 Greetings and welcome to the Introduction to Astronomy. In this week's special topic we are going to discuss quadrature and what that means. So you may have heard this term, and it is one of those used to discuss the various positionings that can occur within the solar system. So here we have an image of our solar system with the Sun at the center, here's our Earth's orbit, inner, and here's the orbit of Mars, one of the outer planets. Now we have various different positionings that can occur. Mars can be in the same direction as the Sun in the sky. So from this position, as seen in this direction, this position, Mars would be at conjunction. It would be in the same direction in the Sun in the sky. From the opposite side, if Mars is in this direction, then we would see Mars at opposition or opposite to the Sun in the sky. So what does quadrature mean? Well quadrature occurs when we have these at a right angle. So the object forms a 90 degree angle with the Earth and the Sun. So here is the Earth's Sun line, and something perpendicular to that we would see quadrature here in two different positions. So the moon or Mars could be at quadrature. So this occurs generally if we're talking about the planets for a superior planet when the elongation, which is its angle here between the Sun and the planet, is equal to 90 degrees. And this will occur twice. So this will occur two times during the orbit. It will occur at a western quadrature when the planet is to the west of the Sun in the morning sky or at eastern quadrature when it is east of the Sun in the evening sky. So we can see those two times this will occur when that angle will be 90 degrees. Now how does this work for the moon? How about the moon, because the moon can be at quadrature as well. Well this is not the time of first or third quarter moon, which might be your first thought. It is when the angle is 90 degrees, but because things are not infinitely far away, the first third quarter moon will not occur at exactly that same time. So that angle of being 90 degrees will not be at the same time as the first or third quarter moon. So that concludes this discussion of quadrature and what that means in terms of positioning of a planet or the moon in the sky. So let's summarize here. What is quadrature? Well it is when the superior planet and the earth and with the earth and sun, so these three objects make a 90 degree angle. It occurs twice, once in the evening and once in the morning sky as the planet orbits. And the moon can also form a 90 degree angle, so be at quadrature, but this will not be at first and third quarter as you might expect. It will actually be a little bit off of those just because the sun is not infinitely far away. So that concludes this discussion of quadrature. 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.