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

"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"

Gosh.

It's almost as if it's not really a science at all.

It's almost as if they're - y'know - making shit up as they go along, or something.

A quick google/wiki search reveals that it's actually worse than that, even.

An astrological age lasts approximately 2,150 years.

The earliest date proposed for the start of the age of Aquarius is the year 1447, according to the "calculations" of someone called Terry McKinnell. Another astrologer, John Addey has "calculated" that it has not actually started yet, but will in fact begin in the year 3597.

Now, the astute among you will note that those two dates are 2,150 years apart. It's hard to imagine how much wronger at least one of them could be.

I've identified my earlier confusion.

The precession of the equinoxes is about the earth's tilt relative to the stars, and apparently this changes on a 26,000 year cycle.

Whereas obliquity - the tilt relative to the plane of the orbit - changes over 41,000 years.

The two are different but linked.

On a related note, while I (and every other person who knows how to use a compass properly) knew that the magnetic north pole wanders about the place a bit, I only just found out that the *actual* north pole itself - the point on the earth's surface through which the axis of rotation passes - actually moves about by tens of metres due to earthquakes and other internal "adjustments". This boggles my mind a bit... I like that.

It gets even more mind-boggling:

"The calculations also show the Japan quake should have shifted the position of Earth's figure axis (the axis about which Earth's mass is balanced) by about 17 centimeters (6.5 inches), towards 133 degrees east longitude. Earth's figure axis should not be confused with its north-south axis; they are offset by about 10 meters (about 33 feet). This shift in Earth's figure axis will cause Earth to wobble a bit differently as it rotates, but it will not cause a shift of Earth's axis in space — only external forces such as the gravitational attraction of the sun, moon and planets can do that."
http://www.nasa.gov/topics/earth/features/japanquake/earth20110314.html

One of the questions on Christmas University Challenge for 'old grads' was, which planet in our solar system has the most nearly circular orbit of the sun.

The answer was Venus.

I guessed Mercury, being closest to the sun, as did the Cambridge guy. But Mercury is very small and maybe gravitationally affected by Venus, which is next closest.



Trinity won the whole thing. Superb team all with a wide range of knowledge...and worked well together...sort of bouncy. Paxman clearly loved them.

Oh god wrong thread. But you know its meant for the 'Why is the earth's orbit of the Sun elliptical?' thread.



I wanted to forget the humiliation of this one.

No humiliation. It's a perfectly easy mistake to make. Few enough people even understand the concept of axial tilt at all - many think the season happen because in winter we're further from the sun. (How they think that works for Australia having summer when we have winter I don't know...)

It's a bizarre mistake.

I'm still getting over it.

But when I did that modelling thing to finally understand it...it really was up there for me as a sudden insight.

They really ought to do make sure all school kids see it...and for christ sakes give them 5 minutes to take it in.

(I was somewhat disappointed by the Christmas lectures for kids at the Royal Institution programmes this year to do with the human brain.

It was all whizz bang...and 'lets have a round of applause'. You could hardly get over one insight, before another was rammed down your throat).

It's really not a bizarre mistake at all.

Here's another little factoid you might find interesting on the subject: Uranus's obliquity to the ecliptic is 89 degrees.

What that means in English is that the planet is practically lying on its side, which means its "seasons" are *extreme*.

Here on earth, the poles are the "land of the midnight sun", because at those very, very high latitudes the sun doesn't set for days or weeks. But because our tilt is slight, that only applies really quite near the poles. On Uranus, it applies to pretty much an entire hemisphere. In "midsummer", the sun not only doesn't set, it barely even moves from directly overhead. You only get something like a usable day/night cycle on Uranus close to the equinoxes.

Of course all that's academic because
(a) from that far out the sun is just a bright-ish star in the sky and
(b) Uranus is a gas-giant without a solid surface we could reach.

Still, it's a weird one.

> the sun not only doesn't set, it barely even moves from directly overhead.<

A bit like we Brits hoped for our Empire...on which the sun would never set...

But thanks I do find this stuff fascinating.

(I heard a thing about Titan, (a Saturn moon; (I know you'll know that, but for everyone else)), the other night, I tend to listen to BBC all night radio these days. It actually has seas composed of methane because of the temperatures there. I think NASA are planning to go back there.

Aside.

I stayed with a family in the Russian Forest during 'White Nights" a few years ago. Not quite perpetual daylight...but never properly dark. Sitting outside in the wee hours reading...was pretty special though.