NavList:

   

Reply

A similar discussion occurred in May 2006 on the old Nav-L list, and I
posted this:

[quote]

As already explained by George Huxtable, the month to month variation in
the declination of Polaris is largely caused by the annual aberration.

Let's first examine the declination with respect to the true equator and
equinox of date *without* applying aberration. In the table below, the
first column of declinations shows a practically constant increase of
17″ per year. This is the effect of precession and nutation, that is,
the changing orientation of Earth's axis. (For some point elsewhere on
the celestial sphere the geometry could be quite different, possibly
resulting in very little change in declination during a year.)

In the second column of declinations I have applied annual aberration
but not precession and nutation. That is, Earth's axis is held fixed.
You can see a cyclic fluctuation, with Polaris returning to the same
place after one year.

The last column combines both effects. The values here essentially
duplicate the original posting.

             °  '  "     '  "     '  "
2006 05   +89 17 35   +17 34   +17 34
2006 06   +89 17 36   +17 24   +17 25
2006 07   +89 17 38   +17 17   +17 21
2006 08   +89 17 40   +17 15   +17 20
2006 09   +89 17 41   +17 18   +17 25
2006 10   +89 17 42   +17 25   +17 33
2006 11   +89 17 43   +17 35   +17 44
2006 12   +89 17 44   +17 45   +17 55
2007 01   +89 17 46   +17 53   +18 04
2007 02   +89 17 48   +17 55   +18 08
2007 03   +89 17 50   +17 52   +18 07
2007 04   +89 17 51   +17 44   +18 00
2007 05   +89 17 52   +17 34   +17 51

[end quote]

The table ignores annual parallax, but it's only .008 arc seconds for
Polaris. That is, its apparent position will shift .008 arc second
relative to some extremely distant background object, as your viewpoint
moves from the Sun to Earth.

Like all stars, Polaris also has "proper motion", a slow drift in its
position relative to a space-fixed coordinate system. It's too tiny to
be apparent at 1 second precision during one year.

I used the Hipparcos catalog as the basis for the table. The data for
Polaris may be viewed by entering its Hipparcos Identifier, 11767, on
this page:
http://www.rssd.esa.int/index.php?project=HIPPARCOS&page=hipsearch

Note the high quality of the data: the standard errors for position,
parallax, and annual proper motion are all tenths of a milli arc second!

The original post is here, but the table formatting is messed up, at
least in my browser:
http://www.i-DEADLINK-com/lists/navigation/0605/0222.html

-- 
I block messages that contain attachments or HTML.


--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply