 Greetings and welcome to the Introduction to Astronomy. In this video we are going to talk about time and the calendar and how those are used in astronomy to be able to tell how we use astronomy really to tell time. Most of what we use for general measurements of time such as the day, the week, the month, and the year all come from astronomical events and have their basis and that is one of the earliest jobs of the astronomer long ago was to be able to keep track of the calendar. So let's get started here. And first of all in terms of keeping time what the early astronomers did was to develop a calendar that was based on astronomical events. So that they were able to use things like the day, the week, the month, and the year which all have astronomical significance. The day is related to the rotation of the earth. The earth takes about 24 hours to spin once on its axis and that would cause the sun to rise and set over the course of a day defining our day from sunrise to sunset. The month was based on the phases of the moon. Our moon goes through a cycle of phases and it takes about 30 days and that is the basis of our month is the length of the cycle of the phases of the moon. And the year is the revolution of the earth around the sun. That's not something we can see directly but we can see the changes in the patterns of the constellations over time and that changed on a yearly basis which gave us our measurement of a year. The most, the one that's at least obvious would be the week what in astronomy is seven. So we have seven here. There were seven objects that move among the stars and that would include the sun and moon and the five known planets to ancient astronomers. So seven objects there and one of them is named for one day of the week is named for each of these objects. So Sunday and Monday for example. But also each of the planets are named for one of the days. So let's look at each of these in a little bit more detail and what we see first of all in terms of measuring the day how do we measure a day? Well how long is a day? Well there are several different measurements of a day. The earth's rotation is actually 23 hours and 56 minutes. So it takes a little bit less than 24 hours for the earth to spin once but the solar day is 24 hours. This is what we use. We use the solar day as our measurement of time not actually the earth's rotation. Because we don't see the earth rotating we can measure this relative to the stars. The sidereal day is measured relative to the stars but that's not something we can easily see directly when a star gets back to the same position in the sky. We easily see when the sun is back to the same position in the sky. So we measure everything relative to the sun. Now why are the two different? Why is there this four-minute difference between the solar and the sidereal day? And that's shown in the image here that if you have the earth here rotating and moving around the sun at the same time. So the earth rotates and relative to the very distant stars when it moves from this position to this position now some distant star off in the distance here is back in the same position relative to the relative to the horizon. However the sun is now in a different direction. So seen in the images here at the first image the sun and the star would be exactly lined up. In this case 23 hours and 56 minutes later one sidereal day later the star is back in the exact same position but the sun hasn't quite gotten there yet. So the earth actually has to rotate a little bit more just a small amount and that corresponds to the four minutes that it takes for the sun to get back in the same position and that's what we use for the day. So we use what we call the mean solar day is the average length of a solar day. Now let's look at some concerns here. What are some concerns with the so using solar time? Well one of the difficulties is that things do not move consistently. So when we're looking at solar time here we can have apparent solar time which is the time given by a sundial. So where the sun is in the sky. However this is not the same from day to day and can actually vary by about a maybe a half an hour or so depending on what day of the year it is. The solar time can change a little bit because of the earth's motions. So what we really look at and what we use for time here on earth is the mean solar time and the mean solar day is exactly 24 hours. So that is the average that's what we average out to the solar day but it really does depend. We also use time zones on earth and that is means that time is not the same everywhere on the earth simultaneously. So for example on the east coast of the United States it could be noon but on the west coast it could be nine in the morning. So east coast say here it might be noon 12 12 o'clock but on the west coast it is 9 a.m. it is still much earlier and that is done to make things consistent so that noon does occur during the middle of the day when the sun is up and midnight occurs when the sun is down. So the sun that's using the solar time so we divide it into 24 regions instead of consistently changing it. Apparent solar time would change consistently whereas mean solar time we use the time zones essentially divide the earth into 24 hour regions and that minimizes the number of time change changes you might have otherwise as you're traveling you might have to adjust for solar time you'd have to adjust your clocks all the time as you're moving further east or west time would constantly be changing. So what was nine o'clock could become 9.05 if you've moved a little bit to the east or west the times are consistently changing so we have very specific zones where these changes occur. Now another thing that you've heard of is what we call daylight savings time. This is nothing really to do with astronomy other than that it is an adjustment to the time and it is a shifting the time so the whole idea is to move the sunlight time into the evening. So we go on daylight savings time during the summer and that gives fewer daylight hours in the morning the sun will rise a little bit later but it will set a little bit later at night as well giving more daylight hours in the evening. It really does not change anything all it is is done in offset we take away one hour in the fall and we add back one hour in the spring so the whole nothing changes in the long run we just are really shifting when the times are occurring. Now let's look a little more about the calendar here so what do we look at with our calendar? Really we want to look at the times and our calendar is very complex so when we look at the calendar here it can be very complicated because they are determined by astronomical motions that are not related to each other so why do months have different numbers of days in them why are there not an even number of days in a year why are they not why we sometimes have leap years that have an extra day in them and that is because they are not particularly lined up and these are primarily the day the month and the year the day we know is the rotation of the earth relative to the sun and there are 29 about 29 and a half days in an average month so it averages out around 30 we have some days with 30 some days with 31 and a month with 28 and averaging everything out will come out to about 29 and a half the number of days in a year is 365 and a quarter and that gives us the leap year so these fractional values that we end up do give us the fact that we have some months with 30 and some with 31 days and that we have the leap year where we add an extra day in every four years which takes care of most of this fraction fractional portion here adding in that extra day every fourth year will kind of become very close to rounding out the year but it is a difficulty because they are not directly related to each other so they don't fit very easily when we can't adjust them we can't just make them be 30 months or 30 days in a month because that would that is going by the motion of the moon we can't change the phase cycle of the moon we can't change the revolution of the earth around the sun and make it exactly 365 days and that's what it would take to eliminate some some of these problems so let's look at some of these early calendars and early ideas of the calendars that we had so stonehenge is one example that we look at there are a lot of alignments there with the rising and setting of the sun and the moon and this was done thousands of years ago so we don't have any documentation to know what they were trying to do but we do see that there are these alignments the main one is the rising of the summer solstice sun that rises at an exact spot over any over a special stone if you're standing at the middle of this the summer solstice sun will rise exactly over this one stone and that is the only day that will happen so that would signify the beginning of summer and likely we believe stonehenge would have been used as a very early example of a calendar the Mayans gave us a very complex calendar not based on the sun or moon but actually based on Venus so a little bit differently done there and we have the Chinese calendar which is a 12 which is based on the 12 year cycle of Jupiter if you've ever looked at the Chinese New Year they go through a cycle of 12 different animals for each one for each year where does that come from well Jupiter has a 12 year cycle and that's where that comes from and that gives us the 12 year cycle of the Chinese zodiac so let's look in a little bit more detail at the ones that we at the calendars that we use and those are we'll look here at the Julian calendar first the Julian calendar was introduced by Julius Caesar and the year was approximated as being 365 and a quarter days so if you remember from the previous slide it was actually 365.2422 pretty close to 365 and what Caesar did was to add in a leap year every fourth year to keep the calendar from shifting otherwise you'd be you'd be have this extra quarter of a day every year so we had to make adjustments and what was done was to change this to use this and the problem that we found is that there was an 11 minute difference now 11 minutes doesn't sound like all that much but from the time of Caesar until 1582 when the modification was made that spring was actually starting 10 days earlier that 11 minute difference here had added up to 10 days over over more than a thousand years and it would continue to grow because it was adding 11 minutes every single year you had your leap year every four years and you needed one just a little bit less than that every three in a fraction years and of course you can only have a leap year or not have a leap year so what was done was to change this was done in 1582 and the Gregorian calendar which we use today is dropped first of all it dropped 10 days out of the year to bring things back into sync so to get back to the day that was wanted so essentially what happened is that October 4th became October 15th those 11 days that we had added in that 11 11 days we added or 10 days that we added in relate to that 11 minute change that added up over the course of all those years so we went from October 4th to October 15th that one year so October 5th through October 14th did not exist in 1582 and this changed how and they also changed how the leap year were yes it kept Julian Julius Caesar's idea that every fourth year was a leap year except we had to get it a little bit closer we had to take that century years had to be divisible by 400 to be a leap year so that would mean that 2000 was a leap year because it's evenly divisible by 400 but 1700 1800 and 1900 were not leap years so that means that 2100 2200 and 2300 are not going to be leap years because they are not easily divisible or not evenly divisible by 400 the year 2400 will be another leap year so that brings us very close to matching up that number of the actual number of days in a year which was 365.2422 that gets us very very close to that number and it'll still it still slowly drifts but it's a much longer period time it's not something that you will notice in hundreds or even not much in terms of thousands of years because we have a very very close approximation for the number of days in a year now so let's finish up here with a little summary of what we've looked at in this lesson we did talk about the day being based on the rotation of the earth so that is that and there and that is based on the rotation of the earth relative to the sun so not just the rotation of the earth itself it's actually the rotation relative to the sun that gives us that's why we have that four-minute difference between the solar and the sidereal days that we talked about that is because the earth is moving around the sun at the same time it is rotating time is based on these unrelated astronomical motions we talked about the motion of the earth spinning on its axis the motion of the moon around the earth and the motion of the earth around the sun so there are not an even number of days in a month or days in a year they're not even and we use things like leap years to make this adjust and to keep the calendar from slowly shifting so that concludes our discussion of time and the calendar and we'll be back again next time for another topic in astronomy so until then have a great day everyone and i will see you in class