This Month in the History of Astronomy - April
- Apr 2, 1889 - Harvard Observatory's 13" refractor arrives
on Mount Wilson, a month later beginning a long astronomical legacy
at this site which housed the largest telescopes in the world from
1908-48: the 60" for the first decade followed by the 100". This
latter mirror is still the largest solid ever cast in plate glass;
weighing 4 1/2 tons it's just 13 inches thick. Subsequent large
mirrors were cast in borosilicate glass (pyrex) so they wouldn't
change shape so much as the temperature changed during a night;
the larger mirrors also evolved in the direction of being made
lighter (and stiffer) by removing some (or most) of the inside
glass on the underside of the mirror, making it more a structured
object rather than just a big disk of glass.
- Apr 3, 1842 - Hermann Vogel, the first to discover
spectroscopic binary stars (Algol and Spica), where an unseen
companion star can be deduced from the periodic Doppler shifts
in the spectral lines of the observable, brighter component. (In
the case of Algol the companion was already known to be there
because it is an eclipsing binary. At radio wavelengths the Sun
is a member of a sort of spectroscopic binary system as seen by
an outside observer, with the earth the brighter componenet, due
to our TV and military radar emissions.
The study of spectroscopic binary stars led Vogel to the further
discovery of interstellar calcium absorption lines, which are
stationary in the spectrum, part of the gradual realization that
insterstellar space is not a perfect vacuum and is not therefore
perfectly transparent. Vogel was also the first to study the
evolution of the specturm of a nova. He spent the bulk of his
career at the Potsdam Astrophysical Observatory near Berlin.
- Apr 7, 1991 - The Gamma Ray Observatory (GRO) was
- Apr 8, 2009 - First light for the Kepler spacecraft,
a 0.95 meter Schmidt telescope with 42 CCD detectors at its focal
- Apr 11, 1862 - William Wallace Campbell, pioneer
observer of stellar motions and radial velocities, and director
of Lick Observatory from 1901 to 1930. He was president of the
University of California and the National Academy of Sciences,
and the building on the UC/Berkeley campus housing the astronomy
department is named after him. In 1919 Campbell was involved in
the highly publicized observations of a total solar eclipse
looking for the deflections of starlight predicted by Einstein's
General Relativity. (see May 29)
- Apr 11, 1901 - Donald H. Menzel, born in Florence,
Colorado, and raised in Leadville, he was a Ph.D. student of
Henry Norris Russell and had a long and distinguished career,
part of it as the director of the Harvard College Observatory.
- Apr 14, 1629 - Christian Huygens, Dutch scientist and
one of the preeminent scientists of the 17th century. Besides
developing a theory of light and getting the first patent for a
pendulum clock, Huygens was the first to discern Saturn's rings
and he also discovered Saturn's largest satellite, Titan. (See
the 25th of last month.)
- Apr 15, 1707 - Leonard Euler, famed Swiss
mathematician who lost the sight of his right eye while
observing the sun (1733-4).
Euler studied under Jean Bernoulli in Basle, earning his master's
degree at age sixteen. He had positions in both St. Petersburg,
Russia (Naval College, Academy of Sciences), and Berlin, Germany
(Academy of Sciences); he knew both Frederick the Great (Germany)
and Empress Catherine the Great (Russia).
Euler made advances in celestial mechanics, and the inclusion of
tidal forces in analyzing the earth-moon-system for the first time,
but is perhaps best known for having the base of natural logarithms,
the transcendental number "e" (2.71828...), named/abbreviated after
He was also responsible for practically inventing spherical
trigonometry, for bringing integral and differential calculus
close to their modern forms, and for standardizing notation by
consisently using symbols like π, i (imaginary/complex
numbers), and Σ (summation).
- Apr 22, 1724 - Immanuel Kant, German philosopher whose
contributions ranged into the natural sciences and astronoly.
Kant's paternal grandfather was a Scottish immigrant who had germanized
their last name from Cant, according to Kant, though later scholars
At barely the age of thirty Kant developed Newton's mathematical physics
to show how the moon's tidal interaction with the earth would slow down
the earth's spin and eventually lead to the moon's synchronous rotation
with the earth. The next year he published Universal Natural History
and Theory of the Heavens, in which he laid out his "nebular
hypothesis" for the formation of the solar system from a giant spinning
gas cloud. This would naturally explain why all the planets orbit in
roughly the same plane and in the same direction. Kant further speculated
that the Milky Way was a giant spinning disk of stars that had also
formed out of a gas cloud, an even larger gas cloud, and that the faint
"nebulae" then being turned up in the largest telescopes of the day were
examples of the same thing, which turned out to be correct even though
it wouldn't be soundly established for almost another 175 years. Kant
also went on to work on the theory of the winds (the Coriolis force)
and was a big advocate for the nascent fields of both geography and
- Apr 22, 1891 - Sir Harold Jeffreys, astrogeophysicist and
the first to hypothesize the Earth's liquid core; Jeffreys also made
contributions to our understanding of tidal friction, nutation,
general planetary structure, and the origin of the solar system.
- Apr 23, 1858 - Max Planck, pioneer quantum physicist.
The Planck equation describes the amount of light energy emitted
by a "blackbody" as a function of the its temperature (on an absolute
scale) and the wavelength (or frequency) of emission. Blackbodies
are perfect absorbers of light and approximate stars over a wide
range of conditions. Other astronomical objects also radiate like
blackbodies over some or all parts of the spectrum, a notable
example being nebulae such as the Orion Nebula in the radio part
of the spectrum, where the temperature properly refers to that of
the electrons involved in the flourescence process which makes the
visible light we see.
- Apr 25, 1923 - Francis Graham Smith, pioneer British
radioastronomer who was also Director of the Royal Greenwich
Observatory during the development of Las Palmas Observatory in
the Canary Islands, which includes both optical and radio facilities.
His early investigations with Martin Ryle and his radio interferometer
were of the Sun, but the techniques developed and the expertise gained
led to the discovery and identification of both Cygnus A (a Type II
supernova remnant) and Cassiopeia A (a double lobed radio galaxy).
Smith was also involved in the systematic radio sky survey that led to
the 3C catalog (the 3rd Cambridge radio source catalog)
in 1959; the first quasar, 3C 273, was turned up a few years later
(1963) while characterizing the contents of the catalog.
Smith was also involved, again along with Ryle, in early studies of
the use of artificial satellites for navigational purposes (1957).
At the Jodrell Bank radio facility from 1964 to 1974, Smith mainly
studied pulsars, being the first to show their radiation is strongly
polarized, which led to his development of the 'relativistic beaming'
mechanism or model for pulsar radiation (1970). He also made some
of the first measures of the magnitude of interstellar magnetic
fields in the Milky Way.
- Apr 26, 1933 - Arno Penzias, Nobel Prize winner for his
part in the discovery of the cosmic microwave background radiation.
- Apr 28, 1774 - Francis Baily, a British explorer and
stockbroker until turning to astronomy at the age of 50. Baily
helped found the Royal Astronomical Society of London, revised
star catalogs, and studied meteorology. He's most remembered for
his observations of the May 15, 1836 solar eclipse, an annular
eclipse visible in Scotland, and his explanation of the phenomenon
at the beginning and ending of totality, now known as Baily's
- Apr 28, 1900 - Jan Hendrick Oort, who quantified the
Milky Way's rotation characteristics and proposed a vast, spherical
resevoir of comets (the Oort Cloud) surrounding the Sun and
stretching nearly half way to the nearest stars.
- Apr 28, 1906 - Bart Jan Bok, Dutch-born American astronomer
who had a distinguished career studying the structure and dynamics of
the Milky Way, and who, with his astronomer/wife Priscilla, authored
a classic book on the subject. As director of the Univ. of Arizona's
Steward Obs., Bok was also influential in the siting of Kitt Peak
National Observatory in the Tucson area.
- April 28, 1928 - Gene Shoemaker, American geologist
turned astro-geophysicist, who was central in the development of
the new field of planetary sciences.
Raised in Los Angeles, Shoemaker enrolled in high school at age
thirteen and graduated in just three years. Entering Cal Tech, he
got his bachelor's degree before turning twenty (1948), and his
M.Sc. degree, also from Cal Tech, the next year.
He would then go to work for the U.S. Geological Survey, searching
for uranium deposits in Utah and Colorado, which had been found to
be associated with volcanic vents. This led him to Meteor Crater in
NE Arizona, at that time accepted as a volcanic crater -- though its
discoverer (1891), Daniel Barringer, had thought it formed from a
meteor impact. Shoemaker's investigations used the shapes of craters
formed by atomic bomb tests in Nevada to show Meteor Crater was
created explosively, the case being sealed in 1960 (the year he got
his Ph.D. from Princeton) by his discovery there, with Edward Chao,
of shocked quartz.
In the 1960's, Shoemaker turned to the moon, creating its first
geologic map, using telescope photos made by Francis Pease, and
then worked with Urey and Kuiper on the Lunar Ranger imaging
program, which was preparatory to the Apollo moon landings, as
were the Lunar Surveyor missions, where he led the imaging team.
Shoemaker then became the lunar geology principal investigator
for Apollo's 11 thru 13. He became well-known for his appearances
with Walter Cronkite as an affable and moustachioed lunar geology
TV expert during coverage of the early Apollo missions.
While continuing as a consultant with NASA the rest of his life,
Shoemaker joined Cal Tech in 1969 and began the search for
earth-crossing asteroids. He would go on to be involved in
the discovery of 820 asteroids and comets, including several
families of earth-crossers -- the Apollo asteroids among them.
Comet Shoemaker–Levy 9 had already been captured by Jupiter's
gravity and tidally disrupted into more than a dozen fragments when
it was discovered in 1993; it was quickly realized that its orbit
would cause the pieces to impact Jupiter, which it did in July of
the next year.
Shoemaker was unfortunately killed in an auto accident three years
later while searching remote Australia for impact craters. In 1999
the Lunar Prospector spacecraft carried his ashes to the moon.
- Apr 30, 1777 - Karl Gauss, German mathematician, physicist,
and astronomer. Something of a precocious talent, Gauss was only fourteen
when he so impressed the Duke of Brunswick that the latter supported
him generously for the next fifteen years. By twenty-two he had earned
his doctorate, and was both professor of mathematics and director of
the Göttingen Observatory by age thirty, where he remained the
rest of his working life of nearly five decades, despite offers from
several foreign math/science powerhouse countries of the time.
During his teen years he developed the principle of least squares,
and, as a founder of what would later be called number theory, Gauss
did the first work on the frequency of prime numbers. By his early
twenties he had made the first discovery in Euclidean geometry in
2,000 years -- that a 17-sided polygon could be inscribed within a
circle using only a ruler and compass -- and had also proved a
fundamental theorem of algebra, that every equation has a complex
(or imaginary) root, and that such numbers can be described
analogously to points in a plane.
He next turned to astronomy for about the next decade. Inspired by
the discovery (1801) and subsequent 'loss' of the first asteroid,
Ceres, as it passed on the far side of its orbit behind the sun, he
developed the now classic method for determining an object's six
orbital elements from three observations, which he published in
He then extended this work into the basic theory of orbital
"perturbations", which Leverrier and Adams used several decades
later to analyze the motion of Uranus and correctly deduce the
existence and location on the sky of Neptune (1846).
In his fifties, Gauss got involved in the study of crystallography,
optics, mechanics, and capillary action. He worked with Wilhelm Weber
on electricity and magnetism, and the two invented the first telegraph
(1833). The unit of magnetic flux density (magnetic field strength) is
known as the Gauss in honor of this pioneering research, while Weber
has another unit named after him. (The first use of the letter "c"
-- for "celeritas", meaning "fast" -- for the speed of light was in
an 1856 paper by Kohlrausch and Weber.)
Discoveries and other firsts
- Apr 1, 1960 - First weather satellite, Tiros 1, launched.
- Apr 1, 1998 - NASA's TRACE (Transition Region and Coronal
Explorer) satellite is launched, providing the first high-resolution
observations of the sun from space.
- Apr 2, 1845 - First photo of the Sun taken.
- Apr 3, 33 - the date of a partial lunar eclipse rising
over Jerusalem, which biblical scholars Humphreys and Waddington
(1983) concluded was the most likely date for the crucifixion of
- Apr 3, 1966 - First lunar orbiter, Luna 10.
- April 10, 531 - One of the most spectacular meteor showers
of the first millennium, lasting about two hours, and due to a series
of close approaches to earth by Halley's Comet at its most recent three
apparitions -- in 295, 374, and 451 -- the April 1, 374, passage being
the second closest known. The earliest known record of the Eta Aquariid
meteor shower was made in 74 B.C. by Chinese astronomers, and there is
evidence from Mayan records that they used the timing of this shower to
determine the length of what we'd call the sidereal year.
- Apr 10, 2019 - The Event Horizon Telescope (EHT) project,
consisting of a collaboration of 200+ scientists at radio observatories
around the world with millimeter wavelength capabilities, releases the
first ever image of a blackhole -- the super-massive blackhole at the
center of the giant elliptical galaxy M87 in the Virgo Cluster. See
the book Einstein's Shadow (Seth Fletcher, 2018) for full
details on this latest application of Very Long Baseline Interferometry
(VLBI) at very short wavelengths.
- Apr 11, 1960 - First radio search for extraterrestrial
civilizations, started by Frank Drake (Project Ozma).
- Apr 11, 1986 - At 65 million km (0.43 AU), Halley's Comet
is closest to Earth for this apparition.
- Apr 12, 1961 - First man in space, Yuri Gagarin, makes
one orbit aboard Vostok 1.
- Apr 12, 1981 - First Space Shuttle (Columbia) launch.
- Apr 14, 1969 - Nimbus 3 was launched, the first weather
satellite with an infrared spectrometer to measure earth's atmospheric
composition and temperature profile. In all there were seven Nimbus
earth observing platforms launched over a fourteen year period
(1964-78), and Nimbus 3 was the first to test a radioisotope
thermoelectric power generator; these kinds of power units were
subsequently used on many missions to the outer planets, where
sunlight is so dim that solar panels are impractical.
- April 16, 2019 - Abraham Loeb and Amir Siraj at
Harvard report the discovery of the first meteor of interstellar
origin, based on a back-tracking of its well-observed
- Apr 17, 1912 - The unusual hybrid eclipse of the sun
just two days after the sinking of the Titanic in the nearly new
moon North Atlantic.
A hybrid eclipse is a combination annular and total solar eclipse,
where the moon and sun are almost exactly equal in angular size, so
that the difference in the distance to the moon from different places
on the curved surface of the earth can swing the eclipse one way or
the other. Such eclipses start and end annularly, with a period of
totality centered on the eclipse midpoint, which experiences the
eclipse at local sundial noon and so is at the center of the region
on earth closest to the moon (so it appears slightly bigger).
For this eclipse, which was about as close to not being a hybrid
eclipse as it's possible to be, the brief period of totality (only
2 seconds max, and but 1 km wide) occured in Spain and Portugal.
The beginning of the period of slight annularity passed just to the
NW of Paris -- it's first eclipse of such a magnitude in 188 years
-- before continuing NE through northern Europe and NW Russia. The
other end of the annular eclipse touched the region in N South
America where Venezuela and Brazil meet before heading out across
the ocean towards Europe.
This Saros (#137) had one more hybrid eclipse in it (1930). All
eclipses in the cycle subsequently have been and will be annular,
including the one on the Summer Solstice in 2020 -- until the 26th
century, when they will become only partial.
- April 18, 2019 - The first molecule thought to have
formed in the early universe, helium hydride (HeH+),
is announced in Nature to have been found out in the
current universe by Rolf Güsten and collegues at the Max
Planck Institute for Astronomy, using NASA's Stratospheric
Observatory for Infrared Astronomy (SOFIA - a modified Boeing
747) to observe the planetary nebula NGC 7027 at a sub-millimeter,
far IR wavelength of 149.137 microns (2,011.57 GHz).
Helium was practically the only element besides hydrogen to have
formed in big bang cosmologies, and it has the highest ionization
potential for any (neutral) atom, 24.6 eV. This corresponds to
a hard-UV wavelength of light of 508.4Å (or less), which
is only emitted in appreciable quantities by the very hottest
objects (~100,000+°K), like the central stars of the youngest
planetary nebulae -- or the universe at an age of several hundred
The molecule is fragile and easily reacts in a cosmic environment
with abundant hydrogen to liberate the helium atom and form an
H2 molecule, this second reaction becoming dominant
when the helium hydride production rate falls off as the
temperature declines. This unusual molecule was first synthesized
on earth in the laboratory in 1925.
- Apr 22, 1715 - The first total solar eclipse predicted well
enough and far enough in advance (by Edmond Halley) that someone --
Monsieur le Chevalier de Louville, of the Royal Academy of Sciences
in Paris -- could travel in advance to observe it, in this case to
London, which hadn't been in the path of totality for 575 years.
- Apr 24, 1970 - China launches its first satellite.
- Apr 25, 1990 - The Hubble Space Telescope is deployed from
the space shuttle Challenger, having been launched the day before.
- Apr 26, 1920 - Famed Shapley-Curtis debate on the nature
and distance of spiral nebulae, aka galaxies, takes place in Wash DC.
- Apr 27, 1961 - Explorer XI was launched, the first satellite
with a gamma ray detector. Only energies greater than 100MeV could
be detected, and in all less than 100 photons were counted. The
result implied there was no prodigous matter-antimatter annihilation
going on in the universe (proton-antiproton in this case). The result
was mentioned by Kennedy in his State of the Union speech (1/31/1962);
Dirac had first raised the speculative possibility of there being
cosmic antimatter in his 1933 Nobel address.
©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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