1800-1900: Developing a New Understanding of Physics in Astronomy

After a large amount of data, theorizing, and screwing with physics we come to the 1800’s.  Here we bring further data/discoveries, but we also come to a point where physics starts to really become confusing.  Especially when we throw in calculus because, well, it’s integral.  The advent of spectroscopy and discoveries about Optics shed light on many aspects of stars and Astronomy.  Lastly, there were some shocking aspects of electricity and magnetism that had to be looked into.

To start we still have the Herschel family.  William Herschel’s son, John Herschel, was a prominent Astronomer in his own respect.  He wrote about chemicals and photography in 1819, and in 1822 he published a minor work on calculating eclipses of the moon.  But more importantly, Herschel worked with double or binary stars.  He made a whole catalog of them (taken from his father’s work on calculating parallax), and also studied the common center of gravity for a binary system.  One of his most important discoveries was that a force coming from the Sun seemed to be acting on Halley’s Comet, leading to the discovery of solar winds.  After this foray into Astronomy, Herschel continued his work with photography.  He is never actually recognized as its inventor, but he did coin the word “photograph” in 1839.

From all the results of John Herschel’s work on photography, someone had to go and use it.  That man would be Henry Draper.  In 1880, he took the first picture of a nebula (the Orion nebula to be exact).  His overall contribution to Astronomy is improving upon astronomical photography and combining this with photographing stellar spectra.  This was essential to the fantastic vistas provided today; the banner on the top of this blog wouldn’t exist if it wasn’t for the many great high-quality photos of nebulae.

We can now go closer (let’s call this a blueshift) to Christian Doppler.  Usually, he is noted for discovering the Doppler principle/effect/shift, but he’s most notable for that since he didn’t do too much more.  Some of the things he found about it were actually incorrect, but nonetheless, it was an extremely important principle.

Now we will discuss the important discoveries made in spectroscopy during this time.  It really started with Joseph von Fraunhofer in 1814.  He found that dark lines were created when light from a continuous spectrum was absorbed.  Fraunhofer found that light from the Sun had these lines, and by comparing the solar spectra to lines created by gaseous elements, he showed that the Sun was really a hot sphere of gases.  About 50-60 years later, Gustav Robert Kirchhoff and Robert Wilhelm Bunsen explained the cause of these lines.  Kirchhoff is also known for his work with thermochemistry and thermal emission. From this we go to William Huggins. We are unsure whether he was the most huggable man as his name may suggest, but he developed the technique of spectroscopy to look at the stars and accurately show their composition.  This combined with the Doppler Effect created a fantastic new way to look to stars and their properties.

All the Fraunhofer lines from the Sun, just not on one line since that wouldn’t fit

Hopefully we’ve excited you (like hydrogen electrons), since Johann Jakob Balmer is the next man who worked with the up-and-coming field of spectroscopy.  His main accomplishment is the Balmer series of Hydrogen spectral lines.  This shows that Hydrogen has certain spectral line emissions in the visible wavelength, and they can be calculated.  This is important in connecting measurements of the Doppler effect on an object with spectral lines, or it can show the presence of Hydrogen in an object.  Interestingly, by the time Balmer published his very important papers he was 60, and 72 by the time he finished publishing his work, so it’s never too late to do something great.  In addition to him, it is necessary to mention Johannes Rydberg, who showed that the spectral lines produced by atoms can be related and their energies can be found using a formula he created.  The reason we mention Rydberg is that Balmer created his own formula to show the Balmer series, but it was actually a special case of the Rydberg formula.

There are three last people who did some physics-related nonsense. They are Ludwig Boltzmann, Josef Stefan, and Wilhelm Wien.  This nonsense was all about blackbodies (objects that emit energy “perfectly”) and stars are generally treated as blackbodies.  Stefan showed how to determine the power of the radiation emitted by a blackbody and determined the approximate temperature of the Sun.  Boltzmann, a student of Stefan, showed how this can be found mathematically, and also predicted a variety of atomic properties.  Lastly, there is Wien who developed a law that shows that the wavelength emitted by a blackbody changes with temperature.  Altogether, these people did some hot stuff.

Then there is Williamina Fleming.  She was a housemaid to Edward Pickering until one day Pickering got fed up with his male assistants at the Harvard Observatory, and legend has it, declared that his maid could do a better job.  She did.  Fleming classified over TEN THOUSAND stars according to the strength of their hydrogen spectral lines (this classification would be improved by Annie Jump Cannon in the 1900s).  She also discovered many nebulae, novae, and variable stars, including the famous Horsehead Nebula, but she was denied credit for the discovery.  Fleming was later put in charge of the Harvard Computers, also known as “Pickering’s harem”, a group of women who worked as human computers and made many important discoveries in astronomy.

The Horsehead Nebula. Not that easy to find!

Now that we have gotten past a majority of the physicists there was still work being done with distances and planets.

Friedrich Bessel is credited with observing over 3,000 stars, but more importantly, he was one of the first people to use parallax to find the distance to a star.  Parallax is possibly one of the most useful tools in any Astronomer’s arsenal since it is so simple to use, and so important that it will be explained separately in a later post.

Johann Gottfried Galle is next mentioned mainly because he found Neptune using data from Urbain LeVerrier (who may be more important since he also predicted its orbit), but it is nonetheless the last planet of our Solar system.  He also found and studied  414 comets and a crater on the moon.  For this, Galle has a ring of Neptune named for him.  On the subject of Neptune, it is also worthy to mention William Lassell, who discovered Neptune’s largest moon Triton.

To continue with the moon-discovering, Asaph Hall found Mars’ moons Phobos and Deimos, named for gods of fear and terror.  But the only thing that’s scary in this situation is why it took so long.  This is because these moons are REALLY small, as in only a few kilometers across.  Besides this, Hall found the orbits of satellites for other planets, double stars, the rotation of Saturn, and the mass of Mars.

The last thing to mention would be the Yerkes Observatory in Chicago, made in 1897.  It was a giant refracting telescope, able to take vast collections of photographic plates.   This is considered almost a transition to modern Astronomy, because the telescope was state-of-the-art technology and could be used to get the best and brightest photos of space.

The telescope at the Yerkes Observatory. A person doesn’t look like much compared to it!

Other people to mention:

  • Samuel Heinrich Schwabe – Effectively found the sunspot cycle
  • James Clerk Maxwell – One of the most famous physicists of the 1800’s, best known for his equations involving electricity, light, and magnetism.  He also predicted Radio waves and explained that Saturn’s rings weren’t solid or fluid, but rather a bunch of small, separate, solid particles.
  • Lord Kelvin and Hermann Von Helmholtz – worked with star formation, created the Kelvin-Helmholtz mechanism to explain how stars produce energy through gravitational collapse

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1600-1800: Bringing Order to the Heavens

We go to the observatory in France and listen to the stories of science, reaching for the stars.

Thus we continue our exploration of the universe.

A lot can happen in 200 years.  Certainly people know of a variety of revolutions (American, French, etc.) that occurred.  What of science?  Through these years many discoveries are notable in the sciences.  Possibly from Kepler we could call it a Scientific Revolution!

We will split this into the 1600’s and the 1700’s, where we discuss a variety of people.  A majority of the discoveries involve the Solar System, but these years made some stellar improvements to Physics as well.


At this time the telescope was improved by a variety of people, which allowed for a growth in our knowledge of the Solar System and its contents.  Gregory James designed the first reflecting telescope known as the Gregorian telescope.  The first actual refracting (Galilean) telescope was made by the Dutch Hans Lippershey, Zacarias Janssen and Jacob Metius who created lenses.  Refracting telescopes like the ones Galileo made and used were more common and used by ancient Astronomers.  Now reflecting telescopes are used more since it was found eventually that lenses had limitations in viewing objects.  John Flamsteed cataloged over 3000 stars, such an enormous number that the catalog was even needed by Newton and Halley.  There is also Christiaan Huygens, a famous physicist who worked with telescopes and Optics to find Saturn’s rings and its moon Titan.  For this he gets a spacecraft named after him, which  is fittingly supposed to visit Saturn.

Cassini-Huygens spacecraft

Artist’s rendition of Saturn and spacecraft.

Another famous Astronomer who worked with Huygens was Giovanni Domenico Cassini.  The spacecraft that was named for Huygens is actually called the Cassini-Huygens since Cassini also made multiple observations of Saturn and Titan.  He was actually more of an Astronomer than Huygens, since he ended up observing four of Saturn’s moons, discovered the Great Red Spot of Jupiter with Robert Hooke, and found differential rotation in Jupiter’s atmosphere.  Cassini and Jean Richer even managed to use parallax to calculate the distance to Mars, an important use of a method of distance calculation (which will be explained later).

So yes, there were definitely people besides Galileo, Newton, and Kepler who were important during the 1600’s.


But the 1700’s had some extremely important people as well.  One of the most important would be Charles Messier.  By looking at the sky for comets, he found many comet-like objects that weren’t actually comets.  For this he made a catalog of 110 objects, most of which turned out to be nebulae and star clusters.  This catalog could be said to be as important as the telescope since it organizes the objects and explains where they are; that surely puts us in our place!  This list is still used by Astronomers today, especially amateur Astronomers, and for the Science Olympians out there you would know it from some of the Deep Sky Objects.  We have this man to thank for helping make our lives somewhat easier in finding information on them.  Messier sadly doesn’t have any fancy satellites named for him.  He just gets a crater, and a whole catalog of objects with his name tacked on instead.

Entire Messier catalog

^ This whole thing.

Next is Edmond Halley.  Now, we all know about the famous Halley’s Comet.  He got his name attached to one of the world’s most famous comets for calculating the orbit of that very comet.  No easy task since he figured it out by looking to four observations and their calculated orbits that weren’t all his own.  He sadly didn’t see his prediction come true for the comet’s next appearance, but Halley did many other things too.  Similar to Messier, he constructed a catalog of stars of the Southern Hemisphere for 341 stars and observed the transit of Mercury across the Sun.  Halley may have also persuaded Newton to publish some of his works on celestial mechanics.  He also looked towards the famous Transit of Venus to determine the distance from the Sun to the Earth.

Pierre-Simon Laplace was an important mathematician and Astronomer.  He made a lot of math, but this is an Astronomy blog, so we’ll ignore most of his math work.  He restated the “nebular hypothesis” of the origin of the solar system, which pretty much says that a giant cloud of stuff (hydrogen and whatnot) condensed into a star with dust forming the planets as well.  Laplace also was one of the first scientists to theorize black holes and gravitational collapse.  Yes, he theorized black holes almost 100 years before modern physicists did, by working with a man named John Michell who also described gravity’s effects on light.  Of course, Michell himself was one of the first to propose binary stars, so obviously these people were pretty intelligent for their day.

Lastly is the Herschel family.  The family was more known for its music, but there were three famous for Astronomy.  Possibly the most famous is William Herschel.  By reading books on Astronomy and Optics, he learned to become a skilled telescope maker and sky observer by creating mirrors for observation.  Herschel also investigated the proper motion of stars, and from this he discovered that our solar system is moving.  He proposed that other universes exist, hinting at the idea of multiple galaxies.  He’s most well-known for discovering Uranus and two major moons of the planet.  Yes, by 1781 there were still undiscovered planets, but we can thank this man for an inappropriate planet-related joke.  For all this Herschel was able to meet Napoleon (yes, that Napoleon), Laplace, and Messier.  It’s appropriate that Herschel met Messier since Herschel made his own catalog of objects, and in the same year that he discovered Uranus he received a copy of Messier’s catalog which spurred on his interests.  But there isn’t really a better reward than getting a satellite named after you.  Oh wait, Herschel got one of those too. It works in the infrared range. Because he discovered infrared light.

William and Caroline Herschel

P. Fouché. Caroline Herschel Taking Notes as Her Brother William Observes on March 13, 1781, the Night William Discovered Uranus, (Brooklyn Museum)

To continue with his family, he had a sister.  Caroline Herschel was sick with Typhus from a young age.  Despite surviving this she wasn’t able to do much in the opinion of her father since she couldn’t grow any taller.  Since her brother had sympathy for her, she had multiple discussions with him about Astronomy, and eventually she decided to help him.  Caroline made a major contribution to the discovery of Uranus with her brother, but she also did her own observations with a small telescope and found a total of 8 comets in 11 years along with 3 nebulae.  Discovering comets and nebulae may not sound so significant, but many Astronomers at the time (including the notable Messier) were searching for these objects. Caroline Herschel is recognized as the first woman with a scientific position specifically in Astronomy and received a pension for it.  There may be no satellite named directly for her, but she certainly showed that despite not learning as much, she was a capable observational Astronomer.  This is especially unique since she lived in a time where women were expected to be married off and become housewives.

Others to mention:

  • Johann Heinrich Lambert – First to propose Milk Way’s disk-like shape
  • Johann Daniel Titius – Found asteroid belt and with Johann Elert Bode created a law for the distances of planets
  • Giuseppe Piazzi – Discovered the dwarf planet Ceres and has a crater named for him
  • Heinrich Wilhelm Matthias Olbers – Calculated comet orbits, discovered asteroids and proposed the existence of the asteroid belt.  Most famous for Olbers’ Paradox, which states that since the night sky is dark, the universe and the number of stars in it cannot be infinite and static.  This contributes to support of the Big Bang model that the universe is always changing.
  • Ole Rømer – had to be mentioned both for the fancy O and for measuring the speed of light, which was improved on by James Bradley
  • Leonhard Euler  Said incorrectly that light must travel through an “aether”.  But to make up for this, he calculated the orbits of comets, the parallax of the Sun, and tons of other useful math (or maths for our British readers).  Also, his last name is pronounced similar to “Oyler” or “Oiler”, please don’t make the mistake.  A Geometry or math(s) teacher may be mad if you do.
  • Alexander von Humboldt – first detailed study of Leonid Meteor Shower

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Kepler’s Laws

Tycho Brahe was a man with an awesome moustache:

Evidence he was a man with an awesome moustache, possibly a crazy, crazy man

Despite this, this post is titled “Kepler’s Laws” since Brahe’s version of the Solar System is both confusing and wrong, but he did have one.  So, we will give him credit for creating the data used by Johannes Kepler (Brahe’s assistant, shown below), who found some fascinating aspects in the orbits of planets in our Solar System.  Kepler was given the task of understanding the orbit of Mars by Brahe†, but little did Brahe know, Kepler would be making some laws now, and they are quite eccentric.

A man also with a moustache (obviously it helps with Astronomy), but maybe the beard helped make him slightly less crazy than Brahe

Before we go into Kepler’s laws a major aspect of them is the ellipse.  These have a few important properties, essentially being non-perfect circles.  The more “ellipse-like” and unequally round an ellipse is, the more eccentric it is,  going from zero (a circle, a special kind of ellipse) to one (a parabola, not an ellipse).  The orbits of bodies around stars tend to be elliptical, since it is difficult to make things perfectly circular in nature.  Interestingly, the orbits of planets in our Solar System are oddly less elliptical than orbits normally would be.  Confusingly, this would then be called “less eccentric”.  Another aspect of ellipses is the major and minor axes, the major being longer than the minor.

There are three laws of Planetary Motion.

1. The ellipses:

Kepler noticed that Copernicus had planets which had circular orbits  with epicycles, both of which made no sense compared to the models he constructed.  With work, Kepler found that the orbits were actually ellipses, with planets orbiting around the Sun at one focus, as shown below.

2. The perihelion and aphelion:

Kepler next found that the area created over equal times between a planet and the Sun is equal around the ellipse.  A planet moves faster near perihelion (when it is closest to the Sun) and slower near aphelion (when it is furthest from the Sun).  This is shown below, greatly exaggerated:

3. A relation forms:

Lastly, the ratio of the squares of the revolutionary periods for two planets is equal to the ratio of the cubes of their semimajor axes.  Far easier shown is: 

where the 1 to the bottom right of the letter represents the quantities for one planet, and the 2 to the bottom right of the same letter represents the quantities for the second planet.  P is revolutionary period, and R is the semimajor axis or half the major axis of the ellipse.

To conclude, the man is quite important.  It is incredibly impressive that he could create highly accurate laws describing large bodies without even technically using a telescope (he used Brahe’s data from sextant observations), such that it can be taught in schools. It may not sound good that there is more to test on because of him, but it is better than  keeping presumptions that create inaccuracies in our interpretation of the universe.  Now it may not seem as necessary, since we can take far better images of space, but whenever Astronomers try to look into space, a model or predictions are made (good science requires hypothesizing and planning).  Kepler’s Laws can be used to check a model of ANY TWO ORBITING BODIES.  He is so respected he even has a satellite named for him looking for planets outside our Solar System.


† Luckily for Kepler and modern astronomy, Mars has one of the most eccentric orbits in the Solar System.


TL;DR — After looking towards the Copernican Model and using Brahe’s data, Kepler was able to greatly help Astronomy with his three laws of Planetary Motions, which help for explaining motions of any two orbiting bodies.  Essentially, instead of circles, he found that planets revolve around the Sun in an ellipse, and he made certain relations from this.

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A Brief History of Astronomy

It’s a warm summer evening in ancient Greece. You stare up at the stars, noticing that they appear to move together across the sky, but there are a few pesky “wandering” stars that seem to have changed their positions since your last observations a few nights ago…

Thus began the scientific study of astronomy.

Plato hypothesized that all celestial bodies moved at constant rates in circular orbits around the Earth — a geocentric universe. The stars were attached to a huge, rotating “celestial sphere”. This agreed with observations that stars always remained in the same place relative to each other.†  However, the retrograde motion of planets (i.e. they appeared to move backwards) threw a bit of a wrench into the works. Originally it was suggested that each planet simply orbited the Earth on a sphere of its own, much like the fixed stars did, but this model just didn’t match up with observations. Hipparchus fixed the problem temporarily by suggesting that each planet orbited on a deferent and also moved on an epicycle, but his model became more and more inaccurate as time went on. Ptolemy further complicated matters by moving Earth away from the exact center of the deferents and adding equants which the epicycles would orbit at a uniform speed…

Ptolemaic Model

A planet moves on an epicycle (small circle) along a deferent (aka “orbit”, the big circle). The epicycle revolves around the equant (black dot), while Earth is offset from the center of the orbit (x).

He wasn’t right either, but his geocentric theory was accepted for nearly the next 2000 years without much complaint — whenever observations disagreed with the Ptolemaic model, the model was “fixed” by adding more circles in order to better match the data (it eventually required 80+ circles).

The Greek astronomer Aristarchus proposed a heliocentric universe at some point around 250 BC, but no one really supported him and heliocentrism was basically consigned to the junkyard of scientific ideas until Nicolaus Copernicus came along in the 1500s. Annoyed by the complexity of the Ptolemaic model and all its circles, Copernicus suggested that a heliocentric model would make everything much less complicated. In the Copernican model, all the planets orbited the Sun in perfect circles — while this model explained many old problems (including retrograde motion), it too had to use epicycles to make theory agree with actual observations. Copernicus knew that his theory disagreed with the geocentric teachings of the powerful Catholic Church, so he waited until the end of his life to formally publish his findings in a book titled Revolutions.

Retrograde Motion

The Copernican Model provided an easy explanation for retrograde motion (Earth is green, Mars is red).

Nearly a century later, Galileo published Dialogue, which compared the geocentric and heliocentric models in a mock philosophical debate, with heliocentrism undoubtedly coming out on top. The Catholic Church was understandably ticked off at Galileo for publishing this (and he had already published other works contradicting Church teachings), so they convicted him of heresy and sentenced him to house arrest to shut him up. Around the same time, Johannes Kepler used data collected by Tycho Brahe to formulate his three Laws of Planetary Motion, which really did make everything MUCH simpler. However, one casualty of Kepler’s Laws was the idea that planets orbited in perfect circles — instead, he discovered that they revolved around the sun in ellipses. How revolutionary!


† We now know that’s exactly not true, because stars move through space more or less independently of each other, but the effect wouldn’t have been noticeable to the Ancient Greeks.


TL;DR — The Greeks proposed a geocentric model of the universe based on what they observed, and basically everyone agreed with geocentrism until the model became ridiculously complicated in order to fit the data. Copernicus found that heliocentrism was a much more elegant solution; other scientists like Galileo and Kepler supported the Copernican Model in both their writings and their work.