Archive for 2010
Dark Days of Winter: 365 Days of Astronomy
Ahh! I found the original post – it had slid back to 2009! Here it is …
By now my first podcast for 365 Days of Astronomy should be live, and here is the post to support it – containing links and images I mention in the podcast. So go listen already!
Also, please excuse the terrible run-on sentences and immense number of “now”‘s in the transcript. I tried to write exactly what I said, and the way I speak is significantly messier than the way I write. And I always thought I wrote the way I spoke. Hmm.
Transcript!
Telephone ring.
Hello, this is Alice. Oh hey hi, I’m glad you called. Yeah, yeah you’re right. Yesterday, January 3 was perihelion – the Earth’s closest point to the Sun. Pretty cool that that happens in winter, isn’t it? Yeah, I know, kinda mind-blowing.
Anyway the real reason I wanted you to call, I wanted to talk about the fact that January 3 was also the latest sunrise of the year. Yeah no, not December 21 the solstice, but January 3. Yeah, I always thought that the latest sunrise and the earliest sunset took place on the solstice because that’s the shortest day therefore it should have the latest sunrise and the earliest sunset. That makes sense, right? But it’s not true! The earliest sunset takes place weeks before the solstice, round about December 6th here in Seattle. That’s the earliest sunset. And the latest sunrise isn’t all the way until January 3.
So it’s kinda weird about why this is. It has to do with this thing called the equation of time. Now, you can represent the equation of time as and equation, but you can also see a representation of it by looking at an analemma. So let me tell you a little bit about how you get an analemma.
Start with noon. Think about where the Sun is at noon. Point out the window, where is the Sun at noon? Now, I hope you’re not pointing straight up because most people in the world don’t actually get to see the Sun straight up over their heads at noon – ever, any time of the year. Now, there are some. Everybody who lives between the Tropics of Cancer and Capricorn gets to see it at least one day out of the year. But the rest of us, we don’t get to see it. Generally it is going to be, if you’re in the Northern Hemisphere, it will be directly above South. Some number of degrees above South will be the highest point that the Sun gets to. And if you live in the Southern Hemisphere it will be some number of degrees above North that you’ll be able to see the Sun at noon.
Now, when it gets to that highest point in its path across the sky, that’s called astronomical noon. That’s the definition of astronomical noon. Next time you see it right there at its highest point look at your watch: probably isn’t reading noon, because we have time zones and all kinds of things like that. But also, even more importantly, that’s not the noon that really matters.
We’ve got two different kinds of time that we’re dealing with. Apparent solar time, which is what I just told you about. It’s noon when the Sun is at the highest point in the sky. You can read this with a sundial a little bit, you can also read it by measuring the angle of the Sun and making sure that it is exactly halfway across its path across the sky. So you’ve got apparent solar time, but you also have mean solar time. Now, mean solar time is what we really use in terms of determining the number of hours that have really passed. Mean solar time is if you took a clock, a perfect clock, and on the vernal equinox, March 21, you set that clock to noon the second you saw the Sun cross over the meridian – the second you saw the Sun get to its highest point – and then you let that clock run for a year, at the end of that year on the next vernal equinox, March 21, when that clock reads noon, the Sun will be right there exactly where it should be: at its highest point. Okay, so that’s mean solar time. It means that we’re averaging it out over a year.
Now the Sun, as we move around the Sun, it appears to move a little faster or a little slower through our sky because of the equation of time. Most of this is because of the eccentricity of the Earth’s orbit around the Sun. We’re not orbiting in a perfect circle. A little bit of it has to do with the tilt of the Earth, so it’s really a pretty complicated little equation, but the effects are interesting and fun.
So, we have the equation of time affecting how fast the Sun is moving across the sky. (Yes, it’s not the Sun moving, but effectively, from our point of view, while we’re watching the Sun rise and set it looks to us like the Sun is moving.) So, effectively, as the Sun is moving across the sky, some days of the year it moves a little slower and some days it moves a little faster. So, when noon comes, some days it’s behind where it should be. It’s not yet at its highest point. And some days its in front of where it should be, not yet at its highest point or, oops, past being at its highest point actually.
Now, with your perfect watch, if you go out, you set up a camera, and you take a picture of the Sun every day at noon by your perfect watch, what you’re going to see, when you put all those pictures together, is a figure-8 shape. That is the analemma, it is also a great representation of the equation of time. Okay, now if you don’t want to spend a year waiting to see that picture, just Google “analemma” or you can stop by my website: www.alicesastroinfo.com, and I’ll put up a picture for you. Also 365 Days of Astronomy will have a link up to my website from their website if that’s easier for you.
So, how does the equation of time make the earliest sunset happen before the solstice and the latest sunrise happen after the solstice? Let’s get back to that. It’s because the entire day is shifting a little bit. And I keep wanting to say it’s shifting left to right because I’ve laid out the hours on a number line, and that’s how I’m visualizing this. So, why don’t you visualize it with me? And I found it a little too complex to lay out the entire number line for a day, so I’m just using the numbers one through ten: they’re representing hours. I’m pretending we have a ten-hour day. Instead of a 24-hour day, we’ve got a ten-hour day. We’ve just got a number line: one through ten. Also, you’ve got ten fingers, so if you’re sitting on the bus, you can just hold your hands out in front of you and you’ve got that number line that you can look at.
Now, think about this: if we’ve got a solstice that’s four “hours” long, it starts at “three” and it ends at “seven.” So, the Sun rises at “three” and it sets at “seven”. So noon is at “five” there. Okay? So we’ve got a pretty short solstice day there. Now, I’m not even going to go into minutes. I’m going to say everything changes by whole hours. The day after the solstice has to be a little bit longer. So instead of being four “hours” long, it is going to have to be at least five “hours” long. All right? And, let’s go with the one that has the latest sunrise. So let’s say the sunrise is just an hour later so instead of our solstice starting at “three” we have our day starting at “four” and then you’ve got to count a five “hour” day beyond that: six, seven, eight, nine – so the Sun sets at nine. So we’ve shifted our entire day to the right.
Let’s do earliest sunset, okay? So the earliest sunset, to get our earliest sunset it’s going to have to happen before “seven” so it will have to happen at “six” which means our sunrise is going to have to happen at “one.” Now remember, these aren’t real hours we’re working with because we’re just doing a ten-hour number line. I’m just showing you how the whole day is shifting left to right. But, from “one” to “six” is once again a five-hour “day” instead of that four-hour “day” that our solstice was. And our solstice does not have the latest sunrise or the earliest sunset like that. Now, in the real world we have to deal with a lot finer methods of measuring, and it turns out these sunsets are only off by a couple of minutes from each other.
And you can look this up. I get a lot of my information from the U.S. Naval Observatory. They have a couple of great resources: one is “Sun and Moon Data for One Day,” they’ll also give you an entire year’s worth of sunrises and sunsets if you’d like, and so you can look at those. They also have a really great post called “The Dark Days of Winter” which is where I got a lot of the information for this so check that out. But in Seattle, that earliest sunset is 4:18pm. The sunset on the solstice was 4:20pm – so we’re not talking about a big difference here.
Alright, well, we’ve talked about a lot of things today. Lot of vocabulary words, and I hope you go and look some of them up. If you have any more questions give me a call. I will talk to you later. Okay, yeah. Bye!
And in case you didn’t catch that, my name is Alice Enevoldsen, I’m the planetarium specialist for Pacific Science Center in Seattle, Washington – pacificsciencecenter.org and the writer for Alice’s AstroInfo alicesastroinfo.com.
Pictures!
Analemma (actually a tutulemma) from NASA
Links!
Vocabulary Words!
Blerch. I used a lot of jargon in that podcast, but if you weren’t taking notes and remembered that there was a word somewhere in there that you wanted more info about, here are some of the main ideas.
Equation of Time
Analemma
Perihelion
Astronomical Noon
Tropic of Cancer
Tropic of Capricorn
Mean Solar Time
Apparent Solar Time
Meridian
Eccentricity
Today is also the first day of winter quarter, wish my students and me luck on our three-month journey.
~ A l i c e !
