April 30: Johann Karl Burckhardt

Johann Karl Burckhardt

April 30, 1773 – June 22, 1825

Johann Burckhardt was a German-born astronomer and mathematician who later became a naturalized French citizen.

Burckhardt was born in Leipzig where he studied mathematics and astronomy. Later he became an assistant at the Gotha Observatory and studied under Franz Xaver von Zach. On von Zach's recommendation he joined the Paris Observatory, then directed by Jérôme Lalande. Burckhardt focused his studies on the orbits of comets. He joined the Bureau des Longitudes in 1795. In 1799, he became a naturalized French citizen. He was elected to the L'Institut National des Sciences et des Arts in 1804. After Lalande died in 1807, Burckhardt succeeded him as director of the Paris Observatory.

Between 1812 and 1825, he published Tables de la Lune, based on a lunar theory distinct from that of Pierre-Simon Laplace. Burckhardt's lunar tables enjoyed a substantial reputation for a time, and they were officially used for computing the lunar ephemeris in the Nautical Almanac from 1821 to 1861 (superseded in part, as from 1856, for the lunar horizontal parallax, by tables due to J C Adams, and eventually replaced altogether, for issues from 1862, by the tables based on P A Hansen's more comprehensive lunar theory).

The Lunar crater Burckhardt is named in his honor.

April 20: Albrecht Unsöld

Albrecht Otto Johannes Unsöld

April 20, 1905 – September 23, 1995

Albrecht Unsöld was a German astrophysicist known for his contributions to spectroscopic analysis of stellar atmospheres.

Albrecht Unsöld was born in Bolheim (Württemberg), Germany. After school attendance in Heidenheim, Unsöld studied physics at the University of Tübingen and the University of Munich. At Munich, he studied under Arnold Sommerfeld, and was granted his doctorate in 1927. As a Fellow of the Rockefeller Foundation, he was an assistant in Potsdam and worked at the Mount Wilson Observatory in Pasadena, California. He then completed his Habilitation in Munich in 1929. In 1930, he was an assistant at the Institute of Theoretical Physics at the University of Hamburg. In September 1932, Unsöld became Ordinarius Professor and Director of the Institute for Theoretical Physics (and Observatory) at the University of Kiel – a position he held until his emeritus status was granted in 1973, after which he remained scientifically active for 15 years.

While a student at Munich, Unsöld was one of many of a long line of students who helped Sommerfeld explore and advance atomic theory.

At Kiel, Unsöld made an intensive study of the effects of abundances, radiation damping, Doppler shifts, electric fields, and collisions on the formation and shape of spectral lines in stellar atmospheres. His analysis of the B0 star Tau Scorpii, obtained on his 1939 visit to Yerkes and McDonald Observatories, provided the first detailed analysis of a star other than the Sun, and he was able to determine the physics and composition of the star’s atmosphere.

From 1947 to 1948, Unsöld was President of the Astronomische Gesellschaft.

Unsöld edited the Zeitschrift für Astrophysik until it was merged with other European journals into Astronomy and Astrophysics. He was also the author of many books, and his book Physik der Sternatmosphären mit besonderer Berücksichtigung der Sonne was a bible for quantitative stellar spectroscopy and related fields, with special emphasis on the Sun. 

Unsöld received the Bruce Medal (1956) and the Gold Medal of the Royal Astronomical Society (1957). The asteroid 2842 Unsöld is named in his honor.

April 19: Grigory Shajn

THE 2.6 m SHAJN REFLECTOR AND SPECTROGRAPHS

Grigory Abramovich Shajn

April 19, 1892 – August 4, 1956

Grigory Abramovich Shajn was a Soviet/Russian astronomer. In modern English transliteration, his surname would be given as Shayn, but his astronomical discoveries are credited under the name G. Shajn. Nonetheless, his last name is sometimes given as Schajn.

He was the husband of Pelageya Shajn née Sannikova, who was also a Russian astronomer.

He worked on stellar spectroscopy and the physics of gaseous nebulas. Together with Otto Struve, he studied the rapid rotation of stars of young spectral types and measured the radial velocities of stars. He discovered new gaseous nebulas and the anomalous abundance of Carbon-13 in stellar atmospheres.

He became a member of the Soviet Academy of Sciences in 1939, and was also a member of various foreign societies such as the Royal Astronomical Society. From 1945 to 1952 he was the director of the Crimean Astrophysical Observatory.

The double 40-cm astrograph manufactured by the Karl Zeiss Jena firm was mounted at the Crimean Observatory in 1949. The first program was a plan by G. Shajn on a deep spectral and photometrical investigation of the Milky Way. One of the cameras was completed by an objective prism with the angle 6.9°, which registered the spectra of stars. Another camera was used to obtain direct images for stellar photometry.

He also discovered a few asteroids and co-discovered the non-periodic comet C/1925 F1 (Shajn-Comas Solá), also known as Comet 1925 VI or Comet 1925a. However, the periodic comet 61P/Shajn-Schaldach was co-discovered by his wife rather than by him.

The crater Shayn on the Moon is named in his honor.

April 18: Max Waldmeier

Max Waldmeier

April 18, 1912 - September 26, 2000

Max Waldmeier was one of the leading personalities in solar physics of the 20th century.

He graduated from ETH Zurich in 1935 at the age of 23 and got his PhD there two years later on a thesis about the laws that govern the sunspot activity. He was professor at ETH Zurich and at the University of Zurich during 34 years, 1945 - 1979. During these years he also served as Director of the Eidgenössische Sternwarte (Swiss Federal Observatory) in Zurich. In 1948 he organized in Zurich the first General Assembly of the International Astronomical Union that took place after World War II. He served as a member of the Editorial Board of Solar Physics from 1968 to 1977.

Waldmeier founded the Astrophysical Observatory Arosa in 1939, a mountain station where he could carry out his observations of the Sun's corona. This was one of the first coronagraphic stations since Lyot's invention of the coronagraph. In 1951 he built a solar tower telescope in Zurich and in 1957 the Specola Solare in Locarno, a solar station on the southern side of the Alps, where the weather is usually complementary to that in Zurich. He carried out more than 20 expeditions to study the corona at solar eclipses. 

The Swiss Federal Observatory under the direction of Waldmeier served as the World data Center for Sunspots and was responsible for the IAU publication Quarterly Bulletin on Solar Activity. More than 30 observatories around the world sent their sunspot observations to Zurich, where they were reduced together with the Swiss observations (in Zurich and Locarno) to form the Zurich Sunspot Number, carefully calibrated to a common scale. This continuous record of relative sunspot numbers was initiated in 1855 by the first Director of the Swiss Federal Observatory, Rudolf Wolf.

Waldmeier's systematic observations of various aspects of solar activity, from the photosphere to the corona, spanned several decades and have provided the foundation for much of our current knowledge about the Sun's activity cycle. Much of what we take for granted today can actually be traced back to pioneering work by Waldmeier.

April 17: Giovanni Battista Riccioli

Giovanni Battista Riccioli
April 17, 1598 – June 25, 1671

Giovanni Riccioli was an Italian astronomer. He was a Jesuit who entered the order in 1614. He was also the first person to measure the rate of acceleration of a freely falling body.

Riccioli was born in Ferrara, Italy. He devoted his career to the study of astronomy, often working with Francesco Maria Grimaldi. He wrote the important work Almagestum novum in 1651. By necessity, he opposed the Copernican heliocentric theory though praising its value as a simple hypothesis.

He and Grimaldi extensively studied the Moon, of which Grimaldi drew a map. Much of the nomenclature of lunar features still in use today is due to him and Grimaldi. He also observed Saturn, and was one of the first Europeans to note that Mizar was a double star.

Other books he wrote were: Geographiae et hydrographiae reformatae libri (1661), Astronomia reformata (1665), Chronologia reformata (1669) and Tabula latitudinum et longitudinum (published in 1689).

Despite his stated opposition to Copernicus's theory he named the prominent lunar crater Copernicus after him, and other important craters were named after other proponents of the theory: Kepler, Galileo and Lansbergius. Craters that he and Grimaldi named after themselves are in the same general vicinity, while some other Jesuit astronomers have craters named after them in a different part of the Moon, near Tycho. This is sometimes considered to be tacit sympathy for Copernican theory, which as a Jesuit he could not publicly express.

Between 1644 and 1656, he was occupied by topographical measurements, working with Grimaldi, determining values for the circumference of Earth and the ratio of water to land. Defects of method, however, gave a less accurate value for degrees of the meridian than Snellius had achieved a few years earlier. Snellius had been mistaken by approximately 4,000 meters; but Riccioli was more than 10,000 meters in error [Hoefer, 1873]. Riccioli had come up with 373,000 pes despite the fact that references to a Roman degree in antiquity had always been 75 milliare or 375,000 pes.


Image: G.B. Riccioli, Almagestum Novum (1651). The image portrays Urania, the muse of astronomy, weighing up the rival systems of Copernicus, in which the earth moves round the sun, and Riccioli himself, in which the earth remains stationary at the center of the universe. The older system of Ptolemy has already been discarded and lies on the ground alongside.

April 16: John Hadley

John Hadley

April 16, 1682 – February 14, 1744

John Hadley was an English mathematician, inventor of the octant and precursor to the sextant around 1730.

In 1717 he became member (and later vice-president ) of the Royal Society of London.

The octant is used to measure the altitude of the Sun or other celestial objects above the horizon at sea. A mobile arm carrying a mirror and pivoting on a graduated arc provides a reflected image of the celestial body overlapping the image of the horizon, which is observed directly. If the position of the object on the sky and the time of the observation are known, it is easy for the user to calculate his own latitude. The octant proved extremely valuable for navigation and displaced the use of other instruments such as the Davis quadrant.

An American, Thomas Godfrey, independently invented the octant at approximately the same time.

Hadley also improved the reflecting telescope.

Mons Hadley and Rima Hadley on the Moon are named in his honor.

April 15: Leonardo da Vinci

Leonardo di ser Piero da Vinci

April 15, 1452 – May 2, 1519

Leonardo da Vinci was an Italian polymath, being a scientist, mathematician, engineer, inventor, anatomist, painter, sculptor, architect, botanist, musician and writer. Leonardo has often been described as the archetype of the renaissance man, a man whose unquenchable curiosity was equaled only by his powers of invention. He is widely considered to be one of the greatest painters of all time and perhaps the most diversely talented person ever to have lived. Helen Gardner says "The scope and depth of his interests were without precedent...His mind and personality seem to us superhuman, the man himself mysterious and remote".

Leonardo is revered for his technological ingenuity. He conceptualised a helicopter, a tank, concentrated solar power, a calculator, the double hull and outlined a rudimentary theory of plate tectonics. Relatively few of his designs were constructed or were even feasible during his lifetime, but some of his smaller inventions, such as an automated bobbin winder and a machine for testing the tensile strength of wire, entered the world of manufacturing unheralded. As a scientist, he greatly advanced the state of knowledge in the fields of anatomy, civil engineering, optics, and hydrodynamics.

Leonardo's approach to science was an observational one: he tried to understand a phenomenon by describing and depicting it in utmost detail, and did not emphasize experiments or theoretical explanation. Since he lacked formal education in Latin and mathematics, contemporary scholars mostly ignored Leonardo the scientist, although he did teach himself Latin. In the 1490s he studied mathematics under Luca Pacioli and prepared a series of drawings of regular solids in a skeletal form to be engraved as plates for Pacioli's book De Divina Proportione, published in 1509.

The interest in Leonardo's genius has continued unabated; experts study and translate his writings, analyse his paintings using scientific techniques, argue over attributions and search for works which have been recorded but never found. Liana Bortolon, writing in 1967, said: "Because of the multiplicity of interests that spurred him to pursue every field of knowledge ... Leonardo can be considered, quite rightly, to have been the universal genius par excellence, and with all the disquieting overtones inherent in that term. Man is as uncomfortable today, faced with a genius, as he was in the 16th century. Five centuries have passed, yet we still view Leonardo with awe."

April 14: Annie Russell Maunder

The original butterfly diagram drawn by Walter Maunder and Annie S. D. Maunder in 1904

 has been on display for some time at the High Altitude Observatory in Colorado.

Annie Scott Dill Maunder, née Russell

April 14, 1868 – September 15, 1947

Annie Maunder was a British astronomer and mathematician.

Annie received her secondary education at the Ladies Collegiate School in Belfast. Winning a prize in an 1886 internediate school examination, she was able to sit the Girton open entrance scholarship examination, and was awarded a three year scholarship. She studied at Cambridge University (Girton College) and in 1889 she passed the degree examinations with honors, as the top mathematician of her year at Girton, and ranked Senior Optime in the university class list. However the restrictions of the period did not allow her to receive the B.A. she had otherwise earned.

In 1891 she began work at the Greenwich Royal Observatory, serving as one of the "lady computers" assigned to the solar department. There she assisted E. Walter Maunder, and she spent much time photographing the Sun and tracking the movements of sunspots. (The solar maximum was in 1894.) The two were married in 1895, his second marriage, and Annie was required to resign from her job. However the two continued to collaborate, with Annie accompanied Walter on solar eclipse expeditions.

She published "The Heavens and their Story" in 1910, with he husband as the co-author. (She was credited by her husband as the primary author.) In 1916 she became the first woman elected to the Royal Astronomical Society. The investigations of the couple demonstrated a correlation between the variation in sunspot numbers and the climate of the Earth, leading to the discovery that the decrease period of solar activity during the Maunder Minimum likely resulted in the "little ice age".

Walter and Annie did not have any children, although he had five children from his previous marriage. Walter died in 1928, and she died in London, England in 1947.

The Lunar crater Maunder was co-named for Walter and Annie Maunder.

April 13: Bruno Benedetto Rossi

Bruno Benedetto Rossi

April 13, 1905 – November 21, 1993

Bruno Rossi was a leading Italian-American experimental physicist. He made major contributions to cosmic ray and particle physics from 1930 through the 1950s, and pioneered X-ray astronomy and space plasma physics in the 1960s.

Rossi was born in Venice, Italy. After receiving the doctorate degree from the University of Bologna, he began his career in 1928 as assistant at the Physics Institute of the University of Florence where he made his first discoveries regarding the nature of cosmic rays. In 1932 he was called to the University of Padua as professor of experimental physics. There, in addition to teaching and research, Rossi planned the new Physics Institute of the University and oversaw its construction. In the fall of 1938 he was expelled from his position as a result of the racial decrees of the fascist state. Rossi was Jewish and so was his wife, Nora Lombroso (granddaughter of anthropologist, Cesare Lombroso), so they had to leave Italy and traveled to America with brief stays in Copenhagen, Denmark and Manchester, England.

They arrived at the University of Chicago in June 1939 where he was given a temporary position as research associate. Rossi immediately began a series of experiments that yielded the first proof of the decay of a fundamental particle, the mesotron, now called muon, and a precise measurement of its mean life at rest. The latter was achieved at Cornell University where he was appointed associate professor in 1942. During the war Rossi worked first as consultant on radar development at the Radiation Laboratory of the Massachusetts Institute of Technology, and then at Los Alamos as co-director of the Detector Group responsible for development of instrumentation for experiments that supported the development of the atomic bombs.

In the fall of 1946 Rossi was appointed professor of physics at MIT where he established the Cosmic Ray Group to investigate the nature and origins of cosmic rays and the properties of the sub-nuclear particles produced in the interaction of cosmic rays with matter. In the late 1950s, when particle accelerator experiments had come to dominate experimental particle physics, Rossi turned his attention to exploratory research made possible by the new availability of space vehicles. At MIT he initiated rocket experiments that pioneered the direct measurements of the interplanetary plasma. As a consultant to American Science and Engineering, Inc. he initiated the rocket experiments that discovered the first extra-solar source of X-rays, Scorpius X-1. Rossi was made Institute Professor at MIT in 1965.

Among his contributions to the electronic techniques of experimental physics are the inventions of the coincidence circuit (Florence 1930), the time-to-amplitude converter (Cornell 1942) and the fast ionization chamber (Los Alamos, with H. Staub 1943).

Rossi retired from MIT in 1970. From 1974 to 1980 he taught at the University of Palermo. In 1990 his autobiography, titled Moments in the Life of a Scientist, was published by Cambridge University Press. 

April 12: Edward Walter Maunder

Edward Walter Maunder

April 12, 1851 – March 21, 1928

Edward Maunder was an English astronomer best remembered for his study of sunspots and the solar magnetic cycle that led to his identification of the period from 1645 to 1715 that is now known as the Maunder Minimum.

In 1873 Edward Maunder returned to the Royal Observatory, taking a position as a spectroscopic assistant. Shortly after, in 1875, he married Edith Hannah Bustin, who gave birth to six children. Following the death of Edith Hannah in 1888, he met Annie Scott Dill Russell (1868–1947) in 1890, a mathematician with whom he collaborated for the remainder of his life. In 1895 Maunder and Russell married; they had no children. In 1916 Annie Maunder became one of the first women accepted by the Royal Astronomical Society.

Part of Maunder's job at the Observatory involved photographing and measuring sunspots, and in doing so he observed that the solar latitudes at which sunspots occur varies in a regular way over the course of the 11 year cycle. After 1891, he was assisted in his work by his second wife, Annie Scott Dill Maunder (née Russell), a mathematician educated at Girton College in Cambridge. She worked as a "lady computer" at the Observatory from 1890 to 1895. In 1904, he published their results in the form of the "butterfly" diagram.

After studying the work of Gustav Spörer, who had identified a period from 1400 to 1510 when sunspots had been rare ("the Spörer Minimum"), he examined old records from the observatory's archives to determine whether there were other such periods. These studies led him in 1893 to announce the period that now bears his name.

He observed Mars and was a sceptic of the notion of Martian canals. He conducted visual experiments using marked circular disks which led him to conclude, correctly, that the viewing of canals arose as an optical illusion. Also he was convinced that there cannot be life "as in our world" on Mars, as there are no temperature-equating winds and too low mean temperatures. 

In 1890, Maunder was a driving force in the foundation of the British Astronomical Association. Although he had been fellow of the Royal Astronomical Society since 1875, Maunder wanted an association of astronomers open to every person interested in astronomy, from every class of society, and especially open for women.

Edward Maunder was the first editor of the Journal of the BAA, an office later taken by his wife Annie Maunder. His older brother, Thomas Frid Maunder (1841–1935), was a co-founder, and secretary of the the Association for 38 years.

Craters on Mars and the Moon were named in honor of him and his wife Annie.

April 11: William Wallace Campbell

William Wallace Campbell

April 11, 1862 – June 14, 1938

William Wallace Campbell was an American astronomer, and director of Lick Observatory. He specialized in spectroscopy.

Campbell was a pioneer of astronomical spectroscopy, he catalogued the radial velocities of stars, and was a director of Lick Observatory from 1901 to 1930. He served as 10th President of the University of California from 1923 to 1930.

He led a team to Australia in 1922 where he photographed a solar eclipse. The data obtained provided further evidence supporting Albert Einstein's theory of relativity.

Campbell was awarded the Henry Draper Medal (1906), the Gold Medal of the Royal Astronomical Society (1906) and the Bruce Medal (1915).

The Lunar crater Campbell, a crater on Mars and the asteroid 2751 Campbell are named in his honor.

April 10: Sidney Dean Townley

Sidney Dean Townley 

April 10, 1867 – March 18, 1946

Sidney Dean Townley was an American astronomer and geodeticist. During his second year at the university he took a course in astronomy. He was also given a room at the Washburn Observatory and worked nights as an assistant. These would serve to shape his interest in astronomy. 

In his second year as a graduate student he was offered a Hearst fellowship at the Lick Observatory, which he accepted, arriving in 1892. In 1893, however, the fellowship funds were re-committed to an eclipse expedition to Chile, so he had to depart. 

He became an instructor of astronomy, first at the University of Michigan, followed by the University of California. From 1893 until 1898 he worked at the Detroit Observatory, where he studied variable stars and comets. 

By 1897 he gained his Sc.D. from the University of Michigan with a thesis on the "Orbit of Psyche". In 1898 he spent a year on leave to travel through Germany, visiting major observatories in Berlin, Leipzig, and Munich. After his return he began teaching at the University of California, Berkeley, and was appointed director of the International Latitude Station at Ukiah, California. While there he developed an interest in geodesy, particularly seismology.

Townley was a member of the Astronomical Society of the Pacific and served as its president in 1916, and also spent time as director and on the publication committee. He also joined the Seismological Society, and served at various times as president, secretary-treasurer, and editor of the society journal. 

In 1911 he became an assistant professor at Stanford University. In a short time he became full professor, and would remain in that position until his retirement in 1932, thereafter becoming professor emeritus. 

During his career he published roughly 100 academic papers, and edited the contributions of many others. He was widely recognized for his editorial skills. 

The crater Townley on the Moon is named in his honor.

April 9: Charles-Eugène Delaunay

Charles-Eugène Delaunay

April 9, 1816 – August 5, 1872

Charles Delaunay was a French astronomer and mathematician. His lunar motion studies were important in advancing both the theory of planetary motion and mathematics.

Born in Lusigny-sur-Barse, France, Delaunay studied under Jean-Baptiste Biot at the Sorbonne. He worked on the mechanics of the Moon as a special case of the three-body problem. He published two volumes on the topic, each of 900 pages in length, in 1860 and 1867. The work hints at chaos in the system, and clearly demonstrates the problem of so-called "small denominators" in perturbation theory. His infinite series expression for finding the position of the Moon converged too slowly to be of practical use but was a catalyst in the development of functional analysis.

Delaunay became director of the Paris Observatory in 1870 but drowned in a boating accident near Cherbourg, France two years later.

Delaunay was a member of the Académie des Sciences, (1855) and was awarded the Gold Medal of the Royal Astronomical Society, (1870). 

The Lunar crater Delaunay is named in his honor.

April 8: David Rittenhouse

David Rittenhouse
April 8, 1732 – June 26, 1796

David Rittenhouse was a renowned American astronomer, inventor, clockmaker, mathematician, surveyor, scientific instrument craftsman, and public official. Rittenhouse was a member of the American Philosophical Society and the first director of the United States Mint.

Rittenhouse was born near Germantown, Philadelphia, Pennsylvania. He was self-taught and from a young age showed great ability in science and mathematics. At nineteen years old, he started a scientific instrument shop at his father's farm in West Norriton Township, Pennsylvania. His skill with instruments, particularly clocks, led him to construct two orreries, one of which is currently in the library of the University of Pennsylvania and the other is at Peyton Hall of Princeton University. Rittenhouse was one of the first to build a telescope used in the United States.

His telescope, which utilized natural spider silk to form the reticle, was used to observe and record part of the transit of Venus across the sun on 1769-06-03, as well as the planet's atmosphere.

In 1785, Rittenhouse made perhaps the first diffraction grating using 50 hairs between two finely threaded screws, with an approximate spacing of about 100 lines per inch. This was roughly the same technique that Joseph von Fraunhofer used in 1821 for his wire diffraction grating.

At a young age Rittenhouse showed a high level of intelligence by creating a working scale model of his grandfather's paper mill. His uncle was a carpenter in Philadelphia, who died at a young age. When he died, he left young Rittenhouse a set of tools and instructional books. It was with these tools that Rittenhouse became an inventor and carved out a career. Sparked by the making of his first clock as a young boy, he later impacted the United States in many ways, from creating the first observatory in the country to founding the United States Mint.

After Galileo saw the first sign of Earth's neighbor, Venus, in 1610, astronomers who had been studying the planet, chose Rittenhouse as the person to study the transit path of Venus and its atmosphere. Rittenhouse was the perfect person to study the mysterious planet, as he had a personal observatory on his family farm. "His telescope, which he made himself, utilized grating intervals and spider threads on the focus of the telescope." His telescope is very similar to some modern day telescopes. Rittenhouse served on the American Astronomical Society, and this was another factor in being chosen to study Venus . Throughout his life, he had the honour to serve in many different clubs, committees, and much more. One example is the American Philological Society, he began as a librarian, became secretary, and after Benjamin Franklin's death, he became Vice President. Another one of his interests was the Royal Society of London; this was very rare to see a foreign member of this exclusive society.

When Rittenhousee was thirteen years of age, he had mastered Isaac Newton's Laws of Motion and Gravity. As a young boy he loved to build scale models, such as a working waterwheel and a paper mill. Rittenhouse never went to elementary school and was completely self-educated from family books. With his love of tools and his amazing ability to create things he crafted two orreries for Rutgers University in New Jersey. In return for the gift, the college gave him a scholarship to attend the college enabling him to obtain a degree in philosophy. At the age of twenty-eight, he published his first mathematical paper, one of many papers published throughout his life.

His great excitement at observing the infrequently-occurring transit of Venus (for which he had prepared for a year) resulted in his fainting during the observation. In addition to the work involved in the preparations, Rittenhouse had also been ill the week before the transit. Lying on his back beneath the telescope, trained at the afternoon sun, he regained consciousness after a few minutes and continued his observations. His account of the transit, published in the Transactions of the American Philosophical Society, does not mention his fainting, though it is otherwise meticulous in its record.

The Lunar crater Rittenhouse is named in his honor.

April 7: James Glaisher

James Glaisher

April 7, 1809 - February 7, 1903

James Glaisher was an English meteorologist and aeronaut.

Glaisher was an assistant at the Royal Greenwich Observatories at Cambridge and Greenwich, and Superintendent of the Department of Meteorology and Magnetism at Greenwich for thirty-four years.

In 1845, Glaisher published his clue point tables, for the measurement of humidity.
Glaisher was a founder member of the Meteorological Society (1850), and the Aeronautical Society of Great Britain (1866).

He is most famous, however, as a pioneering balloonist. Between 1862 and 1866, usually with Henry Tracey Coxwell as his co-pilot, Glaisher made numerous ascents in order to measure the temperature and humidity of the atmosphere at its highest levels. One one ascent in 1862, he broke the world record for altitude, but passed out around 8,800 metres before a reading could be taken. Estimates suggest that he rose to more than 9,500 metres and as much as 10,900 metres above sea-level.

He had ten children, including the mathematician James Whitbread Lee Glaisher (1848-1928).

The Lunar crater Glaisher is named in his honor.

April 6: André-Louis Danjon

André-Louis Danjon

April 6, 1890 – April 21, 1967

André Danjon was a French astronomer born in Caen.

Danjon devised a method to measure "Earthshine" on the Moon using a telescope in which a prism split the Moon's image into two identical side-by-side images. By adjusting a diaphragm to dim one of the images until the sunlit portion had the same apparent brightness as the earthlit portion on the unadjusted image, he could quantify the diaphragm adjustment, and thus had a real measurement for the brightness of Earthshine. He recorded the measurements using his method (now known as the Danjon Scale, on which zero equates to a barely visible Moon) from 1925 until the 1950s.

Among his notable contributions to astronomy was the design of the impersonal (prismatic) astrolabe now known as the Danjon astrolabe, which led to an improvement in the accuracy of fundamental optical astrometry. Within four years of its introduction (1956), the Danjon astrolabe was being used in more than 30 major observatories. An account of this instrument, and of the results of some early years of its operation, are given in Danjon's 1958 George Darwin Lecture to the Royal Astronomical Society (in Monthly Notices of the RAS (1958), vol.118, pages 411-431).

He was Director of the Paris Observatory from 1945 to 1963.

He won the Gold Medal of the Royal Astronomical Society in 1958. The Lunar crater Danjon is named in his honor.

April 5: Vincenzo Viviani

Vincenzo Viviani

April 5, 1622 – September 22, 1703

Vincenzo Viviani was an Italian mathematician and scientist. He was a pupil of Torricelli and a disciple of Galileo.

Born and raised in Florence, Viviani studied at a Jesuit school. There, Grand Duke Ferdinando II de' Medici furnished him a scholarship to purchase mathematical books. He became a pupil of Evangelista Torricelli and worked on physics and geometry.

In 1639, at the age of 17, he was an assistant of Galileo Galilei in Arcetri. He remained a disciple until Galileo's death in 1642. From 1655 to 1656, Viviani edited the first edition of Galileo's collected works.

After Torricelli's 1647 death, Viviani was appointed to fill his position at the Accademia dell'Arte del Disegno in Florence. Ferdinand II also appointed him engineer with the Uffiziali dei Fiumi— a position Viviani would hold for the rest of his life. Viviani was also one of the first members of the Grand Duke's experimental academy, the Accademia del Cimento, when it was created a decade later.

In 1660, Viviani and Giovanni Alfonso Borelli conducted an experiment to determine the speed of sound. Timing the difference between the seeing the flash and hearing the sound of a cannon shot at a distance, they calculated a value of 350 meters per second, considerably better than the previous value of 478 meters per second obtained by Pierre Gassendi. The currently accepted value is 331.29 meters per second at 0°C or 340.29 meters per second at sea level. In 1661 he experimented with the rotation of pendulums, 190 years before the famous demonstration by Foucault.

By 1666, Viviani started to receive many job offers as his reputation as a mathematician grew. That same year, Louis XIV of France offered him a position at the Académie Royale and John II Casimir of Poland offered Viviani a post as his astronomer. Fearful of losing Viviani, the Grand Duke appointed him court mathematician. Viviani accepted this post and turned down his other offers.

In 1687, he published a book on engineering, Discorso intorno al difendersi da' riempimenti e dalle corrosione de' fiumi.

Upon his death, Viviani left an almost completed work on the resistance of solids, which was subsequently completed and published by Luigi Guido Grandi.

In the 1730s, the Church finally allowed Galileo to be reburied in a grave with an elaborate monument. The monument that was created in the church of Santa Croce was constructed with the help of funds left by Viviani for that specific purpose. Viviani's own remains were moved to Galileo's new grave as well.

The lunar crater Viviani is named after him.

April 4: Joseph-Nicolas Delisle

Joseph-Nicolas Delisle

April 4, 1688 - 1768

Joseph-Nicolas Delisle was a French astronomer.

He was one of the 11 sons of Claude Delisle (1644-1720). Like many of his brothers, among them Guillaume Delisle, he initially followed classical studies. Soon however, he moved to astronomy under the supervision of J. Lietaud and Jacques Cassini.

He entered the French Academy of Sciences as pupil of Giandomenico Maraldi (1709-1788). Though he was a good scientist and member of a wealthy family he did not have much money.

His life changed radically when he was called by the Russian czar Peter the Great to Saint Petersburg to create and run the school of astronomy. He arrived there only in 1726, after the death of the czar. He became quite rich and famous, to such an extent that when he returned to Paris in 1747, he built his own observatory in the palace of Cluny, later made famous by Charles Messier. Also he received the title of Astronomer from the Academy.

Delisle is mostly known for the Delisle scale, a temperature scale he invented in 1732.

The Lunar crater Delisle and the asteroid 12742 Delisle are named in his honor.

April 3: Heinrich Nissen

Heinrich Nissen

April 3, 1839 - February 29, 1912 

Heinrich Nissen was a German Ancient Historian and archaeoastronomer.

Heinrich Nissen studied in Kiel and Berlin under August Boeckh and Theodor Mommsen. After graduating, he travelled in Italy between 1863 and 1867. This research was later published as the major work Italischen Landeskunde (1883 and 1902). 

Nissen first taught as a professor in Strasbourg. In 1884 he was elected as successor to Arnold Schaefers at University of Bonn. There, unlike his predecessor, he exclusively taught Ancient History. He also pioneered Epigraphic research - based upon his study during his years in Italy - and was dedicated to Roman provincial archaeology. 

The term archaeoastronomy was first used by Elizabeth Chesley Baity (at the suggestion of Euan MacKie) in 1973, but as a topic of study it may be much older, depending on how archaeoastronomy is defined. 

Clive Ruggles says that

Heinrich Nissen, working in the mid-nineteenth century was arguably the first archaeoastronomer.

 

Rolf Sinclair says that Norman Lockyer, working in the late 19th and early 20th centuries, could be called the 'father of archaeoastronomy.' Euan MacKie would place the origin even later, stating: "...the genesis and modern flowering of archaeoastronomy must surely lie in the work of Alexander Thom in Britain between the 1930s and the 1970s.

Archaeoastronomy (also spelled archeoastronomy) is the study of how past people "have understood the phenomena in the sky, how they used phenomena in the sky and what role the sky played in their cultures." Clive Ruggles argues it specifically is not the study of ancient astronomy, as astronomy is a culturally specific concept and ancient peoples may have related to the sky in a different way.

Archaeoastronomy uses a variety of methods to uncover evidence of past practices including archaeology, anthropology, astronomy, statistics and probability, and history. Because these methods are diverse and use data from such different sources, the problem of integrating them into a coherent argument has been a long-term issue for archaeoastronomers.

April 2: Francesco Maria Grimaldi

Francesco Maria Grimaldi

April 2, 1618 - December 28, 1663

Francesco Grimaldi was an Italian mathematician and physicist who taught at the Jesuit college in Bologna. Between 1640 and 1650, working with Giovanni Battista Riccioli, he investigated the free fall of objects, confirming that the distance of fall was proportional to the square of the time taken.

In astronomy, he built and used instruments to measure geological features on the Moon, and drew an accurate map or selenograph which was published by Riccioli.

Later physicists used his work as evidence that light was a wave, and Isaac Newton used it to arrive at his more comprehensive theory of light.

He was one of the great geometer-physicists of his time and was an exact and skilled observer, especially in the field of optics. He was the first to make accurate observations on the diffraction of light (although by some accounts Leonardo da Vinci had earlier noted it), and coined the word 'diffraction,' which means "breaking up." He laid the groundwork for the later invention of the diffraction grating. He was one of the earliest physicists to suggest that light was wavelike in nature. He formulated a geometrical basis for a wave theory of light in his Physico-mathesis de lumine (1666). It was this treatise which attracted Isaac Newton to the study of optics. Newton deals with the diffraction problems of Grimaldi in Part III of his Opticks (1704), after having first learned of Grimaldi's diffraction from the writings of another Jesuit geometer, Honoré Fabri.

The astronomical work of Francesco Grimaldi was closely related to the astronomical work of another Jesuit, Giovanni Battista Riccioli who wrote the Almagestum Novum. In 1640 Grimaldi conducted experiments on free fall for Riccioli, dropping weights from a tower and using a pendulum as timer. He found that the square of the time is proportional to the distance of free fall from rest. Riccioli praised especially Grimaldi's ability to devise, build, and operate new observational instruments. Grimaldi's contributions included such measurements as the heights of lunar mountains and the height of clouds. 

Francesco Grimaldi is responsible for the practice of naming lunar regions after astronomers and physicists, rather than after ideas such as "tranquility". With Riccioli he composed a very accurate selenograph, a copy of which adorns the entrance to the National Space Museum in Washington. 

The crater Grimaldi on the Moon is named after him.