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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » observation on chapter 7

Author Topic: observation on chapter 7

 - posted March 26, 2021 07:51 AM      Profile for briangetty           Edit/Delete Post 
I noticed that two of the data tables are not always the same:
1) on the daily pages, the SHA and declination for the 57 stars is not the same throughout the year.
2) your 1978 table for a0, a1 and a2 for Polaris latitude has different values than in my NA for 2021.
I had heard of a precession in the earth's rotation, but I had thought it was in the order of thousands of years. These are at a much faster cycle. What is the cause?

David Burch

 - posted March 26, 2021 10:22 AM      Profile for David Burch           Edit/Delete Post 
These data do indeed change slowly, but we do things in cel nav on a pretty fine scale.

Precession of the earth's axis tilted at 23.45º is once every 26,000 years. This put the star Alpha Draconis at the hub of the sky at the time the great Pyramids—a large shift over a long time—but we can see this as well in modern times.

Our first edition of Emergency Navigation in the mid 80s used the declination of Polaris to be 89º 12' so max Polaris correction was order of 48'. Our 2020 book GPS Backup with a Mark 3 Sextant includes our modern Regiment of the Pole (link below) where the corresponding correction is just 40'. So here we see a motion of the "fixed stars" of some 8 miles on earth.

Another source of star motion is called "proper motion." This is a smaller effect for most stars, but i believe notable on our cel nav scale over reasonable times for some. If you google "stars with largest proper motions" you will learn a lot about that.




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