This Month in the History of Astronomy - September
- Sep 1, 1877 - Francis Aston, British physical chemist who
started under J.J. Thompson at Cambridge in 1910 and would win the
1922 Nobel Prize in Chemistry for his invention of the mass spectrograph,
with which he was able to show that elements are often mixtures of
isotopes of different, nearly whole number masses.
For example, neon has a mass number of 20.2 in the periodic table. Aston
was able to show that this results from a 9:1 mixture of Ne-20 and Ne-22.
Eventually he'd identify 212 naturally occurring isotopes. His later
masses were accurate to almost a part in 100,000, so he was easily able
to show that hydrogen had a mass of 1.008 (in atomic units), not exactly
1 as it had previously been taken to be. He was among the first to
recognize that the slight deviations from exact whole number masses
which isotopes had was the key to the liberation of nuclear energy.
Aston even experimented with gaseous diffusion methods using clay pipe
to try and separate the two neon isotopes: since the atoms of the heavier
isotope move 10% slower, their concentration in the gas should increase
as the atoms of the lighter, faster isotope diffuse through the walls
of the pipe more rapidly. Such technologies would become important two
decades later during the development of the atomic bomb; the one dropped
on Hiroshima was made from Uranium-235 laboriously separated out of
natural uranium, which is mostly (99.3%) U-238.
Because Aston used photographic emulsions as the detector in his mass
spectrographs, he was a photo expert and participated in total solar
eclipse expeditions to Sumatra in 1926 (Jan 14 at Benkoeben, using a
19-foot focal length coronagraph with a direct vision prism in front
of the lens at second and third contact for spectra of the flash), to
Canada in 1932 (Aug 31, at Memphri Magoy), and to Japan in 1936 where
he was clouded out; he had hoped to study the direction of sky polarization
near the eclipsed sun, the object being to detect any changes in the
direction from which scattered light entered from outside the shadow
during the eclipse and also to find evidence for polarized sky light
scattered with the coronal beam by the earth’s atmosphere, as had been
found by Hugh Newall (Professor of Astrophysics at Cambridge) at Guelma,
Algeria in 1905.
- Sep 6, 1899 - Founding of the Astronomical and Astrophysical
Society of America, now the American Astronomical Society (AAS).
- Sep 7, 1914 - James Van Allen, U.S. physicist at the
University of Iowa who discovered the earth's magnetosphere, a region
of charged particles trapped in the earth's magnetic field. Even though
they acquired the name "Van Allen Belts", they are actually more in the
shape of donuts (toroids).
Van Allen was an expert in instrumentation miniaturization who used
captured German V-2 rockets after WWII, as well as Aerobee and high
altitude balloon launched rockets ("rockoons"), to study radiation
in the upper atmosphere. Explorer 4, launched on July 26, 1958, took
Geiger counters up into unexplored regions, in preparation for the
first U.S. orbital satellite launch, and was the first to discover
the high levels of radiation there.
- Sep 10, 1857 - James E. Keeler, pioneer American
spectroscopist and astrophysicist. In 1895, using the 13-inch
Fitz-Clark telescope at Allegheny Observatory, Keeler showed that
different parts of Saturn's rings rotate at different velocities,
which demonstrated that the rings are not solid bodies but instead
are a collection of small objects in independent orbits, an idea
first proposed by James Clerk Maxwell.
Along with George Ellery Hale, Keeler also in 1895 founded and
then helped edit the Astrophysical Journal until his untimely
death five years later.
- Sep 11, 1877 - Sir James Jeans, English astronomical
theoretician. In the first decade of this century Jeans worked out
the fundamentals of gravitational collapse processes, which are of
relevance to the formation of solar systems, stars, galaxies, and
clusters of galaxies.
After the age of thirty-five (1912) Jeans was an independent
researcher and writer/popularizer who held no university or
professorial post, though he was a research associate at Mount
Wilson from 1923 to 1944. He believed in the continuous creation
of matter (1928), the forerunner of the later steady state
cosmological theory alternative to big bang models.
In his early, university associated years (at Princeton and
Cambridge), he worked on the foundations of the kinetic theory
of gases (1904). He next (1905) turned to the problem of the
equipartition of energy as it applied to specific heat capacities
and blackbody radiation. Finding a numerical error in the derivation
of the energy distribution with wavelength as a function of
temperature for blackbody radiation, this improved relationship
is now known as the Rayleigh-Jeans law.
Jeans later took the theory of compressible and incompressible
gases and fluids to a deeper level than had Henri Poincaré
previously, and applied these findings to a variety of astronomical
problems, from the fissioning of a collapsing and spinning gas cloud
into double stars, to the formation of spiral galaxies and planetary
systems. He pointed out the error in Pierre Laplace's theory of the
formation of the solar system, favoring instead the now out-of-favor
idea that gravitational tidal interactions with a star passing close
by the Sun had drawn the planetary material out of it. The slow
rotation of the Sun stood as a formidable objection to the nebular
hypothesis of Kant and Laplace, which proposed that planetary systems
formed out of the same condensing cloud that produced the primary
star. An initial rotation rapid enough to produce the Sun's planets
should have produced a Sun spinning a thousand times faster. The
resolution of this conundrum came by coupling the protostar with
the protoplanetary disk through a magnetic field, which transfered
angular momentum outward so it doesn't end up in the star.
[Hubble (left) and Jeans, at the Cassegrain focus of the 100"
telescope, 1931. The primary mirror did not have a hole drilled in
its center, so a diagonal flat mirror was used a little more than
50" in front of it to direct the converging beam from the
secondary out to a focus at the base of the tube assembly.]
- Sep 12, 1725 - Guillaume Le Gentil, French astronomer who
first discovered the objects later catalogued by Messier as M32, M36
and M38, as well as the nebulosity in M8.
He's probably most famous for his observing-run-from-hell, originally
to India, for the transit of Venus in 1761. This had been predicted by
Edmund Halley, and by having measures from widely separated places on
the earth it was hoped the AU (Astronomical Unit = earth-sun distance)
could be calibrated and the scale of the solar system determined, by
simple surveying trigonometry techniques. Le Gentil took the assignment
to go to India.
Long story short, 1761 was a bust (he was lost at sea for five weeks,
war had broken out between France and England so his destination was
in enemy hands when he got there, etc.), so he decided to stay on that
side of the globe for the June 4, 1769, transit. (Venus transits occur
in pairs several years apart which are themselves separated by more
than a century.) After getting run out of the Phillipines by the Spanish,
he returned to Pondicherry, India, a treaty having put it back in French
hands, built a small observatory and surrounding support village, and
then waited for the day. After a month of clear mornings -- you guessed
it -- he was clouded out. His voyage back was as eventful (or more) as
his trip there had been, so it was October, 1771, before he
made it back to Paris, eleven years after his departure -- only to
find that none of his mail had gotten through, that his wife (who
had re-married) and family had had him declared dead, and had
"enthusiastically plundered his estate", and then, as a kicker, that
he had also been replaced in the Royal Academy of Sciences. It
eventually took the intervention of the King to sort everything out.
The presumed loss of Le Gentil on the expedition was part of the
rationale for the first of the famous Captain Cook's three voyages,
which were funded by Britain's Royal Society, and which also led to
the colonization of Australia, since the 1769 transit could only be
seen from the South Pacific. The observations were deemed that valuable.
- Sep 12, 1962 - President Kennedy delivers his famous lines
at a speech at Rice University saying the US will put a man on the
moon by 1970, the birth of the Apollo program. This was actually a
reiteration of a May 25th speech, but in the interim Congress had
actually authorized funding for the "the large Moon rocket" project,
part of which involved the Manned Spacecraft Center in Houston, on
land donated through Rice. Almost all the NASA facilities for Apollo
were in the South because powerful Southern senators wouldn't vote
for the $$'s unless lots of them were to be spent in their states.
- Sep 13, 1912 - Horace W. Babcock, American astronomer who,
in 1946-7, first discovered magnetic fields in stars other than the
Sun, based on his earlier work on magnetic fields in sunspots. The
particular star observed was 78 Virginis (alternately CW Virginis or
ο Virginis, aka omicron), which is a type A star which has very
narrow spectral lines, indicative of no rotation; however, in several
other regards its spectrum was peculiar and was more typical of stars
with rapid rotation. Babcock reasoned that its rotation axis might
be pointed nearly directly at us, and he then detected its polarized
light, due to the Zeeman effect, indicative of the strong magnetic
fields (1,500 gauss - now thought to be more than 2x that) expected
from a star rotating at 60 km/sec at its equator, which causes the
spectral lines in polarized light to split.
Babcock was Director of the Palomar Observatory from 1964 until
his retirement in 1978, and was the first to study and propose
a viable method for correcting for atmospheric turbulence (star
twinkling) via adaptive optics (1953), which was not realized
technically for routine use until three or more decades later.
His study of the rotation curve of the Andromeda Galaxy in 1939
found an increasing mass-to-light ratio with increasing distance
from the center, but he attributed this to interstellar extinction
(absorption of starlight) or some dynamical effect rather than
"missing mass" (aka dark matter).
- Sep 16, 2016 - The 500 meter Aperture Spherical Telescope
(FAST) in Dawodang, China, now the world's largest radio dish, sees
first "light" by observing the quasar 3C 409. The next day it observed
the pulsar J1921+2153 and needed only 1 minute integration time to
achieve a signal-to-noise ratio of S/N=5,000. The telescope can cover
frequencies from 70 MHz up to 1.8 GHz, and is partially deformable
into a parabolical surface by 2226 cable actuators which can pull
on the 4450 triangular panels comprising the reflecting surface from
- Sep 19, 1819 - (Jean Bernard) Léon Foucault, French
applied and experimental physicist who first accurately measured
the speed of light (1850-62), and also invented both the gyroscope
(1852) and Foucault pendulum (1851) to demonstrate the rotation of
the Earth. In 1845, along with Armand Fizeau, he made the first
detailed photographs of the Sun's surface -- practical photography
having been invented in only the last decade. Ten years later he
got on at the Paris Observatory and developed improved new methods
for silvering and testing mirrors and lenses (1857-58).
- Sep 23, 1882 - James Joy, noted expert in astronomical
spectroscopy at the Yerkes and Mount Wilson observatories, who
perfected the technique of "spectroscopic parallax" -- determining
the distances to stars using spectral type classification, which
gives the star's absolute magnitude (M) and thus its distance modulus,
m-M, once the apparent brightness (m) is measured. Joy, working along
with colleagues Adams and Humason over a two decade long period,
measured over 5,000 stars this way, in the process better calibrating
the H-R Diagram and discovering many spectroscopic and eclipsing
Working also with radial velocities, Joy used 130 Cepheid variable
stars to determine the distance and direction to the galactic center,
with the goal of finding the circular velocity of a star at the Sun's
distance from the center, and thus the rotation period of the galaxy.
With spectra of the famous variable Mira at minimum brightness, in 1918,
he was able to deduce the presence of its faint white dwarf companion
star (Mira B). The system is a slow binary (roughly a 500 year period)
and a very close visual one, first split by Robert Aitken 5 years later,
at a separation of less than an arc-second -- ~100 AU's at an uncertain
distance of ~90 parsecs. In 2007, a protoplanetary disk was discovered
around Mira B.
In addition, Joy was the first to group a diverse collection of
unusual variable stars with differing spectra into what are now
called T Tauri stars (after the prototype). He was president of
the American Astronomical Society in 1949, a few years after being
its V.P., and a few years before retiring at age seventy.
- Sep 27, 1814 - Daniel Kirkwood: Early U.S. astronomer who
first (1857) noticed the gaps in the asteroid belt (at 2.5, 2.95, and
3.3 AU's) which now bear his name. A professor of mathematics at the
University of Indiana, when his work was published in 1866 it included
the mechanism by which orbital resonance with Jupiter clears any object
with these semi-major axes out of these zones by periodic gravitational
perturbations driving them into other orbits. He used the same kind of
analysis on Saturn's rings to first explain the Cassini division, Encke's
division, and the gap between the B Ring and the Crêpe Ring, using
- Sep 28, 1605 - Ismaël Bullialdus, French astronomer. He
published Astronomia Philolaica in 1645, supporting Kepler's
elliptical orbits, and assuming an inverse-square force law (for
gravity) for the first time. Often credited (because of Halley)
with discovering M31, but he expressedly mentioned that this
"Nebula" had been observed 150 years earlier by some anonymous
but expert astronomer. Considered "the most noted astronomer of
his generation", Bullialdus first calculated the period of the
famous variable star Mira (Omicron Ceti).
- Sep, 1692 - (exact date unknown) James Bradley, the
third Astronomer Royal in England, who succeeded Edmond Halley
in the position (1742).
Bradley is mainly remembered for three discoveries. In 1722,
using a 212 foot focal length lens, he measured the angular
diameter of Venus, and thus its diameter, to a precision not
Then, in 1728, while trying to measure the parallax (distance)
of the star Gamma Draconis, he instead discovered the aberration
of starlight resulting from the earth's orbital motion about
the sun. He was able to distinguish aberration from parallax
because of the phase of the cyclical variation in the position
of the star. γ Dra was chosen because it's the brightest
circumpolar star, and thus could be observed throughout the
year. That the effect of aberration swamped the size of the
expected parallax suggested the star was many times farther
away than had been supposed. From the size of the effect
Bradley was able to substantially improve on Ole Rømer's
value for the speed of light, calculated circa 1676-82 from
the time delay or advance of Io going into Jupiter's shadow
(or emerging from it), which varied by more than ten minutes
depending on the relative distances of Jupiter from earth.
Finally, on Feb 14, 1748, Bradley published his results for
the nutation of the earth, based on a complete 18.6 year
cycle of the moon's nodes.
Discoveries and other firsts
- Sep 1, 1804 - Karl Harding, working under Johann Schröter
in Bremen, Germany, discovers the asteroid Juno. Though it was the third
asteroid turned up, Juno is only the 12th in size, at a diameter of just
233 km. It's relatively high brightness for its size indicates a higher
albedo (reflectivity) than is typical for asteroids -- 24% versus
3-10% for the most common, carbonaceous and nickel-iron asteroids
-- which accounts for its relatively early discovery.
Juno has favorable oppositions every thirteen years (2005, 2018, 2031),
when it is near perihelion. It was less than a month from one of these
at the time of discovery. At these times it's about 1 AU from earth
(and 2 AU from the sun), when it's a magnitude 7½ object easily found
in binoculars and small telescopes; it stays brighter than 9th
magnitude for 4-5 months then.
Juno was the first asteroid for which an occultation was observed;
it passed in front of the star SAO 112328 on February 19, 1958.
- Sep 1, 1859 - Solar physicist Richard Carrington
observes the first solar flare ever recorded. An intense aurora
followed the next day. This was one of the most active solar
outbreaks ever recorded. The aurora was visible as far S as
the Caribbean, and the flare was thought to have caused a 5%
reduction in earth's ozone layer.
- Sep 1, 1979 - Pioneer 11 makes the first ever flyby
of Saturn. Afterwards it headed up and out of the plane of the
ecliptic in which the planets orbit in the general direction of
the constellation Aquila.
- Sep 3, 164 BC - What many centuries later would be
known as Halley's Comet makes its closest approach to earth for
this apparition, forty-four days before perihelion, at only a
little more than 10 million miles distance. The event was
recorded on Babylonian tablets, its second earliest known
- Sep 3, 1976 - The Viking 2 lander touches down on
Mars, about 7 weeks after Viking 1 (see July 20).
- Sep 5, 1831 - The first definitive observation of
Jupiter's Great Red Spot. A spot had been observed as early as
1665, by the great Giovanni Cassini, and followed until 1713,
but it's not clear that this was the same storm seen nearly
120 years later and monitored continuously ever since a close
opposition of Jupiter in 1879. The spot has been shrinking
in size the last several decades and in the early part of the
21st century was only about half the size longitudinally that
it had been a hundred years before.
- Sep 5, 1977 - The Voyager 1 spacecraft was launched,
fifteen days after Voyager 2, but overtaking it by December on
its slightly faster and more direct trajectory, and being the
first to reach Jupiter and Saturn before eventually heading off
into interstellar space, passing the heliopause on Aug 25, 2012.
It's expected to operate until about the year 2025, when its
radio-isotope thermo-electric generators will be unable to
produce sufficient power to keep its instruments ON.
- Sep 8, 1966 - The TV show Star Trek premieres,
inspiring a generation's interest in space, astronomy, technology,
special effects, and alternative social systems.
- Sep 9, 1839 - The first ever glass plate photograph was
made by John Herschel (see March 7).
Interestingly, the photo was of the famed 40-foot telescope of
John's father, William Herschel, which had been in disuse for
several decades and was shortly dismantled.
- Sep 9, 1892 - Lick Observatory astronomer Edward
Emerson Barnard discovers Jupiter's innermost satellite,
Amalthea. It was the first moon of Jupiter to be discovered
since Galileo, and was the last moon in the solar system to
be discovered through visual observations.
- Sep 17, 1811 - Thomas Jefferson observes an annular solar
eclipse with a telescope at Monticello several years after his second
term as president ends. The eclipse was unusually long, at 6 minutes
and 51 seconds, something Jefferson was interested in measuring.
- Sep 11, 1985 - ICE, the International Cometary Explorer,
makes the first ever flyby of a comet, Comet Giacobini-Zinner.
- Sep 12, 1959 - The USSR's Luna 2 becomes the first
manmade object to strike the moon.
- Sep 13, 1922 - The highest air temperature ever recorded
at the surface of the earth was measured in Libya at 136°F.
Some doubt it's reliability. Death Valley's Furnace Creek hit
134°F (56.7 °C) on July 10, 1913.
- Sep 17, 1789 - William Herschel discovers Saturn's moon
- Sep 18, 2015 - The first successful detection of
gravitational waves is made, at the Laser Interferometer
Gravitational-Wave Observatory (LIGO). The source is thought
to be a pair of merging black holes with masses roughly 20-30
times the mass of the sun, at a distance of 400 Mpc.
- Sep 19, 1848 - Saturn's moon Hyperion is discovered
by William Boyd.
- Sep 19, 1988 - Israel launches its first satellite.
- Sep 20, 1948 - The first photographic plate (#0) was
taken with the 48-inch wide field Schmidt telescope at Mt. Palomar,
by Hendricks -- who also ground and polished the highly aspherical
corrector plate upon which the design depends. The photo was of M31,
the Andromeda Galaxy, and it was a "keeper": it is reproduced at
2½x in panel #18 of the Hubble Atlas of Galaxies (1960), though
there the date is given as Sep 28/29, and it is credited to Hubble.
(The Sep 20 date is from Osterbrock's book "Pauper and Prince".) The
telescope would eventually complete the massive and still extremely
useful National Geographic / Palomar Observatory Sky Survey.
- Sep 20, 1962 - Capella was first detected in X-rays,
during the first of two Aerobee-Hi rocket flights, the second time
on March 15, 1963. The position was confirmed by a later rocket
flight on April 5, 1974, and it's X-ray luminosity measured as
~104 times that of the Sun. The cause is thought to
be a hot, massive corona surrounding the most massive component
of the four star (two binaries) system.
- Sep 23, 1846 - Johann Galle and Heinrich d'Arrest of
the Berlin Observatory see and identify the planet Neptune for
the first time. The discovery of Neptune was a major accomplishment
in the entire edifice of celestial mechanics and mathematics built up
from Newton's laws of gravity and motion over the previous 160 years.
- Sep 23, 1876 - The tight grouping of interacting
galaxies now known as Stephan's Quintet (Palomar Observatory
Sky Survey photo at right) is discovered by Édouard Stephan
in Marseilles, France, using a 31½" Foucault reflector.
The group is famous for sporting a discrepant redshift: NGC 7320 is
either a nearby foreground object, by chance seen nearly superposed
on the other four galaxies (NGC's 7317, 7318 A & B, and 7319), which
are some 7x farther, or it is part of the system but with a velocity
which differs from the other galaxies by some 6,000 km/s. The
situation is further complicated by NGC 7318B being at a recession
velocity 850 to 1,000 km/sec less than its other three companions,
making it difficult to understand how it could be a gravitationally
bound member of the group and not just passing through its center
(from the backside towards us). Off the left side of the photo,
which is 7 arc-minutes across, along the arc of NGC's 7317-19 is
NGC 7320C, with a velocity similar to 7318B.
- Sep 23, 1962 - The TV show The Jetsons premieres,
initiating a generation of cartoon-watching baby boomers into
a technologically based future.
- Sep 24, 1970 - First unmanned, automated return of
lunar material to the earth, the Soviet's Luna 16 returns with
three ounces of the moon.
- Sep 25, 1992 - NASA's Mars Observer is launched,
the first such probe to Mars in fifteen years, only to lose
communication with earth three days before reaching Mars in
late August 1993.
- Sep 26, 1785 - William Herschel first observes and
describes the three objects along the Perseus/Pegasus border
that would later (1888) be known as NGC 7619, NGC 7623, and
NGC 7626 -- the brightest galaxies in what is now recognized
as the Pegasus Cluster (I) of Galaxies, at a distance of about
50 megaparsecs (Mpc). Some ten months earlier (November 12,
1784) he had noted what's now known as NGC 7469 nearby; even
though it is cumulatively fainter than the other three, it has
a bright(er) nucleus and is now classified as a
Seyfert galaxy -- one of the six
in the original 1943 paper -- and so is easier to spot visually.
- Sep 26th, 2022 - NASA’s Double Asteroid Redirection
Test (DART) intentionally crashes into the binary asteroid 65803
Didymos, specifically the smaller component known as Dimorphos,
the smallest asteroid yet visited by a spacecraft -- albeit at
14,000 mph. Using software based on anti-missile technology,
DART autonomously guided itself to the collision, missing a
"bull's eye" by just 17 meters (50 feet). The Italian-built
minisatellite LICIAcube separated from DART fifteen days before,
and observed the impact and its aftermath (photo at right),
approaching within 35 miles of Dimorphos less than 3 minutes
afterwards. Numerous telescopes, both ground-based and in orbit,
observed the plume of dust from the intercept, as the system
was less than 7 million miles from earth.
Two weeks post-impact NASA announced the preliminary result that
the orbial period of Dimorphos had been reduced by 32 minutes,
to 11 hrs 23 mins. A decrease of only a minute or two was the
minimal expectation, so this was at the upper limit of what was
thought possible. Ejected debris that might settle back onto
Dimorphos in the future could reduce the reduction.
- Sep 27, 2007 - NASA's Dawn spacecraft was launched,
on its way to the two largest asteroids/protoplanets, Vesta
(Jul 16, 2011 to Sep 4, 2012) and Ceres (Mar 6, 2015 to the
mission's end on Nov 1, 2018)
- Sep 28, 1969 - What would become known as the
Murchison Meteorite falls near its namesake town ~100 miles
north of Melbourne, Australia. The well-observed fall, and
its fragmentation into three parts, is famous for totalling
nearly 100 kg of carbonaceous chondrite material rich in
organic compounds and water, the largest fragment being
almost 7 kg.
Somewhere around 100 different amino acids have been found in
the meteorite, five times the number used by life on earth,
so the Murchison Meterorite often comes up in discussions of
the possible seeding of future life on earth with pre-biotic
In 2020, geochemists identified bits of silicon carbide that
date to 7 billion years old, meaning they were interstellar
dust particles before becoming part of the original asteroid
the meteorite derives from; it's also the oldest known stuff
- Sep 30, 1880 - Henry Draper makes the first photo of
the Orion Nebula (M42).
- Sep 30, 2016 - The Rosetta spacecraft ends its mission
to Comet 67P/Churyumov-Gerasimenko by landing in the comet's Ma'at
region before shutting off. Rosetta measured ~3x more deuterium in
the comet's ice than water on earth contains, casting doubt on the
long-held hypothesis that earth's water originally came from comets.
©2002-2022, Chris Wetherill. All rights reserved. Display here does
NOT constitute or imply permission to copy, republish, or redistribute
my work in any manner for any purpose without prior written permission.
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