Sunday, February 11, 2018

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.

Sunday, January 14, 2018

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. 

Wednesday, December 6, 2017

What Is Ephemeris Time?



Now largely replaced by more accurate atomic clocks, does the monitoring of ephemeris time has become a largely esoteric astronomical exercise? 

 By: Ringo Bones 

In recent years, modern atomic clocks and Global Positioning System based timekeeping have largely relegated the computation of ephemeris time – also known as terrestrial time – into something of an arcane esoteric astronomical exercise. Despite its lesser relevance to contemporary ultra-accurate timekeeping, it is worth noting that the “timekeepers” at the US Naval Observatory still do this with refreshing regularity. 

Ephemeris time may be defined by the motion of any planet of the Solar System or by any of their satellites. Astronomers have formerly defined ephemeris time by the orbital motion of the Earth around the Sun. It may be obtained by observing the position of the Sun with respect to the stars; however, in practice this is difficult to do and it is obtained from the orbital motion of the Moon above the Earth. 

This concept of astronomical time is based on the monthly motion of the Moon among the stars may be considered to form a clock wherein the stars represent the hour marks and the Moon represents the pointer. To utilize the Moon for this purpose, its ephemeris must be calculated with great exactitude and its position must be precisely determined by observation. The ephemeris of the Moon is based upon the mathematical researches of Ernest William Brown (1866 – 1938) at Yale University. 

The position of the Moon has usually been determined from meridian transits and from occultations of stars. A more recent method is to photograph the Moon among the stars with a dual-rate camera developed by William Markowitz at the US Naval Observatory in 1951. By means of a central, dark, plane-parallel glass filter, which is tilted during an exposure of about 20 seconds, the image of the Moon is given an artificial motion which cancels the normal motion of the Moon with respect to the stars. Thus, the Moon is held fixed relative to the stars and sharp images of both the Moon and the stars are obtained. Since the positions of the stars are known, measurement of the plate gives the position of the Moon with respect to the stars. This is entered in the lunar ephemeris and the corresponding ephemeris time is obtained.

Saturday, June 18, 2016

Ramadan 2016: The Longest Fast for Muslims in the Northern Hemisphere?



Given that the Muslim world still follows their lunar based calendar, will this year’s Ramadan be the longest duration fast for those living in the northern hemisphere? 

By: Ringo Bones 

Since it began back in June 6, this year’s Ramadan fast could be the longest in 33 years for Muslims living north of the equator because the holy month has been scheduled according to the Muslim calendar which is lunar based – and it lasts until July 6 for this year’s scheduled Eid-al-Fitr. Over the last few years, the scheduled observance of Ramadan have crept up until this year that it coincided with the height of the summer season in the northern hemisphere. 

By this year’s summer solstice, June 21, Muslims observing Ramadan in Denmark would have to fast for 21 hours because daylight hours could last that long in those latitudes during the summer solstice while Muslims in London will have to fast for 19 hours during the height of summer. While Muslims residing in major metropolitan areas of the southern hemisphere, like those in Johannesburg, South Africa, and Buenos Aires, Argentina will only have to fast for 9 ½ hours this coming June 21 because it is winter in the southern hemisphere. Given that the majority of dry land on planet Earth Is located in the northern hemisphere, more Muslims will be observing Ramadan in the backdrop of the summer season. The scheduling “conflict” is largely due to how the Muslim calendar got established. 

 The formation of the Muslim calendar could be traced back to the Era of the Hegira – i.e. the Muslim Era - which began on July 16, 622 AD, after the flight of the prophet Muhammad from Mecca. The year 2016 of the Gregorian calendar is the year 1437 of the Muslim Era. Before the time of the prophet Muhammad, who died in AD 632, the Arabs had a lunisolar calendar, using lunar months with intercalations something like those of the Jewish calendar. It is believed that the abuse of the old calendar led the prophet Muhammad to eliminate the added months and use a typically lunar calendar. In this system, whose year one is A.D. 622, the day and the synodic period of the moon are the only natural units used, the seasons not being taken into account. The lunar month was used as 29 ½ days and the year had 12 months of 29 or 30 days alternately. In a cycle of 30 years the final month had 29 days in 19 of the years and 30 in the other 11. The Muslim calendar is used extensively throughout the Near East and Middle East, though in 1925 Turkey abandoned it in favor of the Gregorian calendar.   

Saturday, March 26, 2016

Should There Be A Fixed Date For Easter Sunday?


Given that it is one of the most important holidays of the year and all of Christendom, should there be a fixed date on when should Easter Sunday should be observed on the calendar?

By: Ringo Bones 

It must have been very awkward back in 2014 when Easter fell on the 20th of April which made for a very awkward Easter Sunday celebration. Not only because April 20 have been set aside for the celebration of 420 – i.e. the global movement for the legalization of marijuana not only for medical use but also for recreational use as well and there had been recently unearthed evidences that Jesus Christ used marijuana, but also because April 20 his Adolf Hitler’s birthday which gave a whole new meaning of the “comical euphemism” - Jesus Hitler Christ. Thankfully to the relief of more “conservative Christians” plans are in motion to set aside a fixed date for Easter Sunday and it is safe to bet that it will not be one of “awkward days” between the months of March and April. 

The current Archbishop of Canterbury, Justin Welby, had been in talk with various leaders of the different Christian sects around the world and the preliminary agreement of the talks suggests that most of them are in favor of a fixed date for Easter Sunday. The only group opposed to the proposal of a fixed date for Easter Sunday was the top brass of the Coptic Orthodox Church. But according to the Archbishop of Canterbury, the final decision to establish a fixed date for Easter Sunday will probably happen 5 to 10 years from now. But why is it that the celebration of Easter Sunday doesn’t have a fixed date? 

During the early days of the Christian church prior to the reign of Pope Victor I (189 – 198 AD), the Western Churches, as a rule, kept Easter on the first day of the week in the beginning of Springtime, while many of the Eastern Churches conforming to the Jewish rule of celebrating Passover, observed Easter on the 14th day of the Jewish month of Nisan. Through the energetic efforts of Pope Victor I, the latter practice gradually disappeared. But another problem came to the fore: granted that Easter was to be kept on Sunday, how was that Sunday to be determined? 

The Council of Nicaea in 325 AD paved the way fro the final settlement by ruling that Easter is to be observed by all on the same Sunday, that this must be the Sunday following the 14th day of the paschal moon, and that moon was to be accounted whose 14th day followed the vernal equinox. Because of the differences in the systems of chronology followed in various places, however, the decrees of Nicaea did not immediately remove all difficulties nor win universal acceptance. The Gregorian correction by Pope Gregory XIII of the Julian calendar then in use in 1582, moreover, introduced still further discrepancies. 

Throughout Western, Christendom the corrected calendar is now universally accepted and Easter is solemnized on the first Sunday after the full moon following the vernal equinox as first suggested by an English monk named Venerable Bede back in the year 700, with the result that the earliest possible date for Easter Sunday is March 22 and the latest is April 25. In the East, however, the calendar has not been bought into accord with the Gregorian reform and thus their observance of Easter seldom coincides with the Western date. In recent years, laudable endeavors have been made to fix the date of Easter, but definite results are still awaited. Let’s just hope that the current Archbishop of Canterbury Justin Welby succeeds and his name will be immortalized together with the Venerable Bede.