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Corey S Powell (coreyspowell@mastodon.social)'s status on Thursday, 28-Aug-2025 09:05:50 JST Corey S Powell

Last week I came across a remarkable archival document by astronomer William Herschel. He describes his telescopes as time machines, capable of "penetrating into time" as much as 1,910,000 years into the past.
His specific numbers were off, but his insight was spot on. In the year 1800!
#space #science #history #nature
In conversation about 9 months ago from mastodon.social permalink
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1. Manuscript page by astronomer William Herschel, describing how his telescopes penetrate into time as well as into space.
  https://files.mastodon.social/media_attachments/files/115/080/968/938/962/194/original/3cf634cb465331a1.jpg
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  ShadSterling (shadsterling@mastodon.social)'s status on Thursday, 28-Aug-2025 09:06:26 JST ShadSterling
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  @coreyspowell I hadn’t known that the speed of light had been known before Maxwell, so this prompted me to look up the history, and the estimates before 1800 were pretty good: https://en.m.wikipedia.org/wiki/Speed_of_light#History
  So I guess Herschel’s specific numbers were off because it turns out measuring the speed of light is a lot easier than measuring the distances to stars
  In conversation about 9 months ago permalink
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  1. Domain not in remote thumbnail source whitelist: upload.wikimedia.org
    
    Speed of light
    
    The speed of light in vacuum, commonly denoted c, is a universal physical constant exactly equal to 299,792,458 metres per second (approximately 1 billion kilometres per hour; 700 million miles per hour). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time interval of 1⁄299792458 second. The speed of light is the same for all observers, no matter their relative velocity. It is the upper limit for the speed at which information, matter, or energy can travel through space. All forms of electromagnetic radiation, including visible light, travel at the speed of light. For many practical purposes, light and other electromagnetic waves will appear to propagate instantaneously, but for long distances and sensitive measurements, their finite speed has noticeable effects. Much starlight viewed on Earth is from the distant past, allowing humans to study the history of the universe by viewing distant objects. When...
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  Corey S Powell (coreyspowell@mastodon.social)'s status on Thursday, 28-Aug-2025 09:06:27 JST Corey S Powell
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  Transcript of the Herschel letter:
  In conversation about 9 months ago permalink
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  1. Appendix to the Paper on the power of penetrating into Space by telescopes* After having shewn the extent of the power of my 40-foot telescope to penetrate into Space, I should have added page 84, that this instrument may be said to have also a power of penetrating into time: at least with respect to what is past. For by looking through it at one of the most distant nebula that was visible in it, we may be convinced, by actual inspection, that we see an object, which certainly was in the state we now perceive it, above nineteen hundred thousand years ago; and by viewing others less and less distant so have the same conviction of existing objects, in different periods, brought from that immense duration down to the present time. The proof of this rests upon the known velocity of light, by which we calculate that rays from the star Sirius for instance cannot be less than six years and nearly three months and a half coming to the earth; and that consequently, from such an object as we have pointed out in the paper referred to, a ray of light could not possibly have reached the telescope at the time we are looking into it, had it not begun its flight at least one million, and nine hundred & ten thousand years ago. * See Phil Trans. for 1800 Page 49.
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  Minoru Saba repeated this.

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