This Month in the History of Astronomy - July
- Jul 1, 1916 - Iosif Samuilovich Shklovsky, Russian
astrophysicist especially known for his wide ranging work in the
pioneering field of radio astrophysics. Shklovsky was the first
to work out that the radio wavelength radiation received from the
Sun originated in the ionized gas of the corona (1946). He then
went on to show that the radio wavelength radiation from the
supernova remnant known as the Crab Nebula (M1) was synchrotron
radiation, produced by high energy electrons circulating in
strong magnetic fields. He also developed a way to separate
radio energy detected from the Milky Way into thermal (produced
by hot objects) and non-thermal components (like synchrotron
Shklovsky proposed that cosmic rays produced by supernovae
explosions within about 300 light years of earth in the past
might have been responsible for some of the mass extinction
events found in the fossil record. And, in 1967, before pulsars
were discovered, he studied the optical and x-ray radiation
coming from Scorpius X-1 and determined that it could be
explained by material accreting onto a neutron star.
- Jul 17, 1894 - Georges Édouard Lemaître,
Belgian astrophysicist and cosmologist, who also, during WWI,
witnessed the first use of poison gas (chlorine) in the history
- Jul 19, 1846 - Edward Pickering, pioneering American
observer and Harvard College Observatory director from 1876 to 1919;
this was the era of the introduction of photography in astronomy and
the Harvard plate collection started during Pickering's tenure is
still a valuable archival source of data.
Pickering was the first to adopt Norman Pogson's 1854 suggestion for
a magnitude scale which is essentially the one still in use today (5
mags -> 100x), though he chose Polaris (at m=2.1) as the fundamental
reference star, not realizing that it is slightly variable. The
first great catalog of stellar magnitudes, containing 4,260 stars,
was published in 1884. By 1908 the number was up to 45,000 stars,
and it is estimated Pickering made 1.5 million visual photometric
measures during its compilation.
Pickering was also responsible for the introduction of stellar color
indices (c.1890), which compare brightnesses at blue and yellow
wavelengths. After it was recognized that stars radiate thermally,
like hot, dense solids, the color index became central to knowing
what a star's photospheric ("surface") temperature was:
Pickering made advances in spectroscopy such that it became possible
to observe several stellar spectra simultaneously, leading in time
to the Henry Draper Catalog (1918), with 225,320 stars, which is the
basis for all modern stellar spectroscopic classification. HD numbers
are still commonplace in observational astronomy for relatively
bright stars -- brighter than about 9th magnitude -- since on average
there are several (~5) per square degree of sky.
As if all that wasn't enough, Pickering also was instrumental in
the construction of the first all-sky photographic atlas (1903),
utilizing a southern observing station in Peru run by his brother.
It consisted of 55 wide field plates and went down to the 12th
Besides being awarded the Royal Astronomical Society's Gold Medal
twice, Pickering became publicly well known in 1909 via an
article in the magazine Popular Astronomy, when, in the wake
of Percival Lowell's promotion of the idea that there was intelligent
life on Mars, he suggested the practicality of signaling them with
sunlight reflected off a mirror a half square mile in area.
- Jul 22, 1784 - Friedrich Bessel, German astronomer and
mathematician, who was the first ever, in 1837, to measure a star's
parallax. (The star was 61 Cygni and the parallax was a mere 1/3rd
of an arc-second). This ended a debate that dated back two millenia
to the Greeks (Aristotle) about the distances to the stars. Bessel
is also remembered for the mathematical functions that bear his name
(1817), which appear in many areas of mathematical physics.
Bessel's early work on the orbit of Halley's Comet (1804) so impressed
Heinrich Olbers that a position was found for him, and four years later
the Prussian government put Bessel in charge of constructing the first
large German observatory (1810-13), at Königsberg, where he was
professor of astronomy and director until his death (1846). There he
compiled accurate positions for 50,000 stars, and is also credited with
the first two astrometric binaries: uncovering the invisible companions
Sirius B and Procyon B by wiggles in the motions of the primary stars.
Before he died, Olbers said his most important contribution to astronomy
had been in recognizing and furthering Bessel's genius.
- Jul 25, 1573 - Christoph Scheiner, German astronomer who
made early studies of sunspots, made improvements to the helioscope
and telescope, and invented the pantograph (1603-05) -- a favorite
device of Thomas Jefferson 150 years later, which copies documents,
plans, or drawings to any scale.
Scheiner started observations of the Sun in 1611 with a well-mounted
telescope he himself built, and was perhaps the first to understand
the utility of projecting the image onto a white screen rather than
viewing it directly through an eyepiece. He was the first to measure
the Sun's period of rotation using sunspots, as well as noting the
brighter patches known as faculae.
His early observations of sunspots were communicated to Galileo and
Kepler using a pseudonym (because of his employment by the church),
though he incorrectly thought they were small planets in close orbit
about the Sun. Galileo somehow figured out Scheiner's identity and
hinted he was guilty of plagiarism, even though confirmation of
Galileo's discovery of sunspots was coming in from several other
venues as well.
Scheiner observed both Mercury and Venus at inferior conjunction
and concluded they orbited the Sun, but held to the geocentric
view that the earth was stationary at the center of the universe.
Through years of observations he was able to be the first to measure
the tilt of the Sun's sunspot rotation axis relative to the ecliptic,
at 7½° -- only ¼° higher than the value in use
- Jul 25, 1920 - Chushiro Hayashi, Japanese astrophysicist
and cosmologist who made important early contributions to the theory
of a hot Big Bang. He was also a pioneer in modeling star formation
and stellar evolution, and is the namesake of the pre-main sequence
stellar evolutionary 'Hayashi tracks' in the Hertzprung-Russell
In 1950, two years after Alpher, Bethe, and Gamow had first worked
out the details of how nuclear synthesis in a very early, hot, dense
universe would have built up heavier elements due to neutrons
combining with protons, and in decreasing proportions as their mass
increased (as is observed - gold is rare, carbon common), Hayashi
showed that the weak (nuclear) interaction would have 'frozen' the
ratio of neutrons to protons (at ~1:6) two or three seconds after
t=0, making the resulting primordial helium abundance in the universe
independent of density, and dependent almost entirely on the exact
nature of the weak interaction.
In 1970 Hayashi won the Eddington Medal, awarded by the Royal
Astronomical Society for investigations of outstanding merit in
theoretical astrophysics, and in 2004 the Bruce Medal, awarded
by the Astronomical Society of the Pacific for outstanding
contributions to astronomy over a lifetime.
- Jul 30, 1878 - Joel Stebbins, U.S. astronomer who
first developed techniques for photoelectric photometry -- the
measurement of the brightness of stars (and other objects)
with electronic light meters.
Stebbins got his Ph.D. at the Lick Observatory (1901-03) and
observed there at times until the age of 80, while being at the
Washburn Observatory and University of Wisconsin from 1922-48;
inbetween he was at the University of Illinois.
From 1909-25 he worked on the development of the photo cell,
using it to measure light curves of eclipsing binary stars. By
the 1930's he had moved into measuring the effect of interstellar
absorption on starlight and the distribution of interstellar dust
in the Milky Way. He also made early determinations of how this
absorption depends on the wavelength of light.
Discoveries and other firsts
- Jul 1, 1770 - Lexell's comet passes a mere 2.3 million km
from Earth, less than 9 times the distance to the Moon.
- Jul 1, 1917 - The 100" mirror arrived on Mt. Wilson.
Businessman John D. Hooker donated the funds for the glass, which
was the same as that used for the wine bottles made by the Saint
Gobrain Glassworks in France. This was the last regular glass
large telescope mirror made before Pyrex became the standard.
- Jul 2, 1967 - The Vela gamma-ray satellite was launched
with the intention of detecting nuclear bomb explosions but became
famous for its serendipitous discovery of natural gamma-ray bursters.
GRBs, the brightest electromagnetic events known to occur in the
universe, were almost a complete mystery for decades, until it was
possible to catch a few in other wavelength bands simultaneously.
This placed GRBs in distant galaxies, where they could possibly be
due to supernovae forming black holes or colliding/merging neutron
star explosions, or stars being tidally torn apart by close
passage by a black hole.
- Jul 3, 2005 - The Deep Impact spacecraft crashes an 800
pound ballistic projectile into Comet Tempel 1 with the energy of
about 5 tons of TNT -- to kick up ices and dust from its surface
for its flyby mothership to analyze.
- Jul 4, 1054 - The bright supernova recorded by Chinese
astronomers and known today as Messier 1, the Crab Nebula, is
thought to have detonated on this day. It was visible during the
daytime for about two weeks, and was likely also seen (and recorded)
by Anasazi astronomers in the Four Corners area, based on rock art
- Jul 5-6, 1687 - The publication date of Isaac Newton's
Principia, with his three famed laws of motion. Edmond
Halley was instrumental in getting the work to press, only 600
copies of the 500 page book were printed, and the estimate is
that Newton's style of writing was such that there were only
about 100 people in the world who were able to understand it.
- Jul 9, 1979 - Voyager 2 made its closest approach to
Jupiter. Over approximately the next decade it would fly by the
other three gas giant planets in the outer solar system, being
the first to both Uranus and Neptune.
- Jul 11, 1991 - A total solar eclipse passes over Mauna
Kea, Hawaii, and the largest telescope in the world (the Keck I
- Jul 14, 1965 - First ever flyby of Mars: Mariner 4.
The spacecraft sent back 22 pictures showing a heavily cratered,
barren, almost lunar surface, finally putting to rest the notion
of there being any advance lifeforms or civilization on the planet.
- Jul 14, 2015 - The New Horizons spacecraft, almost
a decade after its launch in 2006, became the first spacecraft
to fly by Pluto. Two of its five moons (Kerberos and Styx) were
only discovered after launch (in 2011 and 2012), and so were only
barely able to be worked into the imaging plans, and two of the
other three (Nix and Hydra) had only been discovered by the Hubble
telescope about six months before launch.
- Jul 16, 1850 - The first photograph of a star (Vega)
other than the Sun was made at the Harvard Observatory.
- Jul 16, 1994 - The first of nearly two dozen fragments
from Comet Shoemaker-Levy 9 impacts Jupiter, leaving the most
visible features in the planet's atmosphere ever seen.
- Jul 16, 2186 - Future history... the total solar
eclipse with the longest duration between 2000 B.C. and
3000 A.D., 7 minutes and 29 seconds, will occur.
- Jul 17, 334 - The Sicilian Firmicus is the first to
report seeing solar prominences during an eclipse of the sun
(annular in this case).
- Jul 17, 1963 - The Nuclear Test Ban Treaty is signed,
prohibiting the detonation of nuclear devices in the atmosphere.
In order to monitor compliance with the treaty, the U.S.
subsequently launches the first gamma ray detectors into orbit.
In 1967 these inadvertently discover the first of many cosmic
gamma ray sources, which are even today among astronomy's
- Jul 17, 2017 - In what was called "the most challenging
stellar occultation in the history of astronomy," NASA's New
Horizons team, in a remote region of Argentina, observed in the
direction of the distant Kuiper Belt object known as 2014
MU69 using a "picket fence" of 24 mobile telescopes,
of which five recorded the occultation. Only the Hubble Space
Telescope can detect the 22-40 km diameter object directly (using
reflected sunlight), which is over a billion miles farther from
the sun than Pluto. The observations help refine the orbit of
2014 MU69 for the Jan 1, 2019, flyby by the New
- Jul 18, 1860 - First wet plate photo of an eclipse of
the sun, the new process being 30x faster than a daguerreotype.
- Jul 18, 1980 - India launches its first satellite.
- Jul 19, 418 - The historian Philostorgius reports the
first comet discovered during a total solar eclipse in Asia Minor.
- Jul 19, 2020 - The shortest sidereal day on record, by
1.4602 milliseconds, since atomic clocks started keeping track of
the earth's spin rate. A total of 27 leap seconds have been added
to Coordinated Universal Time (UTC) since the 1970’s to keep it
synched up with the earth's rotation and generally lengthening
days, the last of these being in 2016. Since the Earth is now
spinning faster than at any time in the last 50 years there's
talk of possibly having to add "negative leap seconds", and 2021
is expected to be the shortest year yet measured, to the tune of
~½ millisecond per day. The exact cause is not known but could
be due to material of differeng densities in the core (or mantle)
of the earth rearranging itself in terms of its distance from the
center, changing the earth's moment of rotational inertia.
- Jul 20, 1969 - The first humans set booted foot on the
Moon: Apollo 11 lander with Neil Armstrong and Edwin Aldrin. Most
people don't realize that Armstrong's famous words were meant to be
"A small step for a man. A giant leap for mankind." The small
stumble was the cause of the inordinately long pause between the two
halves of the utterance, while Armstrong realized that on live TV
there was only going to be the one 'take'.
- Jul 20, 1976 - Viking 1 lands on Mars and the first images
ever taken from that planet's surface are returned.
- Jul 22, 1962 - The first ever attempt at an interplanetary
spaceflight, Mariner 1, launched to fly-by Venus, fails less than
5 minutes after launch and just seconds before the upper stage
carrying the spacecraft was set to separate from its first stage
booster rocket. About a month later (see Aug 27) Mariner 2 would successfully complete the mission.
- Jul 26, 2006 - The first ever observed transit of a
Uranian moon (Ariel) was made by the Hubble Space Telescope (photo
at right), the first opportunity to do so since 1965 because of
the extreme tilt of Uranus's spin axis and the moon's orbit being
more or less in the plane of Uranus's equator.
- Jul 28, 1917 - The announcement of the discovery of a nova
in the galaxy NGC 6946 at magnitude 14.6 with the 60" telescope on
Mount Wilson by George W. Ritchey.
As described by Sandage in the Hubble Atlas of Galaxies (1961),
the nova itself was not important except for it causing Ritchey to get
curious and start carefully inspecting the entire, nearly decade long,
60" plate collection for others like it, which netted two more novae
that had been photographed in M31 in 1909.
At the Lick Observatory, Heber D. Curtis inspected the plate collection
made with the 36" Crossley reflector and found novae in NGC's 4227 and
4321 -- two in the latter, both fainter than magnitude 15½. Within
months others had chimed in and there were then eleven novae known in
seven galaxies, with four in M31. Within two years, new observations had
put the M31 total up to twenty-two.
One of these, known as S Andromedae (the second ever variable star, as
nova were classified then, discovered in that constellation) had been
observed in 1885 by Hartwig in M31 at a magnitude of 5.8 (though Sandage
says 7.2). A decade later, Z Centauri had been discovered by Fleming at
magnitude 8 near the center of NGC 5253. Earlier that same year, VW
Virginus had been found by Wolf at magnitude 12½ in NGC 4424 in
the Virgo Cluster of galaxies.
This confused state of affairs led Knut Lundmark in Sweden to sort
things out in his 1920 Ph.D. thesis, discovering it made sense if there
were two classes of novae: novae of the sort seen in the Milky
Way -- twenty-six were known at that time -- and for which distances
and thus absolute magnitudes could be determined to a greater or lesser
degree; and the class of what are now called supernovae, 12 magnitudes
S And, and the "novae" seen in distant galaxies, were in this latter,
new class, while the new ones being turned up in nearby M31 were of
the former, old class. What was more, the apparent brightnesses
observed only made sense if M31 was 650,000 light years away, and
thus far outside the Milky Way. The supernovae seen in other galaxies
were so faint these objects must be many times (30x) further still.
People were not immediately convinced by these incredible and
revolutionary ideas from a new Ph.D., though the idea of the
possibility of there being two classes of novae had first been
floated by Heber D. Curtis (1917), but it would be four more years
before Hubble, working with the new 100" telescope, turned up
Cepheid variable stars in M31, M33, and NGC 6822 and finally
settled the issue of whether or not the "spiral nebulae" were
"island universes", as well as establishing the reality of the
new class of exploding stars, though it would be several decades
before it was understood what kind of explosions were taking place
in both classes.
- Jul 28, 1851 - The first photograph of the Sun during
a total eclipse is made, a daguerreotype by Berkowski at
- Jul 29, 1878 - A total solar eclipse passes across parts
of the U.S. The eclipse was observed by the distinguished Simon
Newcomb (see March 12), whose sketches
were widely reproduced, and still seen today. The event caused such
a stir that even Thomas Edison going to Denver to see it -- Colorado
had only been a state for less than two years (so attack by "injuns"
was still a widely hyped if somewhat mistaken fear just two years
after Custer's Last Stand) -- was national front page news. The
eclipse was also the occasion for the first known astronomical
observations from the summit of Pikes Peak, by Samuel Pierpont
Langely. The Carriage Road to the top was still a decade in the
future, and the Army Signal Service's Weather Observatory was less
than five years old at the time.
©2002-2021, 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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