 Greetings and welcome to the Introduction to Astronomy. In this week's Astronomy of Other Cultures we are going to continue our study of Egyptian astronomy and look at the calendar that was developed by the ancient Egyptians. So what kind of calendar did they have? Well, the early calendar was what we would call a lunar calendar based on the moon. So a new month begins when the moon is no longer visible. Now this is a difficult type of calendar to use when you base things on the moon and that's primarily because it involves looking for the moon. So you actually had to go out and see when the moon was visible and when it was no longer visible. Now, of course, as we know, every day is not clear. So what would happen? You would have to be able to make estimations on days when it was unusually cloudy but you knew that you would be getting toward that end where the moon would no longer be visible. This remained used for things like religious festivals but not as the civil type calendar. However, they did develop a solar calendar about 4500 years ago which had 365 days. Of course, that's the same length as our day right now, our year right now I should say. Now this was divided into 12 months of 30 days. So everything was exactly even. You had total of 12 months and each of those months has 30 days. Well if you multiply 12 times 30, you end up with 360. So you didn't quite fit all of the days. What do we do now? Well now we put extra days in some of the months to kind of make it even out. In this case they just had the 5 extra days as a festival at the end of the year. Now you'd just have though you'd have your 12 months and at the end of the 12th month you'd have a 5 day festival and then you'd start the new year again, start new year would start over again with the first month. So you'd have those 5 extra days that would give you the 365 day calendar which is pretty close to what it takes for the earth going around the sun. Now we also divided into seasons there and there were 3 seasons not the 4 seasons we have now of 4 months each. So again 12 months we divide them into 4 seasons of 3 months each. This was 3 seasons of 4 months each and they were divided into 10 day decans. So their week would have been 10 days long and there would have been exactly 3 weeks in every month. So much more uniform in many ways than what we use today. You had 10 days in a week and then 3 weeks in a month and then 12 months in a year and the only thing you had is the extra was that little bit at the end those 5 extra days that were needed to be able to account for the 365 day calendar. Now one of the common things that is looked at with Egyptian calendar is how it was set and how it was related to the flooding of the Nile. Now what they looked for, one of the things that was looked for was the relationship of Sirius to the Nile flooding and it was what we called the helical rising of Sirius. What does helical mean? It really means we are seeing Sirius in the morning sky just before sunrise when we can first see it and pick it out of the sun's glare that was generally related to about the time the Nile flooded. Now that's a little bit iffy as to how accurate that would have been. The Nile flooding was not completely regular so it wouldn't occur exactly at the same time but it was roughly a guide but may not have been the overall basis for a calendar that was used by the Egyptians. So let's go ahead and summarize a little bit of what we looked at with the Egyptian calendar. We looked at the early one was a lunar calendar and then the civil calendar developed that had 365 days and we briefly mentioned the helical rising of Sirius that could have been the basis for a calendar but is in question. But it did tie in to the Nile flooding at around the same time that you would begin to see Sirius in the morning sky right before sunrise. So that concludes this lecture on the calendar of the Egyptians. We'll be back again next time for another topic in astronomy of other cultures. So until then have a great day everyone and I will see you in class.