The Mouse can’t remember when he first stepped onto the boat, or how he got there, or the name of the cursed river. All he knows is that there are always new souls lined up on the shore, waiting to be ferried across, coins plucked from their eyes to pay the way.

@jimfrost Wonderful shots

@mckra1g I’m putting this in my rotational motion lecture notes. @angst_ridden

Astronomically, January 1st is a meaningless date. It is an arbitrary reference point in a system based on the approximate orbital period of an otherwise unremarkable star that is perhaps only notable for having a life-bearing planet.

But for those of us who reckon time by this system, today is the 99th anniversary of humanity taking a monumental step towards understanding our place in a Universe that is vast and puzzling, but ultimately knowable.

Happy New Year!

Image: Richard Powell

Hubble kept Shapley in the loop as he built the case for separate galaxies throughout 1924, alerting him to the discovery of Cepheid variables in M31, M33, and other spiral nebulae.

Shapley, who had long argued that the Milky Way was the whole Universe, doubted Hubble at first. But he eventually relented in the face of growing evidence.

Upon receiving one of Hubble's letters, Shapley remarked to doctoral student Cecilia Payne-Gaposchkin “Here is the letter that destroyed my universe.”

In the early morning hours of October 6, 1923, Edwin Hubble took a photo plate of M31 showing a Cepheid variable star. He originally mistook it for a nova - you can see where he has crossed out an “N” on the plate and excitedly replaced it with “VAR!”

Applying Henrietta Swan Leavitt’s period-luminosity relationship to subsequent observations, Hubble concluded that the distance to M31 was greater than reliable size estimates for the Milky Way.

Image: Carnegie Observatories

This important property of Cepheid variable stars was discovered by astronomer Henrietta Swan Leavitt. It makes them “standard candles” (a term she coined) that we can reliably use for establishing distances to cosmologically nearby galaxies.

Henrietta Swan Leavitt’s period-luminosity relationship for Cepheids is one of the first rungs on the “cosmic distance ladder,” the collection of methods used by astronomers to measure extragalactic distances.

It was the identification of Cepheid variable stars in M31 and other spiral nebulae that allowed Hubble to prove they were so far away that they must be outside the Milky Way.

A Cepheid variable is a type of star whose brightness waxes and wanes over a period of time that tightly correlates with its maximum brightness.

Measure that period and you know its absolute brightness. Compare that to how bright it appears, and you can estimate its distance.

But there had always been astronomers who suspected that the Milky Way was just one of many galaxies. This view goes at least as far back as Immanuel Kant’s “Universal Natural History and Theory of the Heavens,” which he published anonymously in 1755.

It’s hard to imagine the pre-Hubble view of the Universe, though it was less than 100 years ago.

Just a year before Hubble’s announcement, many astronomers thought the collection of stars that make up our galaxy was Everything.

Astronomer Harlow Shapley was perhaps the leading proponent of the establishment view. He had long argued that spiral nebulae seen by astronomers — shapes any kid would now recognize as galaxies — were just dust clouds inside the Milky Way.

Images: Huntington Library

Hubble’s announcement — Other galaxies exist! Our Milky Way is just one in a Universe full of them! — was made on the third day of the 33rd Meeting of the American Astronomical Society, in a paper read by H.N. Russell. The meeting started on December 30th; I don’t know if Hubble waited for New Year’s Day to be dramatic.

For astronomers, the announcement probably wasn’t a sudden revelation. Hubble had been discussing this result with colleagues, and word had gotten around.

It seems hard to believe, but there are people alive today who were born into what scientists thought was a much smaller universe.

Former US President Jimmy Carter was three months old when Hubble made his announcement.

Betty White, who passed away just a few years ago, was always my favorite example of this. She was about to turn three years old when we realized there are other galaxies.

@cstross @ZachWeinersmith Was going to go with Hieronymus Bosch, but I think Charlie has the right answer.

Edwin Hubble announced #OTD in 1925 that Andromeda and other spiral nebulae were in fact separate galaxies outside the Milky Way, in a paper read to an AAS meeting by H.N. Russell.

The Universe was far larger than what many astronomers had allowed themselves to imagine. It was more than just our little island of stars.

Hubble built on observations by Vesto Slipher, collaborated with Milton Humason, and relied on Henrietta Swan Leavitt’s Cepheid variable work to reach this conclusion.

@ladyteruki Thank you. I am so sorry for what you experienced.

This afternoon we set up the 11yo’s new microscope. While preparing her first slides, she dropped a pair of sharp tweezers on her bare foot and started to bleed. She immediately grabbed a slide, wiped a little blood on it, put it under the microscope, and snapped her first photo!

If you are already in 2024, please no spoilers.

@ojelabii Of course!

Okay if I buy Baldur's Gate 3 how much of a hit is my productivity going to take?

@alfora This is really, really cool. How many pictures? One per chapter?