The Earth's Tilt and Rotation - Perihelion, Aphelion, Tropics, Circles, Daylight, Seasons, The North Star, Etc.

[DonRocks]

1 hour ago, porcupine said:

Do you have a source for this? It doesn't make sense, given that perihelion is in early January and aphelion is in early July. Or does it have something to do with Kepler's second law? I am confused.

No, I was writing quickly, and just assumed that the two Equinoxes would be when the Earth was exactly 1 AU away from the sun, but now that you mention it, there's no particular reason I should have assumed this. The 12-hour days have to do with tilt, and not distance.

I also assumed that the Winter Solstice was when the Earth was at the maximum AUs from the Sun, and the Summer Solstice was when the Earth was at the minimum AUs from the Sun, but again, these were just automatic assumptions I made without even thinking about it (in fact, it's just about the exact opposite). This is, quite literally, something you learn in freshman science class, if not before (yes, I took Astronomy because it was an easy A).

Do you think the 1-AU moments are exactly 3 months away from the perihelion and the aphelion? I couldn't find anywhere that says when they occur, although I did find one webpage that said they did occur exactly twice a year.

[porcupine]

Axial tilt is the cause of seasons; day length is the result of rotational speed. As for your final question, I don't know; that's what I was getting at with Kepler's second law, described here. (Short version - Earth is moving through space fastest at perihelion and slowest at aphelion). Certainly you could come to that conclusion through interpolation, but iirc there are a large number of variables to contend with and it's all much more complex than you'd think. For example, the position of the other planets at any given time is going to affect the shape of Earth's orbit, though in some cases only minutely. 

But I'm no astrophysicist. 

You might enjoy Ask an Astronomer.

[DonRocks]

31 minutes ago, porcupine said:

Axial tilt is the cause of seasons; day length is the result of rotational speed. As for your final question, I don't know; that's what I was getting at with Kepler's second law, described here. (Short version - Earth is moving through space fastest at perihelion and slowest at aphelion). Certainly you could come to that conclusion through interpolation, but iirc there are a large number of variables to contend with and it's all much more complex than you'd think. For example, the position of the other planets at any given time is going to affect the shape of Earth's orbit, though in some cases only minutely. 

But I'm no astrophysicist.

You might enjoy Ask an Astronomer.

Elizabeth, one thing I've learned about you is that you rarely write anything that isn't correct. Which is why I'm second-guessing myself in thinking your first sentence is incorrect.

The 24-hour day is the result of rotational speed, yes. 

But the number of hours of sunlight vs. darkness is the result of tilt, which is why we have more sunlight during the summer, and less during the winter - think about 24-hours of sunlight and 24-hours of darkness beyond the Arctic and Antarctic circles - that's a good way to visualize tilt as the cause.

Rotational speed is essentially a constant, is it not? I'm sure there are minor fluctuations in due to various factors, but Earth rotates pretty much at a constant velocity, I believe, and that velocity would necessarily be "one circumference per day.".

And I will add that I am certainly no astrophysicist; I'm an unabashed dilettante who loves to learn about a whole host of things - but that, in and of itself, makes me an expert in nothing. I stress: "I don't argue to win; I discuss to learn," and I wish everyone was like this. If only our legal system was based on "finding out the truth" instead of attorneys pulling out all stops to protect their clients - only then would I have respect for courtroom attorneys, and it doesn't stop there - physicians crank 'em in, and crank 'em out, and if they haven't found out the truth, it doesn't matter if your office visit is over, and you walk out in so much pain that you have tears in your eyes - if only our medical system was designed to help patients instead of seeing the maximum number of people possible - only then would I have respect for physicians who see patients. And it doesn't stop there (I'll shut up now, but I'll also end by saying that *my* goal in life is to help mankind to the greatest extent that I'm capable of).

[porcupine]

14 minutes ago, DonRocks said:

Elizabeth, one thing I've learned about you is that you rarely write anything that isn't correct. Which is why I'm second-guessing myself in thinking your first sentence is incorrect.

The 24-hour day is the result of rotational speed, yes. 

But the number of hours of sunlight vs. darkness is the result of tilt, which is why we have more sunlight during the summer, and less during the winter - think about 24-hours of sunlight and 24-hours of darkness beyond the Arctic and Antarctic circles - that's a good way to visualize tilt as the cause.

Rotational speed is essentially a constant, is it not? I'm sure there are minor fluctuations in due to various factors, but Earth rotates pretty much at a constant velocity, I believe, and that velocity would necessarily be "one circumference per day.".

And I will add that I am certainly no astrophysicist; I'm an unabashed dilettante who loves to learn about a whole host of things - but that, in and of itself, makes me an expert in nothing. I stress: "I don't argue to win; I discuss to learn," and I wish everyone was like this. If only our legal system was based on "finding out the truth" instead of attorneys pulling out all stops to protect their clients - only then would I have respect for courtroom attorneys, and it doesn't stop there - physicians crank 'em in, and crank 'em out, and if they haven't found out the truth, it doesn't matter if your office visit is over - if only our medical system was designed to help patients instead of seeing the maximum number of people possible - only then would I have respect for assembly-line medical practices. And it doesn't stop there. I'll shut up now. 

Oh, I see what you're getting at - yes, about the number of hours of daylight vs. darkness. I read your previous post too fast.  :-)

I, too, am a dilettante. And an autodidact. With an almost lifelong fascination with astronomy.

[DonRocks]

15 minutes ago, porcupine said:

Oh, I see what you're getting at - yes, about the number of hours of daylight vs. darkness. I read your previous post too fast.  :-)

I read yours too ambiguously - I see what you were saying now. 

One other interesting point (that I'm sure you already know) is that the Tropics (i.e., The Tropic of Cancer in the North and the Tropic of Capricorn in the South) are the northernmost and southernmost points at which the sun can be directly overhead - in other words, they are directly related to the Earth's tilt, and are directly related to the locations of the Arctic and Antarctic Circles (which are the southernmost and northernmost points where 24-hour sunlight and darkness occur).

Related to all of this in a primal way is the question: Why is the Earth's rotational axis oriented towards the North Star? (This isn't a quiz; I honestly don't know the answer).

Another question just popped into my head: Why do compasses point towards the North Pole? And why is the "actual" North Pole slightly different than the "magnetic" North Pole?

[porcupine]

On 12/27/2016 at 10:42 AM, DonRocks said:

Elizabeth, one thing I've learned about you is that you rarely write anything that isn't correct. Which is why I'm second-guessing myself in thinking your first sentence is incorrect.