A Mosaic of Science and Culture

home | project | meetings | works
A Mosaic of Science and Culture  
INSTRUMENTS
  

The nocturlabe or night clock is an instrument used in the antiquity to determine the solar hour during the night, observing the position of some stars in the sky.

 Navigators, astronomers and explorers used it for several centuries in their expeditions and observations, together with the sun clock, to which it was complemented, being the perfect method to find out what hour it was.

The nocturlabe reached its maximum deve­lopment during the 15th century and kept o being used, at least, until 1759 in which it was replaced by mechanical clocks.

1

Method of measurement

To determinate what the time must be taken a very luminous star as a reference and if possible it belonging to a circumpolar constellation. These stars turn in concentric circles around the Pole Star that keeps its position stable regarding the observer.

The chosen stars are: Dubhe and Merak from “Ursa Major” and Schedir from "Cassiopeia”

1

1

1

1

1

1

The nocturlabe consists on three concentric circles. The support or "circle base", classified (conferred a degree) with twelve months of the year in hourly sense. The limbo or "disc of dates " which is divided in 24 hours and fractions, graduated in anti hourly sense and the top or disc  " the index of a quadrant" where appear the stars of reference of the constellations Cassiopeia and Ursa Major.

–      To place the clock vertically and with the mark (N) or arrow that exists in the base, looking downwards. The disc of dates up to making coincide the day chosen with the N or arrow.

–      Then, point the nocturlabe to the North and locate across the central orifice the Pole Star.

–      Finally, observe the sky and turn the index of the quadrant until the drawing of the disc coincides with the real position that it presents in the sky. The arrow of the index quadrant on the limbo indicates us the hour (T.U.) of the observation.

1

The quadrant is a simple instrument, both in its construction as in its use. The quadrant is used to measure angles vertically. It can measure the height of the stars and also the latitude.

Determination of the height of a star

As soon as we have the star centred, the tense thread will indicate on the protractor an angle that will be the height of the star on the horizon. To centre the star, we look through the visor.

At this moment, we hold with one finger the position of the thread to be able to do the reading.

Determination of the latitude

If we measure from the same place, the height of the Pole Star, throughout the year, we observe that its height is always the same one.

There is verified that the height of the polar one is equal to the latitude of the place of observation.

1

1

1

Planispheres arose several centuries ago as an instrument to know the aspect that was presenting the sky in a date, in an hour and in a certain place.

When we look at the sky, we have the feeling of being under a great vault. This enormous imaginary sphere that surrounds us and in which the stars seem to be fixed will be called the celestial sphere and its equator horizon. Above it they are found the four cardinal points.

The horizon divides the celestial sphere in two halves: the north hemisphere placed above the horizon, and the south hemisphere. For all of us the visible hemisphere will be the North.

The point of the sky around which the celestial sphere seems round, if you live in of North hemisphere, it is the Polaris star.

Aspect of the planisphere

The planisphere is formed by two discs.

In the first one it is represented the whole sky that it is possible to see at a concrete latitude during the whole year and its perimeter has a graduated rule in months and days.

The second disc is divided in 2 zones, a transparent one which shows us the visible sky for a certain date, and a dark one which will conceal the part of the sky that we will not see for that one mentioned date. The transparent zone has is an ellipse, and its edges represent the horizon of our sky.

The Sun does not appear in the map, but although it appears its path throughout the year: the ecliptic with ellipsoidal form. The planets never appear in the planispheres, since they change from one day to another and do not coincide of year in year.

Use of the Planisphere

Turn the top disc until it coincides the time with the day of the month.

Place opposite to the Polaris Star and with the planisphere in a high place.

Make coincide the cardinal points of the horizon with the geographical ones. To know of the North direction you must look for Polaris star.

All the stars and constellations that are inside the ellipse are those who can be seen in this moment in the sky.              

Sunsets and sunrises

Using a planisphere we can also find out at what hour the sun will rise and at what hour will set.

We have to pay attention to two special lines: the horizon and the ecliptic. Steps to follow:

Choose a concrete date for example on April the 14th.

Set a line from the centre of the planisphere up to the selected date. This line cuts to the ecliptic at a point.

Turn the move disc until the East Horizon coincides exactly with that one point.

Check that the hour puts exactly in the date that you chose. For the selected we have obtained 7:25 approximately (solar hour).

For the sunset, the procedure is the same, but turning about-face to the planisphere.

1

1

1

1