 Greetings and welcome to the Introduction to Astronomy. In this week's Misconception in Astronomy we are going to look at leap years and find out if every fourth year is truly a leap year. So let's go ahead and look a little bit about what are leap years. So why do we have a leap year? Why is it necessary to change the number of days in a year? Well that's because the definition of the day and the definition of the year don't come out in even. There are not an even number of days in the year. And that's because our day is based on Earth's rotation as we see here taking the Earth about 24 hours to rotate once and the Earth's revolution is the time it takes to go around the Sun. And that is about one year that it takes to make a complete trip around the Sun from December through March through June through September and back to December again. So when we measure this it actually takes 365.2422 days for the Earth to orbit our Sun. So what is the problem then? Well the difficulty is that we are off. So we're off by almost a quarter of a day every year and if we didn't do anything about that the seasons would slowly change when they occur because the number of days we're losing we're just cutting off and losing that quarter of a day. So this was originally fixed back in Roman times Julius Caesar gave us the Julian calendar which says the years that are divisible by four are leap years. So that's why we say that every fourth year is a leap year. However this is not correct because we're still going to be slightly off on these and in fact the Julian calendar is off by eleven minutes per year. Now you may look at that and say what difference does it make if we're off by just eleven minutes. That's not very much it's certainly a lot less than a quarter of a day and you're not going to notice it. However over time this has built up and over from the time of the Romans until the 1500s it ended up being the way there were days off in fact almost two weeks off in terms of the distance between the seasons and the actual calendar so over 15 nearly 1500 years that had built up to be a number of days so an adjustment had to be made to bring this back into line and one thing was to actually change the day. So in 1582 we dropped ten days out of the calendar to make up for all the time we had lost all of the leap years that we have had we had in that time that we should not have and the change that was instituted was to adjust century years that century years are not leap years unless they are divisible by 400. So under this years like 1700, 1800 and 1900 would not be leap years. 2000 was a leap year but 2100 and 2200 will not be leap years because they are not evenly divisible by 400 and that brings everything back into line a lot better and here we can see in a chart as to how this works so here is where June the first day of summer would occur around June 21st and we see that there's a slow change here that each shift is because of a leap year so we move down and then when there's a leap year we jump back up and you see that there's a slow drift over the course of time and then the century year brings that back up here and then we slowly drop down again but otherwise this would continue to rise over time if we did not affect that century year here so here's where we skipped that so instead of jumping way up here as we would normally do on the Julian calendar we let that run for another 100 years before we do the shift and you can see that overall it keeps the days pretty close to within about a day or so of around the middle of June 21st so that is the Gregorian calendar is it perfect no it's off by 26 seconds a year but it's going to take a very long time for that amount to add up to make a very significant difference so let's go ahead and summarize what we've looked at here and we've talked about leap years and that they exist because there are not an even number of days within each year that fractional part that's missing builds up over time and causes the calendar to drift and we can use leap years by adding that extra day will help us minimize this problem with the calendar so that concludes this lecture on leap years we'll be back again next week for another misconception in astronomy so until then have a great day everyone and I will see you in class