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----- Original Message -----

From: Greg R.

To: NavList

Sent: Saturday, January 13, 2007 2:53 PM

Subject: [NavList 2063] Determining Actual Time of Meridian Passages Before Computers

This got my curiosity up last week, and thought I'd toss it out to the list for comments. I realize that it's pretty much a no-brainer in this modern day of computerized calculations and computer-controlled telescopes and the like, but how was it done in years past?

 

My best first-guess for a method of doing it would be something like establishing a true north-south line (probably with a Polaris shot and surveying gear) to establish the direction of the meridian, then leveling and locking some sort of viewing device (maybe with crosshairs on the boresight?) on that line so that it could only move in a vertical arc (i.e. horizontal movement would be locked on the meridian line). Then some sort of reasonably-precise timepiece (chronometer, etc.) could be used to verify the actual time of meridian passage for a celestial object (obviously excluding the sun in this example, unless shades were available on the viewing device).

 

Does anyone know how it really was done back then? Also, does anyone know what (if anything) is done in this modern age to verify the almanac meridian passage times, or are the computer calculations so accurate that they don't need to be verified?

 

--

Thanks,

GregR

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