 Greetings and welcome to the Introduction to Astronomy. In this week's special topic in astronomy we are going to talk about the equinoxes and solstices and learn a little bit about what those mean. So what are the equinoxes and solstices? Well let's look at the equinox first. The equinox in astronomy is defined as the points where the Sun crosses the celestial equator. Let's take a look at that here on our image and we see that we have the celestial equator is the dotted line here and this pink line is what we call the ecliptic or the path of the Sun. So we note that those two circles on the celestial sphere will cross at two points once here in March and once here six months later in September. So we have what we call the vernal equinox in spring and the autumnal equinox in fall. Now what do we mean by equinox? Well equinox means equal night and that's when the days and nights are the same length. So on those days you would have 12 hours of daylight and 12 hours of darkness. Other days you would have different amounts depending on where the Sun is in the sky. So those are the equinoxes. What are the solstices? Well the solstices are the two other extreme points on the celestial sphere. The equinoxes are where you cross the celestial equator. The solstices are the highest point above the celestial equator and the point where you are farthest below the celestial equator here. So this would be the position of the Sun in December. That would be the winter solstice and this would be the position of the Sun in June or the summer solstice. So they are the highest points. June would be when the Sun is highest in the sky and the December would be when it is lowest in the sky. Now this is going over from a northern hemisphere perspective. These would be switched in the southern hemisphere. So in June in the southern hemisphere the Sun would be lowest in the sky. And in the December it would be highest in the sky. Remember that seasons are reversed between the northern and southern hemisphere. But again we're looking at this from the northern hemisphere perspective right now and that would mean that June would be when the Sun is highest and you'll notice that looking out at the sky you would see that the Sun would be higher in the sky in June at noon time and in December at the same time of day it would be much lower in the sky. And that's part of what gives us our seasons. Now solstice itself means sun stop. Now the Sun doesn't physically stop but it stops getting higher or lower. So between the time of the equinox here the vernal equinox and the summer solstice the Sun is continually getting higher and higher in the sky. Around June 21st it reaches its peak that's where the Sun stops getting higher and starts getting lower again. And we'll continue that all the way through September to December when it reaches its very lowest point in the sky. So these are what we use as the first days of each of our seasons. So we can see them a little bit better here. Let's take a look at those. We see the summer solstice here. That is again when the Sun is highest in the sky. And we've turned things a little bit so that this is the path of the Sun. The ecliptic is now tilted here. So in summer the Sun is much higher. Then six months later in the fall it's going to be equal. And then it's going to be down at its lowest point in winter. And then continue back up to the spring equinox. And then the cycle continues year after year. Now we can actually see this in the sky in an image that is taken. And let's take a look at what example of this. This is an example of an image taken of the position of the Sun. It's called a solar graph. And it looks at the positioning of the Sun over the course of a year. It's essentially a pinhole camera that just constantly records the position of the Sun. So each line here going around would be one position of the Sun. Down here at the bottom you'd have winter. Up here at the top you would have summer. So the Sun takes a different path in the sky getting not as high during the winter and getting much, much higher during the summer, part of which gives us our seasonal changes. So let's go ahead and finish up with our summary. And what we looked at this time is we talked about the equinoxes and how they occur when the Sun crosses the celestial equator. We have the vernal equinox in spring, the autumnal equinox in fall. The solstices occur when the Sun is at its highest and lowest points. And these represent the first day of each season. So that's when the seasons start at the occurrence of each of the equinoxes and the solstices are the start of each of our seasons. So that concludes this lecture on the equinoxes and solstices. We'll be back again next week for another special topic in astronomy. So until then, have a great day everyone and I will see you in class.