# Around we goDecember 27, 2019 10:11 PM   Subscribe

Earth and Sun. With the end of the year approaching, perhaps you'd like to brush up on your understanding of how days, seasons, and years are calculated with this interactive essay.
posted by gwint (20 comments total) 61 users marked this as a favorite

This article is a great demonstration. I was recently re-reading stuff on the 'equation of time', which is the figure-8 thing in the middle.

For us in the Northern Hemisphere, Dec 21st is the shortest day of the year and the length of daylight is now starting to get longer, but the earliest sunset actually occurred in early Dec and the sunsets have been occurring later since then, and the latest sunrise will occur about 2 weeks from now in early Jan before the sunrises will start to get earlier.

This happens because the Earth is on the part of its elliptical orbit that is closest to the sun so it is moving the fastest around the sun. The 24 hour day is from noon to noon of the following day and that is how long it takes the Earth to rotate 360 degrees on its axis + 360/365.25 degrees for the angle that the Earth has moved around the sun since the last noon. Since we're in the part of the orbit where the Earth moves fastest, the angle we moved around the sun is slightly larger than 360/360.25 so the Earth takes a few extra seconds of rotating each day to reach the next noon (the sun in effect lagging behind the clock) and this results in late sunrises and later sunsets.

My concern with this and being able to explain it stems from not being a morning person and my job starting at 7 am with the latest sunrise where I live occurring at about 7:15 in Jan. Here's where you can look up your town.
posted by Blue Tsunami at 10:44 PM on December 27, 2019 [10 favorites]

This kind of thing is entirely my jam. I can't do any of the math involved with astronomy, but I totally dig the concepts and I love having them presented in ways that let me envision what I already know even more clearly.

I want to share this with everyone I know, but nobody else I know would give a damn. So yay for anyone else participating in this thread!
posted by hippybear at 11:17 PM on December 27, 2019 [7 favorites]

Wow. I flunked out of science classes and ended up in the visual arts. As a graphic designer, I am extremely passionate about taking complex ideas and making them easy to understand (while hopefully sparking joy in the viewer). This is such a lovely example of that in practice!

This essay helped me understand concepts in a way I haven’t before, not because I didn’t care but because my brain simply seems to panic and shut down when a lot of math gets involved and I really struggle to make myself keep trying and I had so many other things to learn at the time American students cover these concepts (hey, guess who just got diagnosed with ADHD and learned why this shit is so hard for some people! It’s this gal!) So this was great.
posted by the thorn bushes have roses at 11:51 PM on December 27, 2019 [1 favorite]

Fun fact: The entire orbit of the Moon around the Earth would fit entirely within the Sun.
posted by sjswitzer at 12:06 AM on December 28, 2019 [4 favorites]

Thanks for this, this is a great explanation and visualization.
posted by carter at 5:18 AM on December 28, 2019

Pffft. This is all nonsense. All you need to know about the Sun is explained in the good book:
A new sun goes over every day
There's a big pile of them on the other side of that hill
When they cool down people cut them into blocks and that's where we get margarine from
posted by flabdablet at 5:29 AM on December 28, 2019 [5 favorites]

I would have thought I knew more about that, but apparently not.
posted by jacquilynne at 5:54 AM on December 28, 2019

My Dad showed me some of this using an apple, a tangerine and a flashlight. Good stuff.
posted by Hobgoblin at 5:58 AM on December 28, 2019 [2 favorites]

Consider how much effort and intelligence went into obtaining this elementary knowledge!
posted by thelonius at 7:09 AM on December 28, 2019 [1 favorite]

That’s what I was thinking, thelonius! We’re amazing creatures. Well, some of us.
posted by Don.Kinsayder at 8:58 AM on December 28, 2019 [2 favorites]

Well, not me, anyway. I played with the sliders, mostly.
posted by Don.Kinsayder at 8:59 AM on December 28, 2019 [1 favorite]

Pffft. This is all nonsense. All you need to know about the Sun is explained in the good book:

I always thought Calvin's dad explained things best.
posted by Fukiyama at 10:32 AM on December 28, 2019 [5 favorites]

I am kind of ashamed to say how long it took me to understand that the analemma is in fact not symmetric in the north-south direction--I had assumed it had to be and that it was just wrong on all the globes I'd seen. Anyway, it is because of the eccentricity of the earth's orbit combined with the direction of our axial tilt. I assume that there are a couple of times during the 25,000-year precession of the equinoxes that it *is* symmetric. Hopefully someone will be around to appreciate it.
posted by Gilgamesh's Chauffeur at 10:59 AM on December 28, 2019 [1 favorite]

We happen to live at a fortunate point in the 25,000 year cycle in the precession of the equinoxes. Right now, the sun is closest to the earth in the northern hemisphere in the winter and farthest away in the summer. Due to this change in distance, the sun is about 7% more intense in the northern hemisphere winter and less intense in the northern hemisphere summer.

The effect of this is to moderate the seasons somewhat in the northern hemisphere. It is just the opposite in the southern hemisphere. The seasons are slightly exaggerated in the southern hemisphere.

But there is more to it than that. About 68% of the globe's land is in the northern hemisphere. Land tends to experience more extremes over the seasons because it gains and loses heat faster than the oceans.

So the northern hemisphere with more land would have more variation between the seasons -- except the fact that the sun is closer in the winter and farther away in the summer. The two effects - sun distance and landmass - tend to cancel.

The southern hemisphere is the opposite. The sun distance increases the seasonal effects but the fact that there is much more ocean and much less land moderates the extremes, so not so bad. The two effects tend to cancel

12,500 years from now everything is reversed. The northern hemisphere will have both land and sun effects working together to cause more extreme seasons. The southern hemisphere will have land and sun effects opposite so even more moderate seasons.

So right now we live in a happy medium for seasonal effects due to the sun's distance and global geography. 12,500 years from now maybe much less so. And 12,500 years ago it could have led to human migration.
posted by JackFlash at 2:55 PM on December 28, 2019 [9 favorites]

Very cool. I look forward to digging into this.
posted by veggieboy at 3:17 PM on December 28, 2019

Oh wow, Jackflash - I never thought of that complication!
posted by notsnot at 5:25 PM on December 28, 2019

I love this stuff! I live pretty close to the arctic circle so yearly changes around here are pretty dramatic/ dynamic. Ive actually drawn the angle of the sun at noon (once each week of the year) on the walls to see how much it varies. Now Im working on mapping out on the floor of the cabin the relative direction of sunrise and sunset during the year.
Will be reading this closely to understand more of it. thanks gwint!
posted by cabin fever at 11:27 PM on December 28, 2019 [1 favorite]

This answered so many questions when I tried to understand this myself using youtube and wikipedia last year. But I still have a few more questions which I think are related to Kepler's Laws:

- Why do the lines on the Analemma cross?
- There seem to be two distinct phases when noon happens early, and when it happens late. Does the earth go slower, then faster, then slower, then faster each year?
- The perihelion and aphelion point seem to be arbitrary, and it's a coincidence as JackFlash explained, that the perihelion is so close to the december solstice. However, Perihelion happens when the earth is closest to the sun - which is also when it's going fastest. And that speed, I think, is what makes the days longer and shorter AKA makes noon happen early and late. So, would the analemma look different 12,500 years from now?
- We talk about the "shortest day" and the "longest day" in terms of hours of daylight. But, there are actual days that are shorter (perihelion) and longer (aphelion). What dates are those, and do they happen the same every year?
- Are the day with the earliest sunrise, earliest sunset, latest sunrise, and latest sunset significant and related to these larger maths? And what days are those? And why?
posted by rebent at 6:44 PM on December 29, 2019

Why do the lines on the Analemma cross?
- There seem to be two distinct phases when noon happens early, and when it happens late. Does the earth go slower, then faster, then slower, then faster each year?

The answer is that the fast/slow effect is the combination of two things -- the eccentricity of the earth's orbit (elliptical) and the obliquity (tilt) of the earth's axis.

The eccentricity effect is annual. The earth moves faster at perihelion and slower at aphelion. If eccentricity were the only effect, then the analemma would not cross. It would just be an oval.

Unlike the eccentricity effect, the obliquity effect occurs twice annually. The sun appears to move faster at both the summer and winter solstices and slower at the spring and autumn equinoxes. So two peaks per year.

When you combine the two together, you get the standard analemma, which crosses the zero point, left to right, four times each year.

Here is a great illustration of the equation of time in wikipedia. The top two graphs show the eccentricity and the obliquity. The lower left shows the combination of the two. And finally on the lower right you can see how that projects onto the analemma.

Note that the time swings back and forth, left and right indicating fast and slow relative to mean noon. And it crosses zero four times each year.
posted by JackFlash at 7:57 PM on December 29, 2019 [1 favorite]

So, would the analemma look different 12,500 years from now?

Not much. All you would have done is swap spring for autumn and summer for winter. The analemma would have the same shape.

But instead if you wait, say, some other time less than 12,500 years, then the analemma would change shape, from a symmetrical figure eight to an upside down analemma with the fat part on the top and skinny part on the bottom. It has to do with the shifting phase difference between the eccentricity peaks and the obliquity peaks.
posted by JackFlash at 8:15 PM on December 29, 2019 [1 favorite]

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