 So, I'm sure a lot of you are wondering what we do with the hour angle, because it's not something that we really ever used before in regular everyday language, right? But because so many of our mathematical calculations are in terms of angles, remember angles are coordinates in the solar reference, in the solar favorable reference. So when we normally have, you know, 24 hour time, we need to convert that time into something on the order of 360 degrees, or in our case, minus 180 degrees to plus 180 degrees, right? And so we're going to figure out a way to convert 24 hours of time to degrees, and those degrees are going to be found as the hour angle, right? So time is going to be in terms of decimal hours, so time in decimal hours. And we're going to make sure that we do that in a 24 hour time frame, so that if I were talking about 1.30 in the morning, I would represent this as 1.5, right? If I wanted 1.30 in the afternoon, based on the 24 hour system, that would be 13.5, right? Okay, so we have that understood. So in order to get the hour angle of time, right, we're going to start with time is going to be the hour angle times the conversion of 1 hour per 15 degrees of rotation of the earth, right? So that's going to give me my decimal hours, multiply them there, such that if I wanted to have the hour angle, I would have time times 15 degrees per 1 hour, in which case my time is in hours. The units of hours would cancel out, and I'd be left with a units of degrees, right? Now one of the things that you're going to find in your problems is the calculation of day length, and the day length in the textbook is shown by calculating the hour angle of the sunset. And we do sunset because it's a positive angular value. Basically if I were to, here let me do a quick diagram, if I were to put noon, right, 12 o'clock noon here, anytime before noon would be negative, anytime after noon would be positive. So this is where I have my negative 180 degrees going into the morning, you're looking back before noon, so you have negative degrees going into the afternoon, you're going after 12, so you're adding degrees, so it's positive. So I have a positive value for the sunset, right? And so if I wanted to calculate the sunset, we find that I need to calculate the arc cosine, or the inverse cosine of the negative tangent of the declination times the tangent, the latitude, v. And I might have switched these two guys around in the textbook, but you're going to get the same answer. So that will give me the time of sunset, or the hour angle of sunset, and the hour angle of sunrise is going to be the negative, the sunset. And then the last part that you're looking for hours in a day, right, is going to be two times, because the sunset hour angle times the conversion of one hour per 15 degrees, that's going to give you the number of hours in the day, and you're going to want that for one of your answers as well. Okay?