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Re: SHA vs RA
From: Gary LaPook
Date: 2018 Jan 14, 22:03 +0000

It made it simple for stationary astronomers to locate selected stars through their fixed telescopes. You have a clock that keeps local sideral time and then straight south, on your meridian, will be found stars having the same right acension as you your local sideral time. To point your telescope at a different right ascension just subtract your local sideral time from the star's right ascension and set that value, in hours, minutes, and seconds on you telescope mount setting circle. If you have a sophisticated telescope mount then the setting circle is moved by clockwork to maintain local sideral time so you can then just set the index to the star's right ascension and there it is. Of course you also set in declination.

From: David Pike <NoReply_DavidPike@navlist.net>
To: garylapook@pacbell.net
Sent: Sunday, January 14, 2018 10:00 AM
Subject: [NavList] Re: SHA vs RA

How about:

Astronomers sat by their telescopes waiting for the selected body to reach apogee at their position, so they wanted to know how long they had to wait after the First Point of Aries passed. Hence RA is measures eastwards in hours and minutes. Like Frank says, Air Navigators particularly didn’t have time for all that messing around. They just wanted something to add onto LHA Aries to get LHA Star for a convenient time. Hence SHA is measured in degrees west of Aries. It’s as if RA convention allies itself with time to reach a particular position in the sky, whereas SHA convention allies itself with position in the sky at a particular time. DaveP

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