Calculations of the Hourly Angle of the Sun

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You can calculate the hourly angle of the sun by observing a few key characteristics throughout the day. Calculate the hourly angle of the sun with help from a research scientist and one of the world's leading experts on star formation in this free video clip.

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Video Transcript

Hello, my name is Harold Yorke, I am a research scientist in Pasadena, California, and I'm going to explain how to calculate the hour angle of the sun. So let me first define what I mean by the hour angle of the sun. This is the angle that the earth would have to rotate so that the sun appears to be at high noon. In other words, the sun is at the highest distance above the horizon that it will get during the day, at high noon that angle is 0 degrees. An hour before high noon the angle is minus 15 degrees. You take the time it takes to get to high noon and you multiply by 15. At 1:30 of your time, that would be an hour and 1/2 times 15 and the angle is 22.5 degrees. Now how do we get the local time to use in this equation because the time that your clock shows is something different. The best way to do it is to have a sundial and if the sundial tells you it's 10:30 in the morning then you multiply 1.5 hours by 15 degrees. If you don't happen to have a sundial then you will have to convert the clock time to the local solar time and there's 4 steps. You first convert the clock time to something called Universal Time. In Pasadena, California we are in Pacific Standard Time or Pacific Daylight Time and that conversion factor is 8 hours during standard time during winter and seven hours during daylight time. So I would have to take my time on the clock and add seven hours. I then convert the universal time to the mean local solar time and that depends on the longitude. Here in Pasadena where I have a longitude of 118.14 degrees so I divide 118.14 by 15 to get that correction. Then this is only the mean solar time, the sun is not a very good clock it sometimes speeds up and slows down during the course of the year. So we have to use something called the time equation to convert the mean local solar time to the local solar time, this is the time that a sundial would show. Once you have the local solar time, then you can can calculate the hour angle of the sun. So I hope you enjoyed hearing how to calculate the hour angle of the sun. Thank you for watching.


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