 Greetings and welcome to the Introduction to Astronomy. In this week's special topic we are going to look at sidereal and synodic periods and discuss what that means for different objects. So what do we mean by a sidereal or a synodic period? Well, a sidereal period is measured with respect to the stars, while a synodic period is measured with respect to the sun. Now how does that work? That actually gives us different measures for things like days or months, or the rotational period of the planets, or for the revolution periods of the planets around the sun. So let's start looking at the sun here first. So we have a video clip to look at here, and let's go ahead and watch that. And what we see is, as this moves, a sidereal day is relative to the stars, a solar day is relative to the sun. So as Earth rotates here, it's not staying still. It's moving slowly around, and we'll see those white lines connect first. There is the sidereal day, but the sun is not yet back into the same spot. That takes about four minutes more. Now it's been greatly exaggerated here for you to be able to see that. But that's the difference between the solar and the sidereal day, which is about four minutes long. And that makes the difference in what we see in terms of the days. The solar day is what we use. That is our standard timekeeping. The sidereal day is what we're actually rotating relative to the stars. And as you may have noted, this has nothing to do with leap years. Leap years are just that the day and the year are not equal multiples of one another. Now how about the moon? Well the same thing happens for the moon. We have a sidereal period for the moon and a synodic period for the moon. The synodic period is what we're used to. That is the phases, so that is the cycle of phases, which is about 29 and a half days. The sidereal period is about a little over 27 days. Now the reasoning is the exact same thing that we looked at in our video clip here. When the moon is orbiting around the earth, it takes almost a month. So in that time the earth-moon system has moved around the sun and it takes a couple of extra days to get it back to the same orientation relative to the sun to get the same phase of the moon. Now we can also look at this for planets. So what about the planets? Well here we have another thing we can look at. We have two planets and the sun shown here. And we see how these work. We watch the orbits as they go around. And we will see that there they line up again after a certain amount of time, but that is not how long it takes the planet to go around the sun. That is getting them back into the same orientation because they are both moving at the same time around the sun and are moving at different rates. So let's take a look at Venus here. Venus has a synodic period of 584 days and a sidereal period of 225 days. It takes it more than twice as long to get back into the same positioning relative to earth than it does for the sun. So it takes it much longer because earth and Venus are moving at the same time. For something much farther away such as Neptune, a synodic period of 368 days and the sidereal period is over 60,000 days, nearly 165 years. And this is very close to earth's orbital period. Why? Because in one earth year Neptune doesn't move a whole lot. So it's going to stay very close to earth's orbital period. It's essentially earth coming around and having to just go a little bit farther to catch up to Neptune. Whereas Venus and earth are moving at the same speed, so or very close to the same speed. So we get a little bit of a different in the values there that the synodic period is going to be longer than the sidereal period. They're both moving and it takes a longer time, kind of as we see in the video clip here, for them to get back into the same position relative to one another. So let's go ahead and finish up with our summary. And what we've looked at is that we can measure periods relative to the stars, the sidereal period, or the sun, the synodic period. These will have different values because earth is moving in its orbit, as well as the planet is moving if we're talking about another planet. And these will occur for various objects in the solar system. We can measure their periods relative to the stars. That's how long they actually take to go around the sun one time. Or we can measure them relative to the sun. And that is what we call the synodic period to get back to the same orientation, the same earth, sun, planet orientation. So that concludes this discussion on sidereal and synodic periods. We'll be back again next time for another special topic in astronomy. So until then, have a great day, everyone, and I will see you in class.