Full transcript
0:00so the next thing we're gonna do is
0:02define a day and yes I'll admit that
0:05probably sounds silly I mean everyone's
0:08used to what a day is but in astronomy
0:11we actually end up with two different
0:13types of day the so called solar day
0:15which is what you're used to and the
0:18sidereal day okay so a solar day again
0:21is from one noon to the next 24-hour day
0:25that we're all used to sidereal day well
0:29to think of sidereal it's really
0:32relative to the Stars but I usually
0:35define sidereal day as to think 360 then
0:38talk about sidereal day insider a month
0:40so when you think sidereal think 360
0:43degrees one full rotation so a sidereal
0:47day would mean when the earth does one
0:50full 360 degree rotation well you might
0:56be surprised to know that that's not
0:57what our day is the normal day we're
1:00used to is when the earth is actually
1:02rotated more than 360 degrees and to
1:05understand that we have this nice little
1:06image here so we're gonna think this red
1:10line or the line inside the circle is us
1:13we're standing on earth we're looking up
1:16at noon looking not looking at the Sun
1:18you shouldn't look directly at the Sun
1:19always a bad idea but right above us
1:22would be the Sun it's noon well a day
1:25later after we've done a full 360 degree
1:29turn here we are but we're not lined up
1:32with a Sun yet and the reason is because
1:34remember earth is rotating around the
1:37Sun and so it's actually moved in that
1:39time and I mean this is exaggerated but
1:42it's moved in that time and so we
1:45actually have to go further than a full
1:46360 degree rotation to get lined back up
1:50with the Sun to get back to our noon and
1:52the difference is approximately four
1:54minutes so a sidereal day is about three
1:5723 hours of 56 minutes and now it might
2:00again seems silly why do we necessarily
2:03care well in astronomy we want to be
2:06able to sometimes look at the same
2:08object stars at the same time and so
2:12from
2:13day two annexed those stars are gonna
2:15actually ride I'm gonna actually grow or
2:18it sorry it actually rise so from one
2:22day to the next those stars are not
2:24gonna rise at the same time according to
2:26our normal solar day they're gonna
2:28actually rise again 23 hours and 56
2:32minutes later so if I want to study the
2:34same star the same conditions I'm not
2:37using the solar day and over time this
2:39fact will actually get quite drastic I
2:42mean sure one day Foreman it's not a big
2:44deal but over time that's gonna add up
2:46and actually let's take a quick look at
2:48an example so what I got simple example
2:53right here star rises at 8 p.m. we don't
2:56care what the star is we're just hold us
2:58star rises at 8 p.m. in 30 days what
3:03time will it rise so how are we gonna
3:06work this out well the main thing to
3:08keep in mind is this idea that every day
3:12there would be a difference of four
3:14minutes every day four minutes per day
3:17great so then we think well in our case
3:21we have 30 days great so we want to
3:24think of how much this will be in 30
3:27days well we can actually multiply this
3:29out and our case the days will cancel
3:32out and we can think in essence we're
3:34getting four minutes times 30 or 4 times
3:3730 minutes so in other words 120 minutes
3:40and as long as we remember that there's
3:4260 minutes in an hour that's the same as
3:43saying two hours great the last thing we
3:48have to remember is that it's actually
3:50rising earlier because that rotation is
3:53actually rising earlier so what's two
3:56hours before 8 p.m.
3:57well 6 p.m. so that would be the answer
4:02to this question and if you're looking
4:03for another example our book has one on
4:05it's example 4.3 on page 115 so if
4:09you're struggling with this definitely
4:11take a look I think this would make a
4:13great question you're gonna see an
4:15assignment won't be shocked to see it on
4:17a midterm so make sure you know how to
4:19do it well one other concept I want to
4:23talk about is idea of mean solar time
4:27well we're used to the idea of a our
4:3024-hour clock and we know what time it
4:32is we look at her phone we see the time
4:34but that wasn't always practical and in
4:38fact for a long time every city had
4:42their own time because they would look
4:43at well our noon is when the sky Sun is
4:46right above us so in the 1800's if you'd
4:50been around they would have actually had
4:52different times in each city each city
4:55would have had its own unique time based
4:58on how the Sun rose for them
5:01now eventually the world kind of
5:04realized we had to figure something out
5:06and I mean what we ended up with was
5:08these standardized time zones we have 24
5:12time zones 4 and everyone is just a
5:14difference by one hour and this was by
5:18about 1900 most of the world was using
5:20this so but before that and again in
5:23really early 1800s every city would have
5:26had slightly different time of their own
5:28so it's a kind of a neat idea I thought
5:32but one thing that we have to consider
5:36as well is as you travel from one time
5:38zone to another again we have these 24
5:41time zones that evenly split up the
5:43planet and so we roughly have averaged
5:46out certain regions to have their own
5:48time zones well if you kept traveling
5:49around and around the planet and we're
5:51doing it in less than 24 hours
5:53technically you could just keep gaining
5:55days so of course that couldn't be
5:57allowed to happen because again as you
6:00go across time zones you have to change
6:01by an hour and if you kept doing it well
6:03again you're not gonna just keep gaining
6:05days and just be out of sync with the
6:06rest of humanity so what we had to do is
6:09invent the International Dateline a
6:11made-up area where this is where we go
6:14from one day to another
6:15if you travel from one side to the other
6:17you would be going from one day to
6:19another