The actual speed of light is pretty close to 3 x 10⁸ m/s – a crossing time of a bit more than 16 minutes. Rømer was off by about 30%.

50 years later, Bradley would get c to within ~1% when he explained stellar abberation via Earth's orbital motion and a finite speed of light.

But it was Rømer's measurements that made it clear that light was not an instantaneous phenomena.

His data suggested that light travels the diameter of Earth's approximately circular orbit in about 22 minutes. That implies a speed of 2.25 x 10⁸ m/s.

(Variation in the time between Jovian lunar eclipses had been noticed and scrupulously recorded by Cassini, who also considered a finite speed of light as the likely explanation. The clear pattern in Cassini’s observations were what alerted Rømer to the phenomenon.)

But when Jupiter was on the far side of the Sun from Earth, Rømer measured the time between eclipses as slightly longer. When Jupiter was on the same side, the time was a bit shorter.

Rømer carefully recorded variations in this period around the central value of 1.769 Earth days.

After accounting for other effects, he concluded that the variance must be due to a finite, rather than instantaneous, travel time for light.

Image: Linda Hall Library

Before Rømer, most everyone assumed that light moves instantly between distant points. People looked up at the stars and thought they were seeing the universe as it is, not as it was.

Rømer noticed that the time between successive eclipses of Io by Jupiter varied as Earth moved around its orbit.

These eclipses happen regularly, as Io moves in its own orbit around Jupiter, and their timing is more or less evenly spaced.

Every story the Universe tells is a history.

Danish astronomer Ole Rømer was born #OTD in 1644. His observations of Jovian moons confirmed light propagates at finite speed. This profound conceptual shift opened the door for work by Huygens, Fresnel, and Maxwell.

Portrait: Jacob Conring / Wikimedia Commons

There is also an AIP oral history with Charlotte Moore Sitterly, from a 1978 interview by David DeVorkin. These are always excellent!

https://www.aip.org/history-programs/niels-bohr-library/oral-histories/4784

"She was a brilliant scientist." – Vera Rubin

In 2010, Rubin published her recollections of Charlotte Moore Sitterly in the Journal of Astronomical History and Heritage.

You can read it here: https://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?db_key=AST&bibcode=2010JAHH...13..145R&letter=.&classic=YES&defaultprint=YES&whole_paper=YES&page=145&epage=145&send=Send+PDF&filetype=.pdf

She assembled the first version in 1933, then a revised version in 1945. In 1950, while working at the National Bureau of Standards, she published "An Ultraviolet Multiplet Table."

Both works are still considered essential references for astronomers doing spectroscopy. Like most important reference works, they've gone through multiple editions. You can find (and download, if you need spectra) them via Google Books.

https://books.google.com/books?id=BWe85L1gBhIC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false

Charlotte Moore Sitterly collected all the data she could find – both published and unpublished – on atomic spectra, assembling it into "A Multiplet Table of Astrophysical Interest."

https://ui.adsabs.harvard.edu/abs/1945CoPri..20....1M/abstract

Astrophysicist Charlotte Moore Sitterly was born #OTD in 1898.

She compiled extensive data on optical and later UV spectra, and her spectroscopic tables are still in use today.

Image: AIP Emilio Segrè Visual Archives / Michael A. Duncan

The City of Evanston approved construction projects that are killing pedestrian traffic around a local book shop. Now the store is losing thousands. No warning, just the expectation that small businesses would eat the loss. Help them out, order some books!

https://www.bookendsandbeginnings.com/

Look I love Halloween, but my neighbor had this up with three days of summer left.

@Pappy I’m doing the marathon. Volumes 1-10.

My big weekend plans: Doing a very, very long calculation, and watching about 15 hours of short documentaries about the geography, history, and culture of the part of southern Appalachia spanning East Tennessee from the Smokies to the Cumberlands.

"The Heartland Series” ran on WBIR (Knoxville’s CBS affiliate) from 1984-2009. When I was a kid it was filler that I had to sit through while waiting for primetime TV. Now, as an adult, I'm going to mainline it.

https://www.youtube.com/watch?v=4gPCpuxCodY

@wikicliff You've got it. The name Δs² is physicist notation, to remind us that what we're really doing is taking two copies of a “four vector” – an array containing the Δt, Δx, Δy, and Δz info for the two events – and combining them according to a specific rule.

@HomerHarlequin Thanks!

@richardgrant Thanks!

@tehstu Thanks!

Minkowski had formulated this picture of spacetime, with its unusual geometry governed by Δs², by 1907.

There is of course a lot more we can squeeze out of the spacetime interval, but I can't get into that right now because I have to go teach my students about the paths followed by light rays spiraling around a black hole!

So this is our notion of causality, which is essential for physics.

Timelike and lightlike separated events can have a cause-and-effect relationship because there’s no question which one happened first.

Events with spacelike separation cannot have a cause-and-effect relationship.

Material objects must move slower than c, and everything happening to them at one instant is the effect of what was happening the previous instant. Therefore, material objects move along timelike trajectories.