In the previous issue [1], we learned that to meet the needs of ocean navigation, the British invented the marine chronometer to help captains accurately ascertain the longitude of the ship to ensure safe navigation. In spite of the high accuracy of the marine chronometers, they needed to be calibrated regularly, and this came to depend on a time service provided by major ports.

How does the Observatory measure time?
The HKO shoulders this important responsibility in Hong Kong. At the time of its establishment in 1883, its main purpose was to determine "local time" through astronomical observations using a transit instrument (Picture 1). Simply put, the moment when the sun passes directly to the north or south [2] of a location is local noon. This is the basic principle of the sundial (Picture 2), and the time measured is called "apparent solar time". However, apparent solar time is not uniform because the Earth's orbit is not circular and its revolution speed varies with its distance from the Sun. There is also an effect due to the tilt of the earth's rotation axis. Therefore, a more uniform time called mean solar time was introduced. For example, "Greenwich Mean Time" that we are quite familiar with is the mean solar time of the longitude of the Airy Transit Circle (the longitude of the "Prime Meridian" mentioned in the previous issue) at the Royal Observatory at Greenwich. Similarly, the time service provided by the HKO during its early years was based on the mean solar time of the longitude of the Observatory's transit instrument, i.e., what this article will focus on ─ the Hong Kong Meridian.

Picture 1: A transit instrument with a diameter of 3 inches made by Troughton & Simms. A similar one was used by the HKO during its early days for its time service based on astronomical observations. Unfortunately, it was lost during WWII. (Source: Orwell Park Observatory [3] )

Picture 2: The equatorial sundial in the Forbidden City in Beijing. (Source: Wikipedia [4] )

In 1884, the HKO built the Time Ball Tower (Picture 3) beside the Marine Police Headquarters in Tsim Sha Tsui. The 6-feet diameter time ball, hoisted on the mast on the top of the Time Ball Tower, provided an accurate time service for ships arriving and departing the port.

Picture 3: The first-generation Time Ball Tower at the Marine Police Headquarters in Tsim Sha Tsui. The signal mast used to hoist typhoon signals can be seen beside the tower. (Source: SHUN Chi-ming)

The Time Ball Tower was brought into service on 1 January 1885. With the assistance of personnel from the Marine Police Headquarters, the time ball was hoisted to half-mast at 12:55pm, and to the top at 12:57pm, every day. Finally, it was dropped at exactly 1pm by the standard clock at the HKO through circuit control (Picture 4).

Picture 4: The time ball service announced by the Observatory in the Hong Kong Gazette on 1 January 1885. The Hongkong Civil Time mentioned in the notice is the mean solar time of Hong Kong (Source: The University of Hong Kong Library)

Then how was the "mean solar time" accurately measured by the Observatory with the transit instrument? First of all, as mentioned in the previous issue, it is necessary to determine the local time through astronomical observations in order to accurately measure the longitude of ships at sea [5], and vice versa: if we already know the local longitude [6], by using the transit instrument to observe the exact time, according to the Observatory clock, when a specific star crosses the meridian, we can check the clock’s accuracy.

Due to the Earth's rotation, all stars in the sky revolve around the north celestial pole of the celestial sphere relative to the observer on the ground (Picture 5). Thus, by checking the position of a certain star on the celestial sphere in the astronomical almanac, one could determine the time when the star passes to the north or south of the observer's meridian. However, since the earth also revolves around the sun, there would be a difference of about 4 minutes per day between the "sidereal time" derived by the above method and the "mean solar time". Therefore, to obtain Hong Kong Civil Time, the Observatory needed to convert the "sidereal time" into "mean solar time". Of course, astronomical observations could only be carried out under favourable weather conditions, and so they could not be made every day. For example, only 313 observations were made at the Observatory with the transit instrument in 1885.

Picture 5: Due to the Earth’s rotation, the stars on the celestial sphere revolve around the north celestial pole relative to the observer on Earth (Source: Encyclopaedia Britannica)

How was the Hong Kong Meridian established?
As we can see from Picture 5, the meridian is just a virtual curve on the virtual celestial sphere that bisects the sphere into two equal halves at that meridian. You may ask: how were the Prime Meridian at the Royal Observatory in Greenwich and the Hong Kong Meridian at the HKO established? According to HKO's literature [7], the Hong Kong Meridian was first defined by two meridian marks, one on the stone pillar ("North Mark") about 70 feet due north of the Transit Room of the Observatory (Picture 6), and the other on the stone pillar ("South Mark") in Wan Chai Mid-Levels, which is 11,354 feet due south of the Transit Room of the Observatory. The two marks were established in 1884.

Picture 6: The HKO, photographed circa 1905, with the Transit Room next to the Main Building. (Source: SHUN Chi-ming)

Marks with the shape of a cross (North Mark) and a vertical line passing through two circles (South Mark) were installed on the stone pillars to check that the transit instrument was aligned in the north-south direction by aiming it at these two marks, so that the time measurement error could be minimized. The average error of the time service was less than 0.2 seconds, showing a high accuracy considering the technological level at that time.

Meridian marks are found in well-renowned observatories. The meridian marks in Greenwich of England and Paris of France are shown in pictures 7 & 8 respectively.

Picture 7: The meridian mark in Greenwich. (Source: Royal Observatory, Greenwich [8] )

Picture 8: The meridian mark in Paris, France. (Source: Wikipedia [9] )

The first magic stone
I still remember the time I started to study the history of the Observatory and got some impressions of these two Hong Kong meridian marks, but I never imagined that these two stone pillars are still "alive", standing there more than 130 years later to testify the history of the provision of a time service in Hong Kong! The story of searching for the magic stones of the Hong Kong Meridian began on 2 February 2016 when Dr Stephen DAVIES, former Museum Director of the Hong Kong Maritime Museum, came to the HKO in the morning to study a stone pillar located outside the Centenary Building of the Observatory (Picture 9). Although no cross-shaped mark on the stone pillar as recorded in the literature could be found, he still believed it to be the North Mark. I was skeptical, because I had been told by all of my colleagues about the use of this stone pillar, viz, for fixing the physical typhoon signals, since joining the Observatory. Before the Centenary Building was built, there was a signal mast for hoisting the physical typhoon signals beside the stone pillar and an iron hook shaped like a horn with an iron chain attached to it is still on the stone pillar (Picture 10). I never had any doubts about what they had told me.

Picture 9: The stone pillar outside the Centenary Building of the Observatory, which turned out to be the North Meridian Mark. (Source: The HKO)

Picture 10: The horn-shaped iron hook on the stone pillar, with an iron chain attached to it. (Source: The HKO)

However, after taking some measurements, Stephen and our team found that the stone pillar was indeed located about 70 feet north of the old Transit Room, and so we checked the old maps of the Observatory and unexpectedly found the North Mark at the same location on one of the maps (Picture 11). Looking through the old photos of the Observatory, it was discovered that the North Mark stood in a straight line with the Transit Room and a Triangulation Station (or Trig) in front of the main building of the Observatory (Picture 12). As the origin of the Observatory's latitude and longitude (Picture 13), the Trig Station was also closely related to the Hong Kong Meridian, which will be discussed later.

Picture 11: A map of the Observatory in 1983, with the "North Mark" visible outside the Centenary Building. (Source: The HKO)

Picture 12: Aerial photo of the Observatory in 1951. It can be seen that the North Mark stands in a straight line with the Transit Room and the Trig Station. The signal mast for hoisting physical typhoon signals was near the North Mark. (Source: The HKO)

Picture 13: A plaque (left) installed at the Trig Station (right) of the Observatory Headquarters, indicating the longitude, latitude and altitude of the Observatory. The date of the plaque was indicated as July 1953. (Source: The HKO)

The second magic stone
After the first stone pillar was found, Stephen and I already started a discussion on the location of the South Mark via email on the same day. The only information we had was that the South Mark was located 11,354 feet due south of the Transit Room of the Observatory, which should be somewhere near Bowen Road in Wan Chai on the Google Maps (Picture 14). However, after more than a hundred years, with the mountains overgrown with trees, it was easier said than done to find a stone pillar measuring just a few square feet. During our independent visits to explore along Bowen Road, Stephen and I found that there had been landslides in the vicinity in 2005, and some slope stabilisation and flood control works had also been carried out in recent years (Picture 15). Therefore, there was a possibility that the stone pillar had been washed away by the landslides or removed during construction.

Picture 14: The approximate location of the South Mark, near Bowen Road, Wan Chai. (Source: Google Maps)

Picture 15: Some slope and flood control works have been carried out around Bowen Road in recent years. (Source: The HKO)

A year passed by and none of us had any clue. In February 2017, at an exhibition I held with some collector friends at the Central Library, I happened to make the acquaintance with Mr YIP Tsan-pong, a surveyor from the Lands Department. During our chat, I mentioned the search for the South Mark and asked him about the history of the Trig Station of the Observatory in the hope of obtaining his assistance. Good news suddenly came on 12 July of the same year ─ he found the South Mark [10] (Picture 16)! What a surprise!

Picture 16: Photo of the South Mark sent by YIP Tsan-pong on 12 July 2017. (Source: YIP Tsan-pong)

So, how did YIP Tsan-pong find the needle in the haystack? It turned out that he had a secret weapon - aerial photos: Picture 17 clearly shows that there was a square object, which seems to be a man-made structure, in the aerial photo of 1968 (left); whereas in the aerial photo of 1976 (right), the object appeared in the same position, but half of it was covered by trees. After accurately locating it on the aerial photos, YIP Tsan-pong succeeded in finding the stone pillar on the slopes of Bowen Road.

Picture 17: Aerial photos taken in 1968 (left) and 1976 (right). The red squares show magnified details of the spots pointed to by the yellow arrows. (Source: Hong Kong Lands Department)

Stephen and I were overjoyed at the good news. On 22 August of the same year, I went on an inspection visit with my colleagues from the Observatory and found also that there was a trig station on the ground near the stone pillar (Picture 18), indicating that the stone pillar was closely related to surveying. It was found later by YIP Tsan-pong and his team from the Lands Department through measurement that the longitude of the stone pillar was exactly the same as the longitude of the North Mark and the Trig Station of the Observatory. That was proof of the pillar being the South Mark established by the Observatory in 1884, and that the line passing through the North Mark at the Observatory and the South Mark on Bowen Road was the Hong Kong Meridian.

Picture 18: The Trig Station on the ground near the stone pillar, engraved with the words "WDTRIGSTN" and "1893", believed to mean a War Department Trig Station established in 1893. (Source: SHUN Chi-ming)

Since the Hong Kong Meridian passes through the Trig Station of the Observatory, what is the role of this station? Most intriguingly, the Explanatory Notes on Geodetic Datums in Hong Kong [11] published by the Lands Department still says, "The origin (of the Hong Kong 1980 Geodetic datum) is at the old Trig "Zero" (now gone), which was 38.4 feet due south along the Transit Circle of the Hong Kong Observatory in Kowloon. As the latitude of the Hong Kong Observatory determined by astronomical observations was 22°18'13.20"N, the latitude of the Trig "Zero" was thus 22°18'12.82"N. The longitude of the Trig "Zero" was 114°10'18.75"E which was determined in 1924 from simultaneous observations of Bordeaux signals made at Greenwich and Hong Kong." (Picture 19) It is very interesting to note that the longitude and latitude of the Observatory recorded in the document are the same as the longitude and latitude of the plaque installed at the Trig Station (Picture 13), so how can it be said that the Trig "Zero" Station is now gone? Is the Trig Station we see today no longer the original Trig "Zero" Station? This mystery was finally uncovered, but I'll keep you in suspense for now.

Picture 19: Page A2 of the Explanatory Notes on Geodetic Datums in Hong Kong [11] published by the Lands Department, clearly shows that the Trig "Zero" is "now gone". (Source: Hong Kong Lands Department [11] )

The story should have come to an end with two stone pillars built in 1884 for defining the Hong Kong Meridian successfully found. But new questions always cropped up! I told Stephen that a report [12] published in 1914 mentioned that the Observatory had set up two more stone pillars in 1913 to replace the original two, mainly because the smoke from ships in Victoria Harbour would obscure the South Mark, while refraction at the sea surface would also make the image (in the telescope) unstable. One of the two new stone pillars was located at the Observatory, 72 feet due south of the transit instrument (referred to as the "New South Mark" in this article), and the other 16,655 feet north of the Transit Room (referred to as the "New North Mark" in this article), that is, on the hillside of Beacon Hill in Kowloon. In spite of the great difficulties involved in finding this stone pillar, hidden deep in a mountainous area, Stephen said with absolute conviction: "We must go and look for it!"

The third magic stone
It was not difficult to find the New South Mark at the Observatory Headquarters, but it is a pity that there is only a stone base left under today's thermometer shed (Picture 20). The distance between the base and the Transit Room is consistent with the literature records.

Picture 20: The red arrow under the thermometer shed at the Observatory Headquarters indicates the location of the New South Mark (left); the red dotted lines (right) outline the extent of the New South Mark's base. (Source: The HKO)

The last magic stone
As for the New North Mark, we first had to determine its location. YIP Tsan-pong played the same old "trick" again. From the aerial photo of Beacon Hill in 1963 (Picture 21), he found a square object, yet had mixed feelings: the good news was that the location was consistent with the literature records, while the bad news was that it was not easily accessible. One had to walk across steep hills and blaze new trails to reach the destination. Stephen had extensive experience in mountain climbing, but I did not, so I convened an "expedition team", comprising Stephen as well as colleagues from the Observatory and the Lands Department, to explore the climbing routes in our spare time. One of my colleagues even went to the site ahead of others to see which route would ensure a higher chance of success before actually climbing it. In any case, safety has always been our top priority [13].

Picture 21: An aerial photo of 1963. The red square shows magnified details of the spot pointed to by the yellow arrow. (Source: Hong Kong Lands Department)

The expedition team finally set off on 14 January 2018 with the goal of finding the path first and then returning in February if necessary. We were a group of 6: 3 from the Observatory, 1 from the Lands Department, Stephen, and a photographer of "Cool Met Stuff" hoping to film the first-hand exploration process for producing educational videos. The expedition started at 9:30 am, but found a dead end after about half-an-hour walk, so everyone began to cut through thorns, clear roads, pass through dense bamboo forests and climb down along mountain streams... when we reached the last 100 metres, it was really difficult to move a single step, as there were dense thorny vines everywhere! Nevertheless, as the saying goes, God helps those who help themselves, and at 12:46pm Stephen took the lead and found the New North Mark in the vines (Picture 22), which was located within just three metres of YIP Tsan-pong's estimated coordinates – really accurate indeed!

Picture 22: The expedition team photographed at the New North Mark. Upper row from left to right: Stephen DAVIES, SHUN Chi-ming, LEUNG Wai-yeung; lower row from left to right: YIP Wing-sing, SO Wing-yeung. (Source: SHUN Chi-ming)

The best fish swims near the bottom. Although we had put a lot of markings on the trees along the way to make it easier to identify the location when returning, the original trail could not be found when crossing a mountain stream on our way back. As time was passing by very quickly, at the end of our wits we decided to make a last attempt and climb up from a height of about 200 metres to the fifth section of the MacLehose Trail at a height of about 350 metres. Finally, we were able to return to the starting point without mishap before sunset (Picture 23), and successfully completed the mission: all the meridian marks had been rediscovered. If you are interested in learning more about the story of looking for the magic stones, especially this expedition, please watch the "Cool Met Stuff" episodes [14] produced by the HKO.

Picture 23: The expedition team after successfully returning to the starting point, with YIP Tsan-pong who also came to cheer them on. From left to right: YIP Wing-sing, Stephen DAVIES, SHUN Chi-ming, YIP Tsan-pong, a photographer unwilling to show his face, LEUNG Wai-yeung, SO Wing-yeung. (Source: SHUN Chi-ming)

The mystery of the Trig Station
As mentioned above, a trig station at the Observatory's Headquarters represents the origin of the Observatory's latitude and longitude, which are exactly the same as the latitude and longitude of the Trig "Zero" described in the document from the Lands Department. But the document says that the Trig "Zero" is now gone. If so, what is the trig station we see today (Picture 13)? The mystery was ultimately uncovered in December 2020, by coincidence. In November 2020, Stephen DAVIES, YIP Tsan-pong and I were finalising a paper on the Hong Kong Meridian. The article mentioned that although the Transit Room had been demolished, the transit instrument, according to the literature, was installed on a Portland stone slab, which was placed on a brick pier buried in the ground about 5 feet deep [14]. Through a simple archaeological excavation, the brick pier could be found and the exact location of the instrument determined. Then the distance between today's Trig Station and the transit instrument could be accurately measured, so that we could ascertain if that had been the Trig "Zero", which should be 38.4 feet south of the transit instrument according to the document from the Lands Department. In the meantime, to facilitate greater publicity among the public, the Observatory was preparing to build a new stone meridian on this ground, for which it was necessary to cut the concrete pavement, therefore I suggested to my Observatory colleagues about the possibility of digging a little deeper into the pavement to find the brick pier of the transit instrument, if possible. We also gained support from colleagues from the Antiquities and Monuments Office, who provided their professional guidance during the excavation. Finally, on 3 December 2020, we successfully excavated and found the red brick pier at the site (Picture 24) as well as a shock-absorbing brick cylinder surrounding the pier (Picture 24). According to the literature [15], the cylinder was used to isolate the transit instrument from ground vibrations so that the accuracy of astronomical observations could be improved. After field measurement by YIP Tsan-pong and the team from the Lands Department, the Trig Station, at exactly 38.4 feet away from the central point of the pier, was proved to be the original Trig "Zero". According to historical documents, the Trig "Zero" was established by the Public Works Department on 2 February 1929 [16], some 24 years earlier than the year of 1953 inscribed on the plaque at the station.

Picture 24: The red brick pier (top) and part of a shock-absorbing brick cylinder (bottom) were found during the archaeological excavation. (Source: The HKO)

Concluding Remarks
Through research and exploration efforts for nearly 5 years, the mysteries surrounding the Hong Kong Meridian were solved one after another, and the remnants of its six components were successfully uncovered: 1) The brick pier of the transit instrument; 2) The shock-absorbing brick cylinder surrounding the pier of the transit instrument; 3) The (Old) North Mark (ONM); 4) The (Old) South Mark (OSM) ; 5) The New North Mark (NNM); and 6) The base of the New South Mark (NSM) (Pictures 25 & 26). One single line passes through all of them and the Trig "Zero", the origin of the Observatory's latitude and longitude, namely the Hong Kong Meridian, which extends from Beacon Hill in Kowloon to Bowen Road on Hong Kong Island, is more than 8 kilometres long although, of course, in theory it runs from the North Pole all the way to the South Pole passing through the HKO on the way.

Picture 25: Meridian marks at the HKO Headquarters (Source: Hong Kong Lands Department)

Picture 26: The Hong Kong Meridian. (Source: Hong Kong Lands Department)

It is indeed a miracle that these stone pillars, first built in 1884, then retired and forgotten after WWII, could be rediscovered more than half a century later. Apart from the luck and coincidence that played a role, one more story cannot be left out: when Stephen DAVIES, YIP Tsan-pong and I were about to submit the paper at New Year's Eve in 2020, Stephen received an email from an engineer, Ms Irene OR. She contacted Stephen through a friend and told him that she had discovered the South Mark on Bowen Road as early as 2008, but she didn't know it was a meridian stone pillar at the time until she saw the Observatory's "Cool Met Stuff" episodes [13]. Irene was responsible for a slope stabilisation project on Bowen Road undertaken by the Geotechnical Engineering Office in 2008 [17]. Colleagues on the site noticed the stone pillar and thought that it would hinder engineering drilling and asked her for advice. After visiting the site, she felt that the stone pillar was very special and should be preserved, so she instructed the site colleagues to change the drilling location while contacting the Geotechnical Engineering Office and the Antiquities and Monuments Office. A colleague from the Antiquities and Monuments Office visited the site and kept a record, but had no clue about what the stone pillar really was, so the South Mark continued to be unknown to the outside world until it was discovered by YIP Tsan-pong on 12 July 2017. On receiving Irene's message, we could not help but sincerely thank her and the construction team for spontaneously preserving this historic heritage item of Hong Kong, its true value unknown at the time. We also felt relieved to learn that the stone pillar had narrowly escaped the landslide in 2005 (Picture 27). After all, from what we know, no other such meridian marks have been recorded in mainland China or even all the rest of Southeast Asia.

Picture 27: The South Mark (right) is located within 10 metres of the 2005 landslide area (left). (Source: Google Maps and the HKO)

At this point, it should have been a "happy ending". Unfortunately, we always have some regrets to live with. In January 2020, two mountain hikers found that the New North Mark Stone Pillar had partially collapsed (Picture 28). Though the cause of the collapse was unknown, it can be seen from Picture 22 that as early as January 2018, visible cracks had appeared in the New North Mark due to erosion by plants. In addition, the stone pillar might have become unstable and collapsed due to bad weather conditions, as Hong Kong was hit by super typhoon Mangkhut on 16 September 2018, and Typhoon Signal No 10 lasted for 10 hours that day. What a pity! However, I remain optimistic in the restoration of the New North Mark. From Picture 28, we can see that some people have already kindly helped to stabilise the remaining stone pillar. I sincerely hope that with a concerted effort we can work together to better preserve the stone pillar.

Picture 28: The New North Mark, partially collapsed. (Source: Daniel WONG)


  1. Please refer to The Council of the Hong Kong Laureate Forum Newsletter Issue 9, May, 2021: Hong Kong Meridian (I).
  2. Depending on the latitude of the observer.
  5. To be exact, in navigation, to determine the longitude of a ship at sea it is necessary to know the time on the Prime Meridian (i.e. GMT). The time of local noon by astronomical observation is then compared to noon GMT and the difference is the observer’s longitude.
  6. The longitude of the HKO was first established in 1881 by Lieutenant Commander FM Green and the Hong Kong Public Works Department using the telegraph method plus surveying. The longitude of the Observatory when the time service was first introduced was 114° 10' 27.9", and was later revised to 114° 10' 18.75" in 1926 based on the radio technique.
  7. W Doberck (1886): “Report on the Astronomical Instruments at the Observatory and on the Time Service of Hong Kong in 1885”, Hong Kong Observatory.
  10. Similar to the North Mark, the vertical line passing through two circles as recorded in the literature could not be found on the stone pillar.
  12. TF Claxton (1914): “Report on the Time Service (Appendix to Hong Kong Observations, 1913)”, Hong Kong Observatory.
  13. Due to safety considerations, the author does not recommend that hikers look for the New North Mark at Beacon Hill.
  15. W Doberck (1886): The Hong Kong Observatory, Nature, 14 October 1886, pp 572-573.
  16. TF Claxton (1930): Report of the Director of the Royal Observatory, Hong Kong, for the Year 1929, Hong Kong Government.
  17. The project was related to the landslides in 2005 mentioned above.

Mr SHUN Chi-ming, Former Director of the Hong Kong Observatory