Photographic Zenith Tube: The Forgotten Rotational Time Measurement Tool?

If the GPS system ever fails, will the Photographic Zenith Tube be a useful alternative in the measurement of rotational time?

By: Ringo Bones 

During the months leading to Operation Desert Storm, US Navy navigation personnel patrolling the Persian Gulf at the time were ordered to take sextant measurements every 6 hours at night to determine if the then Iraqi strongman Saddam Hussein had acquired technology to screw up the GPS navigation system. The constellation of 24 navigational satellites in geosynchronous orbit that has since became an indispensable tool in navigation with an error rate of plus/minus 30 feet or less. Are there also other tools – as in astronomical time or rotational time determining tools that we can use to check if somebody’s screwing up with the proper functioning of our GPS system? 

The Photographic Zenith Tube – or PZT – is one of the more accurate tools used to measure rotational time. It was designed by Frank E. Ross back in 1909 and was adopted for time determination at the US Naval Observatory in 1934. The PZT is a telescope of 15-foot focal length and 8-inch aperture pointing directly to the zenith. A small photographic plate is placed about ½-inch below the lens. A basin of mercury located at half the focal length below the lens reflects the light from the stars, which comes to a focus at the plate. The lens and photographic plate can be rotated 180-degrees as a unit about a vertical axis. 

Four exposures of a star were taken, of 20 seconds duration each, with the lens in alternate positions. A motor drives the plate so as to track the star and compensate for its diurnal motion and time pulses are generated by the moving plate carriage. By means of a chronograph it is possible to relate the impulses to some particular clock. The photographic plate is developed and the images are measured. By combining these measures with the chronograph readings, it is possible to determine what the clock read when the star was on the meridian.

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.