Determining Rotational Time

Given that mere civilians now have access to the US DoD’s GPS satellite navigational system, is it still important to determine rotational time?

By: Ringo Bones 

During the months leading to Operation Desert Storm, US Navy navigation personnel of vessels patrolling the Persian Gulf at the time were ordered to take periodic sextant readings – as in every six hours during the evening - to determine if the then Iraqi strongman Saddam Hussein had already acquired tech to jam and / or disrupt with the proper operation of the US DoD’s GPS satellite navigational system to everyone using it in the Persian Gulf region. Given this dilemma, can methods of determining rotational time be useful as a double check to find out if the GPS navigational system is still working properly? 

The rotation of the Earth causes the stars to appear to move from east to west. Rotational time is determined by observing the passage of stars across a reference line, such as the local meridian, fixed with respect to the observing station. The “small transit instrument,” a telescope mounted about a horizontal axis that lies in the east-west direction, was chiefly used, formerly to determine rotational time. The telescope can be pointed to any elevation in the meridian. The passage of a star across the meridian is observed in the focal plane of the telescope, which contains a spider thread. The time of passage of the star is indicated with the aid of a chronograph – a device which registers clock impulses along with those generated at the transit instrument. Thus, what the clock read when the star was on the meridian is obtained.