The speed of light was first estimated in 1676 by a Danish Astronomer who was timing the eclipses of Io caused by Jupiter. He noticed the period between eclipses increased with Earth’s distance from Jupiter and guessed it was because light had to travel a longer distance.

Rømer’s determination of the speed of light

Ole Rømer (1644–1710) was already a statesman in his native Denmark some time after his discovery of the speed of light (1676). The engraving is probably posthumous.

Rømer’s determination of the speed of light was the demonstration in 1676 that light has a finite speed and so does not travel instantaneously. The discovery is usually attributed to Danish astronomer Ole Rømer (1644–1710),[note 1] who was working at the Royal Observatory in Paris at the time.

By timing the eclipses of the Jupiter moon Io, Rømer estimated that light would take about 22 minutes to travel a distance equal to the diameter of Earth’s orbit around the Sun. This would give light a velocity of about 220,000 kilometres per second, about 26% lower than the true … Continue Reading (18 minute read)

9 thoughts on “The speed of light was first estimated in 1676 by a Danish Astronomer who was timing the eclipses of Io caused by Jupiter. He noticed the period between eclipses increased with Earth’s distance from Jupiter and guessed it was because light had to travel a longer distance.”

  1. rmp2020

    His name was Ole Rømer. Just FYI.

  2. JimTheJerseyGuy

    Can someone explain to me how, starting from scratch, you could calculate the distance from Earth to Jupiter. Like, I’m in my backyard with only the tools of 1676 available to me? What are the measurements and math involved?

  3. DarthSanity

    Fun fact: he was exploring the timing of the Galilean moons to use them as an accurate measure of time so that ships could accurately determine longitude on the open seas. A sea captain would have an almanac of eclipse information tied to standard time. So, theoretically if a captain saw the eclipse at 2:30 am, and the almanac said it was 8:30 am in Amsterdam, then he knew exactly how far he was from a known point. Latitude was easy to calculate – just measure the angle from the horizon to the North Star or southern cross.

    So it was thought with just a little bit of math longitude could be calculated fairly easily. This was easy to do on land but wasn’t practical on a ship – the mechanics were complex, and eventually Sea captains had precise chronometers available to them they could use instead.

  4. redsky31415

    I tried to replicate that as a high school project (in Germany we have to do a graded science project before we graduate) and worked together with our local observatory. My result was 30% off but it was still pretty cool.

  5. PakinaApina

    What’s even crazier is that black holes i.e. objects whose gravity is so intense not even light can escape them, were theorized as early as 1783 by an amateur scientist named John Michell.

  6. tdotjeh

    Imagine being an Astronomer trying to get a science grant back then. “You want us to pay you to measure the time it takes the moon of Jupiter to eclipse? That’s ridiculous. What useful information will ever come from that??!” Astromer low key discovers the speed of light.

  7. thebigbadpie

    Must’ve been crazy when they figured out light had SPEED

  8. ItIsBack

    just link the veritasium video

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