Cosmological ?s

True north will make its closest approach to Polaris at the end of the 21c.
Does anyone know the exact year? And will it later head back towards Thuban
in draco?

Are the 'dog days' calculated by the rising of the first dog star and the
setting of the second? That seems too easy somehow.

Mike wrote:

True north will make its closest approach to Polaris at the end of the 21c.
Does anyone know the exact year? And will it later head back towards Thuban
in draco?

According to Burnham the closest the pole gets to Polaris will be in
2102, at 27'31" distance from the star. The pole traces out a circle
(about 47° in diameter) in the heavens, so it doesn't really "head
back" but always goes round in the same direction. The precession
cycle will return the pole to the vicinity of Thuban in another
21,000 years or so.

Are the 'dog days' calculated by the rising of the first dog star and the
setting of the second? That seems too easy somehow.

It doesn't sound that easy to me! Every star that's visible from a
given latitude rises and sets every day, so using these phenomena to
define a period of the year requires that they be related to the
position of the sun. And what do you mean by "the first dog star" and
"the second"? The "Dog Star" is Alpha Canis Majoris, AKA Sirius. Are
you thinking of Procyon (which name means "before the dog"), Alpha
Canis Minoris, sometimes called the "Little Dog Star", as the other?
Depending on the observer's latitude, it rises and sets each day
somewhat under an hour before Sirius does.

Anyway, I don't believe there's an official definition for the "dog
days". Some authors say that a period of about forty days, beginning
three weeks before the annual sun-Sirius conjunction and ending three
weeks after, constituted the "dog days". But in general the use of
the term has more to do with hot summer weather -- which we now know
is *not* caused by the star! -- than with its astronomical origins.

--
Odysseus

"Odysseus" wrote in message
...
According to Burnham the closest the pole gets to Polaris will be in
2102, at 27'31" distance from the star.

I would have imagined it closer.

The pole traces out a circle
(about 47° in diameter) in the heavens,

Is it round or oval? Does it go clockwise?

so it doesn't really "head
back" but always goes round in the same direction. The precession
cycle will return the pole to the vicinity of Thuban in another
21,000 years or so.

So it must have been closer to Kochab and Pherkad some 10,000 years ago.

Mike wrote:

"Odysseus" wrote in message
...

The pole traces out a circle
(about 47° in diameter) in the heavens,

Is it round or oval? Does it go clockwise?

Like most circles, it's round. It goes counterclockwise from the
point of view I described; looking down on the earth's north pole
from space the 'wobble' would be clockwise. See

http://www.opencourse.info/astronomy/introduction/03.motion_earth/

(scroll about halfway down) for an animation showing about ten
thousand years of precession as it might be seen from the North Pole.

So it must have been closer to Kochab and Pherkad some 10,000 years ago.

Not as long as that, more like 3500 years -- and not very close
either, about the width of the Little Dipper's bowl from Kochab. Ten
thousand years ago I guess it would have been somewhere between
Draco's head and Boötes.

--
Odysseus

"Odysseus" wrote in message
...
Like most circles, it's round. It goes counterclockwise from the
point of view I described; looking down on the earth's north pole
from space the 'wobble' would be clockwise. See

http://www.opencourse.info/astronomy/introduction/03.motion_earth/

(scroll about halfway down) for an animation showing about ten
thousand years of precession as it might be seen from the North Pole.

Way cool. So the point will be near Errai in 5000ad and Alderamin in
8000ad. The circle is bigger than I presupposed.

Mike wrote:

Way cool. So the point will be near Errai in 5000ad and Alderamin in
8000ad. The circle is bigger than I presupposed.

One way to grasp the size of the circle is to picture the noonday sun
at the summer solstice, imagining it to be located at the top of the
circle. Then the position of the sun at noon on the winter solstice,
47° lower, would be at the 'bottom'. This won't work if you live in
the Arctic or Antarctic, though, because the latter position will be
below your horizon.

--
Odysseus