h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I fully understand how the earth's tilt causes the seasons, because of the change in the amount of solar radiation affecting a certain amount of surface area of the earth..we did that at school.

I looked on Google and it keeps rehashing how the tilt affects the seasons, but I couldn't find where the *reason* for the change in tilt direction changing every 6 months is explained.

Am I missing something very basic?

(PS Is anyone feeling it wobble a bit - at the moment? )

The tilt doesn't change direction, it's the position of the earth in relation to the sun that makes the difference.

http://web.gccaz.edu/~lnewman/gph111/topic_units/Earth_Sun/01_20.jpg
The globes at left and right are the solstices (shortest/longest days) and the globes at top and bottom are the equinoxes (days of equal length).

Imagine an orange with a pencil stuck through the middle. If you move the orange in a circle,*without* turning the orange itself round (so the pencil always points towards the window, say), you've got a reasonable model of the situation.

When viewed externally, the direction of the axis (pencil) doesn't change. If you view it from the centre of the orbit, ie the position of the Sun, it does.

Hey thanks guys I'm going to crack this..by doing the practicals and reading the link.

It feels very weird and embarassing to be a pretty well educated person and have such a wrong idea in your head all your life.

But I suppose it's not quite such a big a deal for people as when it was discovered the earth went round the sun and not vice versa.

Nothing embarrassing about it at all. I still can't tell left from right.

No, really

*carefully whistles past the thread without mentioning the "wobble" of the Earth's precession...*

Good thing you didn't mention anything about precession

Man this amazing. I just did the models in my kitchen with a pen for the axis and a salt seller for the sun and

Like magic before my eyes I understood.

It's brilliant.

This is my first successful, full on totally answered question here.

It was doing the modelling that did it for me. I prefer to work that way round if I can in my head...rather than theory first.

It was seeing the drawings at school of the amount of solar radiation reaching the ground at the differing parts of earth according to tilt, that enabled me to understand that so well.

Thanks Guys.

Excellent!

The ease of modelling is part of the reason why I love physics and related sciences.

I used to love the show, Mr Wizard, for that very reason! Couldn't find his show on it, but this one's pretty good...

http://www.youtube.com/watch?v=Umfrl83pbTs

Seeing as this one is answered completely for me..I want to add a supplimentry..but I'll start a new question, as I don't want to spoil the perfection of this.

Sorry, can't let this one go - post 2 is wrong, the axial tilt DOES change direction. Not by much, and over a period of 41,000 years... but it does.

http://en.wikipedia.org/wiki/Axial_tilt#Values

Post 2 is wrong if you're going to delve deep into the science but it's not wrong for the purpose of the debate. Good to know though. It's always a good thing to be accurate.

The interesting thing about the truth here is that over the course of that 41,000 years, the horoscope changes, because the horoscope is defined as the constellations on the ecliptic, and that ecliptic changes. This was actually used as a minor plot point in one of the Red Dwarf novels - Red Dwarf is MASSIVELY under-rated as science fiction. Until series eight, it was pretty good comedy, but it's not always (I think) appreciated that it was also rock solid brilliant sf. And that is very, very rare. HHGG, for instance, was far, far better comedy than it was sf. Red Dwarf was, IMO, better sf than it was comedy - and it was GOOD comedy.

I think you're confusing the 40,000 year change in tilt with the 26,500 year precession, Hoo, which changes the zodiacal constellations that the sun is in at a particular time of the year.

This is the dawning of the Age of Aquarius.

That's a precession thing I think...so told to me by someone calling themselves an Astrologist.

That's a whole other kettle of fish though.

"you're confusing the 40,000 year change in tilt with the 26,500 year precession"

Hmm. Possibly.

Funny, isn't it? The original question was "what causes the axial tilt to change?" in the context of the seasons, and the original answer was "It doesn't..."

And now we've got to the point where we've established that it DOES change, and it changes the zodiac but not the seasons, and we're discussing which of the several periodic changes is responsible for that change...

(There's another change to the axial tilt that goes on something like an 18 year cycle, although that one's really, really tiny...)

Yes, Effers, that is the Age of Aquarius thing. But the astrologists can't agree on exactly when the age starts, within a thousand years, and they also can't agree on what it means.