September 30: Michael Maestlin

Michael Maestlin

September 30, 1550 - October 20, 1631

Michael Maestlin was a German astronomer and mathematician, known for being the mentor of Johannes Kepler.

Maestlin studied theology, mathematics, and astronomy at the Tübinger Stift in Tübingen, a town in Württemberg. In 1580 he became a Professor of mathematics, first at the University of Heidelberg, then at the University of Tübingen were he taught for 47 years from 1583. In 1582 Maestlin wrote a popular introduction to astronomy.

Among his students was Johannes Kepler (1571-1630). Although he primarily taught the traditional geocentric Ptolemaic view of the solar system, Maestlin was also one of the first to accept and teach the heliocentric Copernican view. Maestlin corresponded with Kepler frequently and played a sizable part in his adoption of the Copernican system. Galileo Galilei's adoption of heliocentrism was also attributed to Maestlin .

The first known calculation of the (inverse) golden ratio as a decimal of "about 0.6180340" was written in 1597 by Maestlin in a letter to Kepler.

Maestlin catalogued the Pleiades cluster on December 24, 1579. Eleven stars in the cluster were recorded by Maestlin, and possibly as many as fourteen were observed. He also observed the occultation of Mars by Venus on October 3, 1590, at Heidelberg.

In Jules Verne's Cinq semaines en ballon (Five Weeks in a Balloon) the character of Joe, the manservant, is described as enjoying, "in common with Moestlin, Kepler's professor, the rare faculty of distinguishing the satellites of Jupiter with the naked eye, and of counting fourteen of the stars in the group of Pleiades, the remotest of them being only of the ninth magnitude."

The Lunar crater Maestlin, the Lunar rille Rimae Maestlin and the asteroid 11771 Maestlin, discovered in 1973, are named in his honor.

September 25: Ole Rømer

Ole Christensen Rømer

September 25, 1644 – September 19, 1710

Ole Rømer was a Danish astronomer who in 1676 made the first quantitative measurements of the speed of light.

Ole Rømer went to the University of Copenhagen in 1662, at which his mentor was Rasmus Bartholin. Rømer was given every opportunity to learn mathematics and astronomy using Tycho Brahe's astronomical observations, as Bartholin had been given the task of preparing them for publication.

After studies in Copenhagen, Rømer joined the observatory of Uraniborg on the island of Hven, near Copenhagen, in 1671. Over a period of several months, Jean Picard and Rømer observed about 140 eclipses of Jupiter's moon Io, while in Paris Giovanni Domenico Cassini observed the same eclipses. By comparing the times of the eclipses, the difference in longitude of Paris to Uranienborg was calculated.

Cassini had observed the moons of Jupiter between 1666 and 1668, and discovered discrepancies in his measurements that, at first, he attributed to light having a finite speed. In 1672 Rømer went to Paris and continued observing the satellites of Jupiter as Cassini's assistant. Rømer added his own observations to Cassini's and observed that times between eclipses (particularly those of Io) got shorter as Earth approached Jupiter, and longer as Earth moved farther away. Cassini published a short paper in August 1675 where he states: This second inequality appears to be due to light taking some time to reach us from the satellite; light seems to take about ten to eleven minutes to cross a distance equal to the half-diameter of the terrestrial orbit.

Oddly, Cassini seems to have abandoned this reasoning, which Rømer adopted and set about buttressing in an irrefutable manner, using a selected number of observations performed by Picard and himself between 1671 and 1677. Rømer presented his results to the French Academy of Sciences, and it was summarised soon after by an anonymous reporter in a short paper, Démonstration touchant le mouvement de la lumière trouvé par M. Roemer de l'Académie des sciences, published 7 December 1676 in the Journal des sçavans. Unfortunately the paper bears the stamp of the reporter failing to understand Rømer's presentation, and as the reporter resorted to cryptic phrasings to hide his lack of understanding, he obfuscated Rømer's reasoning in the process. Unfortunately Rømer himself never published his results.

Rømer never gave a value for the speed of light. However, many others calculated a speed from his data, the first being Christiaan Huygens; after corresponding with Rømer and eliciting more data.

Rømer's view that the velocity of light was finite was not fully accepted until measurements of the so-called aberration of light were made by James Bradley in 1727.

In 1809, again making use of observations of Io, but this time with the benefit of more than a century of increasingly precise observations, the astronomer Jean Baptiste Joseph Delambre reported the time for light to travel from the Sun to the Earth as 8 minutes and 12 seconds. Depending on the value assumed for the astronomical unit, this yields the speed of light as just a little more than 300,000 kilometres per second.

A plaque at the Observatory of Paris, where the Danish astronomer happened to be working, commemorates what was, in effect, the first measurement of a universal quantity made on this planet.

The Lunar crater Römer is named in his honor.

September 22: Pehr Wargentin

Pehr Wilhelm Wargentin

September 22, 1717 – December 13, 1783

Pehr Wargentin was a Swedish astronomer.

When Pehr Wargentin was 12 years old he observed a (total) lunar eclipse which would spark his life-long interest in Astronomy. During his tenure at Frösö trivialskola (elementary school), his teacher deemed him advanced enough to continue directly to Uppsala University. However, Wargentin's father wanted him first to attend the gymnasium (secondary school) of Härnösand, which he did. According to his own account, Wargentin was unimpressed with the purely classical and theological curriculum and the lack of any education in the sciences and did not finish the fourth year.

In 1735, Wargentin matriculated as a student at the University of Uppsala, where he excelled. Olof Hiorter was one of his instructors. He graduated in 1743 and became a docent in astronomy in 1746 and an adjunct in 1748. He was called to Stockholm as secretary of the Royal Swedish Academy of Sciences in 1749 on the death of secretary Pehr Elvius, Jr., and stayed on this post until his death. Wargentin therefore became the first long-serving secretary of the Academy, which had been founded in 1739. He is seen as an important person in leading the Academy to its first golden era. Wargentin also became the first director of the Stockholm Observatory founded by the Academy of Sciences on the initiative of his predecessor, Elvius, and completed in 1753.

Wargentin made studies on the moons of Jupiter and published his first paper on the topic in 1741 in the Acta of the Royal Society of Sciences in Uppsala.

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

September 15: Jean-Sylvain Bailly

Jean-Sylvain Bailly

September 15, 1736 – November 12, 1793

Jean-Sylvain Bailly was a French astronomer and orator, one of the leaders of the early part of the French Revolution. He was ultimately guillotined during the Reign of Terror.

Born in Paris, he was originally intended for the profession of a painter, but preferred writing tragedies, until attracted to science by the influence of Nicolas de Lacaille. He calculated an orbit for Halley's Comet when it appeared in 1759, reduced Lacaille's observations of 515 zodiacal stars, and was elected a member of the French Academy of Sciences in 1763. His Essai sur la theorie des satellites de Jupiter (Essay on the theory of the satellites of Jupiter, 1766), an expansion of a memoir presented to the Academy in 1763, showed much original power; and it was followed up in 1771 by a noteworthy dissertation Sur les inegalites de la lumiere des satellites de Jupiter (On the inequalities of light of the satellites of Jupiter). In 1778, he was elected a foreign member of the Royal Swedish Academy of Sciences.

Meantime, he had gained a high literary reputation by his Éloges of King Charles V of France, Lacaille, Molière, Pierre Corneille and Gottfried Leibniz, which were issued in collected form in 1770 and 1790; he was admitted to the Académie française on 26 February 1784, and to the Académie des Inscriptions in 1785, when Bernard le Bovier de Fontenelle's simultaneous membership of all three Academies was renewed in him.

From then on, he devoted himself to the history of science, publishing successively: Histoire de l'astronomie ancienne (A history of ancient astronomy, 1775); Histoire de l'astronomie moderne (A history of modern astronomy, 3 vols., 1779-1782); Lettres sur l'origine des sciences (Letters on the origin of the sciences, 1777); Lettres sur l' Atlantide de Platon (Letters on Plato's Atlantide , 1779); and Traite de l'astronomie indienne et orientale (A treatise on Indian and Oriental astronomy, 1787). The 1911 Encyclopædia Britannica remarks that "Their erudition was… marred by speculative extravagances."

The lunar crater Bailly is named in his honor.

July 28: Charles Dillon Perrine

Charles Dillon Perrine

July 28, 1867–June 21, 1951

Charles Dillon Perrine was an Argentine astronomer.

Born in Ohio, he worked at Lick Observatory from 1893 to 1909 and then was director of the Argentine National Observatory (today, Observatorio Astronómico de Córdoba) in Argentina from 1909 until 1936.

In 1901, he and George Ritchey observed the apparent superluminal motion in the nebulosity surrounding Nova Persei 1901.

He discovered two moons of Jupiter, today known as Himalia (in 1904) and Elara (in 1905). They were simply designated "Jupiter VI" and "Jupiter VII" and were not given their present names until 1975.

He co-discovered the lost periodic comet 18D/Perrine-Mrkos and several other comets. Antonín Mrkos later named the asteroid 6779 Perrine after him.

He promoted the study of astrophysics in Argentina and pushed for the construction of a large telescope (the Bosque Alegre telescope), which however was not completed until 1942 (he had retired in 1936). He remained in Argentina after retirement and died there, in Villa General Mitre (which has since been renamed to its original name of Villa del Totoral). He is buried in the cementerio disidente in the city of Córdoba.

The lunar crater Perrine is named in his honor.

June 25: Rupert Wildt

Rupert Wildt

June 25, 1905 – January 9, 1976

Rupert Wildt was a German-American astronomer. In 1927 he was awarded a Ph.D. from the University of Berlin. He joined the University of Göttingen, specializing in the properties of atmospheres.

In 1932 he studied the spectra of Jupiter, and other outer planets, and identified certain absorption bands as belonging to the hydrogen-rich compounds of methane and ammonia. The composition appeared consistent with a composition similar to the sun and other stars.

Assuming that the atmosphere was composed of these gases, during the 1940s and 1950s he constructed a model of the structure of these planets. He believed the core of the planets is solid and composed of a mixture of rock and metal, covered by a thick outer shell of ice, overlaid by a dense atmosphere. His model is still widely accepted.

In 1934 he emigrated to the United States, and became a research assistant at Princeton University from 1937 until 1942. He then became an assistant professor at the University of Virginia until 1947, before joining the faculty of the Yale University.

In 1937 he proposed that the atmosphere of Venus was composed of a mist of formaldehyde. His observations of the atmosphere did not find any water at the time, but later balloon-based measurements did show water in the atmosphere and so his proposal was abandoned. In 1940, however, he also hypothesized that the carbon dioxide in the venusian atmosphere trapped heat, a phenomenon later called the greenhouse effect.

From 1965 until 1968 he was president of the Association of Universities for Research in Astronomy. In the period 1966-1968 he also held the post of the chairman of the department of astronomy at Yale, and from 1973 until his death he was professor emeritus.

Asteroid 1953 Rupertwildt and the Lunar crater Wildt are named in his honor.

June 8: Giovanni Domenico Cassini

Giovanni Domenico Cassini

June 8, 1625 – September 14, 1712

Giovanni Domenico Cassini was an Italian/French mathematician, astronomer, engineer, and astrologer.

Cassini was an astronomer at the Panzano Observatory from 1648 to 1669. He was a professor of astronomy at the University of Bologna and in 1671 became director of the Paris Observatory. He thoroughly adopted his new country, to the extent that he became interchangeably known as Jean-Dominique Cassini —although that is also the name of his great-grandson.

Along with Robert Hooke, Cassini is given credit for the discovery of the Great Red Spot on Jupiter (ca. 1665). Cassini was the first to observe four of Saturn's moons, which he called Sidera Lodoicea; he also discovered the Cassini Division (1675). Around 1690, Cassini was the first to observe differential rotation within Jupiter's atmosphere.

In 1672 he sent his colleague Jean Richer to Cayenne, French Guiana, while he himself stayed in Paris. The two made simultaneous observations of Mars and thus found its parallax to determine its distance, thus measuring for the first time the true dimensions of the solar system.

Cassini was the first to make successful measurements of longitude by the method suggested by Galileo, using eclipses of the satellites of Jupiter as a clock.

The Lunar crater Cassini is named in his honor.

January 7: Jovian Moons

NASA/JPL/DLR

January 7, 1610

Galileo Galilei observes the four largest moons of Jupiter for the first time. He named them and in turn the four are called the Galilean moons. Jupiter's 4 Galilean moons, in a composite image comparing their sizes and the size of Jupiter (Great Red Spot visible). From the top, they are Io,Europa, Ganymede and Callisto