or deepen your knowledge about a subject with which you are already familiar.

I recently finished watching the magnificent course “Albert Einstein: Physicist, Philosopher, Humanitarian” by Don Howard, Professor of Philosophy at the University of Notre Dame, former Director of Notre Dame’s Graduate Program in History and Philosophy of Science, and a Fellow of the University of Notre Dame’s Reilly Center for Science, Technology, and Values.

I have taken an interest in Einstein since I first encountered relativity in my early teens, and of course being a physics major in college I became much more familiar with Einstein’s remarkable scientific contributions.  But this course surprised and delighted me with many details about Einstein himself.  Howard obviously has a much deeper understanding of Einstein the person than most physicists do, and his enthusiasm for his subject comes through in every lecture.  I doubt you will find a more thorough treatment of Einstein anywhere short of reading a biography.  Recommended!

As luck would have it, while I was nearing the end of this course, Time came out with an updated reissue of its special edition, “Albert Einstein: The Enduring Legacy of a Modern Genius”.  Great photographs, great text.  Well worth every penny!

Robert Greenberg is music historian-in-residence with San Francisco Performances and has produced a lot of high-quality music courses for The Great Courses.  I am in the process of watching all of them (yes, really, they’re that good!).  Recently, I finished his course on Johannes Brahms, who is probably my all-time favorite composer.

The music of Brahms is well known by many, but how much do you know about Johannes Brahms the person, and the events of his life?  This course is the perfect introduction to those subjects, as well as his extraordinary compositions.

It is amazing to me that no one has yet made a feature-length film about the life of Johannes Brahms (1833-1897).  A historically accurate dramatic portrayal could easily become one of the most significant musical film biographies ever made.  Brahms was one of the greatest composers who ever lived, and he had an interesting life—there is much material to draw upon for the making of this movie.  Greenberg’s course is a great place to begin, and I would also recommend the definitive biography, “Brahms: His Life and Work” by Karl Geiringer.

You’ve just got to love The Great Courses.  This is what television could have been.  PBS is the only thing that even comes close.  I recently completed “The Search for Exoplanets: What Astronomers Know” presented by Joshua Winn, now Professor of Astrophysical Sciences at Princeton University.  Not since Carl Sagan or Neil deGrasse Tyson have I been this excited about an astronomy presenter.  Josh Winn presents his exoplanets course with enthusiasm, precision, and a delivery that really draws you in to the subject.  I hope we see much more of him in the future.

In the Shadow of the Moon

Every once in a while a really great documentary comes along.  In the Shadow of the Moon is one of them. This 2007 British film, which like most documentaries (unfortunately), had a very limited theater engagement, is now widely available for rental or purchase.

It is the remarkable story of the Apollo missions to the Moon, told eloquently by many of the astronauts who journeyed there: Buzz Aldrin, Michael Collins (Apollo 11), Alan Bean (Apollo 12), Jim Lovell (Apollo 8 & 13), Edgar Mitchell (Apollo 14), David Scott (Apollo 9 & 15), John Young (Apollo 10 & 16), Charles Duke (Apollo 16), Eugene Cernan (Apollo 10 & 17), and Harrison Schmitt (Apollo 17).  You certainly get the impression that not only are these guys personable and intelligent, but that they have aged well and still have much insight and wisdom to offer us about the past, present, and future.

The historical importance of this documentary cannot be overstated.  There is nothing, and I mean nothing, like hearing about the first (and still only) human missions to the Moon firsthand from the astronauts who journeyed there.  And, sadly, these pioneering astronauts are not going to be with us much longer. Some have already left us.  In the eleven years since this documentary was released, Edgar Mitchell, the last surviving member of the Apollo 14 crew, passed away in 2016, Gene Cernan, the last man to walk on the Moon, passed away in 2017, and John Young, the longest-serving astronaut in NASA history, left us in 2018.  The seven surviving Apollo astronauts who shared their stories with us in this film are all octogenarians: Buzz Aldrin is 87, Michael Collins is 87, Alan Bean is 85, Jim Lovell is 89, David Scott is 85, Charles Duke is 82, and Harrison Schmitt is 82.

This is a story that needed to be told by those who can tell it best.  There is no narrator, nor is there any need for one.  Kudos to directors David Sington & Christopher Riley, producers Duncan Copp, Christopher Riley, Sarah Kinsella, John Battsek, & Julie Goldman, and  composer Philip Sheppard for making this a film of lasting cultural significance, a film that will be admired and appreciated a hundred-plus years from now.

Space Pioneers

In April 1959, NASA announced the first seven astronauts.  The Mercury Seven are, in order of birth date:

John Glenn (1921)

Wally Schirra (1923)

Alan Shepard (1923)

Deke Slayton (1924)

Scott Carpenter (1925)

Gus Grissom (1926)

Gordon Cooper (1927)

John Glenn, the oldest of the Project Mercury astronauts and the first American to orbit the Earth, was the last to die, in 2016, at the age of 95.

Gus Grissom (1967) – Apollo 1 launch pad fire
Deke Slayton (1993) – brain tumor
Alan Shepard (1998) – leukemia
Gordon Cooper (2004) – Parkinson’s disease; heart failure
Wally Schirra (2007) – abdominal cancer; heart attack
Scott Carpenter (2013) – complications following a stroke
John Glenn (2016) – complications after heart valve replacement, stroke

Walter Cronkite (1916-2009) and Wally Schirra (1923-2007) covered the Apollo moon missions on CBS—far better than anyone else—and I can still remember the events as if they happened only recently.

Want to know who holds the title for longest duration human spaceflight (so far)?  Valeri Polyakov (1942-) entered space aboard Soyuz TM-18 on January 8, 1994 and stayed aboard the Mir space station until returning to Earth aboard Soyuz TM-20 on March 22, 1995.  That’s nearly 438 days (1.2 years) in space!  Moreover, Polyakov, who is a medical doctor, spent over 240 days in space during his first visit to Mir in 1988-1989, giving a total spaceflight time of nearly 1.9 years.

While Polyakov still holds the record for the single longest duration spaceflight, Gennady Padalka (1958-) has spent more time in space than anyone else: 878.5 days (2.4 years)!

Eclipse Comets

A total solar eclipse, such as that which will be crossing America on 21 Aug 2017, would present a great opportunity to discover a bright comet near the Sun.  Has that ever happened?  The answer is yes.

A comet, perhaps magnitude -4 or brighter, was spotted about 1.4° SW of the Sun during the total solar eclipse of 1 Nov 1948.  The editors of Sky & Telescope write in the January 1949 issue, “British Astronomical Association Circular No. 303, dated November 10, 1948, under the title, ‘The Eclipse Comet, 1948 I,’ reads in part:

There can be little doubt that the bright comet now reported seen in the southern morning sky is identical with the one seen during the eclipse of November 1.  The Times of November 2 in the report of the eclipse from its correspondent at Nairobi stated that a bright comet, with a long tail, was seen both by the crew of an R.A.F. aircraft and by observers on the ground.  The head, it was stated by one amateur astronomer, was still visible a few seconds after the Sun began to emerge.

A cable received by Dr. R. d’E. Atkinson, leading the Royal Observatory expedition, reports photographic confirmation of it, saying it was 93′ from the centre of the Sun in position angle 226°, and was very bright, with a tail.

“Harvard Announcement Card 956, dated November 22nd, reads in part:

Dr. Leland E. Cunningham, Students’ Observatory, University of California, Berkeley, writes: ‘New elements have been determined for the bright comet . . . .  These place the comet in position angle 228° and 104′ distant from the sun at the time of the total solar eclipse on November 1, which are in moderate agreement with Atkinson’s observed values of 226° and 93′, respectively.’

“Thus, although Comet 1948 I was missed by northern observers before it passed perihelion late in October, when its tail must have extended into the evening sky after sunset, the total eclipse of the sun provided a favorable opportunity to observe the comet practically a week before southern observers viewed it in their morning sky.  It well can be called the ‘eclipse comet’ of 1948.”

The editors of Sky & Telescope write in the March 1949 issue, “From The Observatory of December, 1948, we quote part of the proceedings of the meeting of the Royal Astronomical Society held on November 12th, at which Dr. R. d.’E Atkinson told something of his recent eclipse expedition to East Africa, and the discovery of the comet during the eclipse.  Dr. Atkinson said:

I propose to speak mainly about the comet which was observed during the eclipse; as far as our own eclipse observations are concerned, I believe they were successful, but the films have not yet been developed.

The comet, though very bright, was not visible at Mombasa, where we were (98% totality), but several newspaper reports from further north referred to it; they did not sound very convincing.  A photograph was published, but as printed it did not actually show the comet; the accompanying description was also based on an error, as I later learnt.  On the journey to Nairobi, sixty hours after the eclipse, I spoke to an eyewitness, whose account disagreed with that in the paper.  It was not until I had seen the photographs taken by the R.A.F. at Nairobi, and had found that they agreed with eye-witness reports at both places, that I realised it must have been a comet; I then made a very rough measurement of its place on the R.A.F. film, and telegraphed Dr. Merton.  As a result of my interest in these photographs, which were taken at 13,000 feet just within and just outside the shadow, the Air Commodore very kindly let me bring the films home for thorough examination.  [On one picture] very much enlarged from a hand-camera snapshot also taken by a member of the crew . . . the tail is clearly visible; visual observers all agreed that it extended downwards until it reached either clouds or the horizon, and it must have been twenty degrees long at least.  The visible part of it does not point away from the Sun at all; any portion which does this must have been extremely foreshortened.  [On another picture] the scale is larger and the definition much better, but the tail is too much underexposed to show except with a magnifying glass.  Viewed in this way, and accepting the idea that the root of the tail will point away from the Sun, one can see enough indications of curvature to make it seem that it is convex to the west; I therefore concluded in my cable a guess that the motion would be westwards, and this has proved correct.  The comet must certainly have been very bright; these pictures were taken with an aperture of f/5.6 and an exposure of 1/300 second; moreover, the head was visible for some 5-10 seconds after the end of totality.  It must certainly have been brighter than Venus.  I have now measured up three separate negatives, and they agree closely in giving a distance from the centre of the Sun of 105.4 minutes, and a position angle of 230°; however, there is some possibility of systematic error, and I have written to the Air Commodore to ask for further details.  If systematic errors can be eliminated, the place should, I think, be useful for orbit determination; it is a week earlier than any other place.”

Thus writes British astronomer Robert d’Escourt Atkinson (1898-1982) about comet C/1948 V1, the “Eclipse Comet of 1948” seen at Nairobi and Mombasa, Kenya on 1 Nov 1948.  It was next observed in the morning sky on 8 Nov 1948, and continued to be followed until 3 Apr 1949.

According to Edward S. Holden (1846-1914), John Martin Schaeberle (1853-1924) discovered a comet-like object on photographic plates taken during the 16 Apr 1893 total solar eclipse, but it has since been determined (Cliver 1989) that this was a coronal mass ejection (CME).

German-born British physicist Arthur Schuster (1851-1934) recorded a comet on photographic plates of the total solar eclipse of 17 May 1882 in Egypt.  The comet moved noticeably during the 1m50s of totality.  It is thought that this comet was a member of the Kreutz sungrazer group of comets.  It has received the designation of X/1882 K1.  The “X/” indicates that there were not enough observations of this comet to determine an orbit.  In fact, the only observations of this comet were during the total solar eclipse.  The comet is sometime called Comet Tewfik—named after the ruler of Egypt at that time in recognition of his hospitality towards the eclipse party.

A comet was discovered during the eclipse of 19 Jul 418 at Constantinople (Istanbul, Turkey) and was observed for four months afterwards.

Seneca the Younger (c. 4 BC – AD 65) writes in his Naturales quaestiones (Natural Questions):

Posidonius, in fact, tells us that during an eclipse of the Sun a comet once appeared which the sun’s proximity had hitherto concealed.

Did Posidonius (c. 135 BC – 51 BC) see this comet, or was he referring to an even earlier observation made by someone else?  With so much of the knowledge of the ancient world lost or destroyed by barbarians and zealots, we may never know.

Clarke, J. 1910, Physical science in the time of Nero; being a translation of    the Quaestiones naturales of Seneca
Cliver, E. W. 1989, Solar Physics, 122:2, 319-333
Federer, C. A. Jr., Sky & Telescope, January 1949, pp. 59-60
Federer, C. A. Jr., Sky & Telescope, March 1949, p. 110, 113
Hetherington, B. 1996, A Chronicle of Pre-Telescopic Astronomy
Kronk, G. W., Cometography, X/1882 K1 (Eclipse Comet or “Tewfik”)
Poitevin, P., Eclipse Comets
Seneca c. 65 AD, Naturales quaestiones, 7.20.4
Vaquero, J. M. 2014, Physics Today, 67:5, 9

Bonner Durchmusterung und Gaia

As our civilization and technology continue to evolve, it seems we take far too much for granted.  We neglect to consider how incredibly hard people used to work years ago to achieve results we today would pass off as almost trivial.  But history has many lessons to teach us, if only we would listen.

As an example, Prussian astronomer Friedrich Wilhelm August Argelander (1799-1875) at the age of 60 began publishing the most comprehensive star catalogue and atlas ever compiled, as of that date.  From 1852 to 1859, Argelander and his assistants carefully and accurately recorded the position and brightness of over 324,000 stars using a 3-inch (!) telescope in Bonn, Germany.  Employing the Earth’s rotation, star positions were measured as each star drifted across the eyepiece reticle in the stationary meridian telescope by carefully recording when each star crossed the line, and where along the line the crossing point was.

Stars Transiting in a Meridian Telescope

One person observed through the telescope and called off the star’s brightness as each star crossed the line, noting the exact position along the reticle on a pad with a cardboard template so that the numbers could be written down without looking away from the telescope.  A second person, the recorder, noted the exact time of reticle crossing and the brightness called out by the observer.  In this way, two people were able to record the position and brightness of every star.

Each star was observed at least twice so that any errors could be detected and corrected.  In some areas of the Milky Way, as many as 30 stars would cross the reticle each minute.  What stamina and dedication it must have taken Argelander and his staff to make over 700,000 observations in just seven years!  Argelander’s catalogue is called the Bonner Durchmusterung and is still used by astronomers even today.  It was the last major star catalogue to be produced without the aid of photography.

Like Argelander’s small meridian telescope, the European Space Agency’s Gaia astrometric space observatory is currently measuring tens of thousands of stars each minute (down to mv = 20) as they transit across a large CCD array—the modern day equivalent of an eyepiece reticle.  But instead of utilizing the Earth’s rotation period relative to the background stars of 23h56m04s, Gaia’s twin telescopes separated by exactly 106.5° sweep across the stars as Gaia rotates once every six hours.  A slight precession in Gaia’s orientation ensures that the field of view is shifted so that there is only a little overlap during the next six-hour rotation.

When Gaia completes its ongoing mission, it will have measured the positions, distances, and 3D space motions of around a billion stars, not just twice but 70 times!

Though electronic computers do most of the work these days, someone still has to program them.  Some 450 scientists and software experts are immersed in the challenging task of converting raw data into scientifically useful information.

I’d like to conclude this entry with a quotation from Albert Einstein (1879-1955), who was born and died exactly 80 years after Argelander.

Many times a day I realize how much of my outer and inner life is built upon the labors of my fellowmen, both living and dead, and how earnestly I must exert myself in order to give as much as I have received.

I love that quote.  Words to live by.