The meridian sighting marks of the Neuchâtel Observatory

Article on the website of Swisstopo

To determine Swiss time the Neuchâtel Observatory observed the passage of the stars above the meridian using a telescope. To make sure that the telescope was correctly adjusted, it was required to scan the north-south axis and identify two marks serving as reference points. These marks still exist today. The southern mark in Portalban has a direct link to swisstopo since it was integrated into the national survey network.

Sighting mark in Portalban (left), sighting mark in Chaumont (right)

What is a meridian sighting mark?

It is a stone construction which had to be visible with the meridian telescope. In order to enable the Neuchâtel Observatory to cover the north-south axis and thus determine the time at which the stars pass over the meridian, two telescopes had to be built. One to the north and the other to the south. If the telescope did not detect these marks, they had to be readjusted.

In 1959, the lack of precision of the meridian telescope and the arrival of atomic clocks sounded the death knell for the use of sighting marks. However, they are now part of the heritage of the Neuchâtel Observatory and swisstopo.

Where are these marks?

The Observatory of Neuchâtel had three sighting marks built. The closest was on the Mail hill, 80 meters from the Observatory. It no longer exists today. The northern sighting mark was erected in Chaumont, 3 kilometres from the Observatory. The third was built in Portalban, on the shores of Lake Neuchâtel, 9.5 kilometres south of the Observatory. Like the mark in Chaumont, the Portalban sighting mark still exists

The southern sighting mark in Portalban

The first mark in Portalban was rather crude. Indeed, in 1861, the Observatory had a black diamond painted on a rock on a white background. The second version of the mark was also created in 1861 and consisted of an obelisk 3.10 meters high. A white diamond surrounded by black was also painted on the stone structure.

In 1927, the Portalban sighting mark was integrated as a first-order fixed triangulation point and as a third-order triangulation point by the Federal Topographic Service (swisstopo). This fixed point was therefore measured precisely, and its coordinates are well known. As a result, the point is protected by federal law and cannot be dismantled. In 1886, a levelling pin was sealed on the sighting mark. swisstopo is responsible for its inspection and renovation. And its condition is checked every 12 years.  

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