 Later in the 1670s, astronomers made measurements of Jupiter's moons when the Sun was nearly between Earth and Jupiter, or when Jupiter is nearly in conjunction. That is a little more than six months later in the year. These measurements are more difficult for astronomers to make. Jupiter and the moons are not in the nighttime sky, and while they are in the daytime sky, you can't really see the moons. It's difficult to distinguish the reflected light from Io from direct sunlight. But astronomers could see Jupiter and its moons on the horizon at dawn, or dusk, when there was not much sunlight, and they began recording the times at which Io passes behind Jupiter's shadow. Now the distance between Earth and Io is very different when Jupiter is in opposition and in conjunction. It differs by the orbital diameter of Earth. It's a huge distance, 2.98 x 10 to the 11 meters. So if we measure the time that it takes to do a little more than six months' worth of Io orbits, which ends up to be 103 orbits, then the time lags due to light travel might be important. So let's do some reasoning with the picture. The clock has started the first orbit when Jupiter is in opposition, and there's a time lag for the light to travel to the Earth. So the Earth sees the star of the first orbit time at T0, that's equal to X1 on C. Later when Jupiter is in conjunction, the Earth sees the end of the 103rd orbit at a clock time of this. The first term is the time that it takes for Io to orbit 103 times. And the second term is the time that it takes for the light that signals the end of the 103rd orbit to travel to Earth. So the time of 103 orbits, as seen from Earth, is this. Now the difference in path lengths of light is simply the orbital diameter of Earth. Indeed in the 1670s, the astronomers' measurements showed that the time to take 103 orbits was 16.5 minutes longer than they expected 103 x 42.5 hours. That is, from Earth it appears as if the Io's orbit is taking longer and longer as it disappears from the night sky towards the morning horizon. Io's orbit time is indeed 42.5 hours, it's fixed at that time. But the apparent slowing down of the orbit is due to the extra time that it takes for light to travel Earth's orbital diameter. It was Romer who collected the numbers together, but he didn't have an accurate orbital diameter of Earth, and he came up with a speed of light of 2 x 10 to the 8th meters per second. That's 2 with 8 zeros after it, or 200 million meters per second. Today when physicists use the correct interplanetary distances, they find that the speed of light in space is 2.997 x 10 to the 8th meters per second. So Romer was off, in the 1670s, by about 33%, but he did get the right order of magnitude for the speed of light, hundreds of millions of meters per second. Often, we round the numerical value of the speed of light to 3.0 x 10 to the 8th meters per second. Light is slower when it passes through air, water, or other transparent media. It is fastest in space, or vacuum. The speed of light in vacuum is a fundamental constant in physics, a constant that is used so often that scientists recall it off the top of their heads. We will encounter the speed of light several times in upcoming videos. Next time, we're going to look at how light reflects off of non-luminous objects, like my books, and we'll also look at reflection from objects with mirror-like surfaces. This is our first example of using a model of light, called the ray model of light.