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1. Robert McNees (mcnees@mastodon.social)'s status on Monday, 25-Sep-2023 14:25:33 UTC Robert McNees

   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

   • Robert McNees (mcnees@mastodon.social)'s status on Monday, 25-Sep-2023 14:27:14 UTC Robert McNees

     in reply to

     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.

   • Robert McNees (mcnees@mastodon.social)'s status on Monday, 25-Sep-2023 14:28:18 UTC Robert McNees

     in reply to

     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

   • Robert McNees (mcnees@mastodon.social)'s status on Monday, 25-Sep-2023 14:28:42 UTC Robert McNees

     in reply to

     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.)

   • Robert McNees (mcnees@mastodon.social)'s status on Monday, 25-Sep-2023 14:29:41 UTC Robert McNees

     in reply to

     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.

   • Robert McNees (mcnees@mastodon.social)'s status on Monday, 25-Sep-2023 14:30:10 UTC Robert McNees

     in reply to

     The speed of light is, to the best of our knowledge, the upper limit on the speed with which any causal influence can propagate in Nature.

     Looking at the stars, we are not seeing things as they are. Even looking at a person standing in front of you, there is a short delay between light leaving them and reaching your eye.

     We see everything as it was and nothing as it is. Every story the Universe tells is a history.

   • kites (kites@mastodon.social)'s status on Monday, 25-Sep-2023 14:52:18 UTC kites

     in reply to

     @mcnees this might be a naive question, but intuitively wouldn't we expect to observe a difference in transit time when Earth was moving away from Jupiter vs towards Jupiter? In other words, when Earth is on the left vs the right in the diagram? I'd expect the observed transit time when Earth is on the top vs bottom to be similar to each other.

   • DocHolliday (wmshakesp3are@mastodon.social)'s status on Thursday, 28-Sep-2023 06:22:21 UTC DocHolliday

     in reply to

     @mcnees not to mention Fig Newton.