What is meridian astronomy?

We can all remember having geography lessons and learning about latitude and longitude (even though most of us have forgotten which is which!). If you look at a globe of the Earth, or at the maps in an atlas, you will see the familiar network of lines of latitude and longitude.

But if you look down from an airliner you do not see black lines running all over the countryside and across the sea. The network is an imagined framework for stating geographical positions, and it is the job of surveyors to make accurate measurements so that the shape of continents and the positions of cities can be plotted out on this framework. Latitude is measured north and south, starting from the Equator (0° latitude) and finishing at the poles (90°).

Longitude is measured east and west, starting from a standard line which, by international agreement, has been chosen so as to run through the Royal Observatory at Greenwich. Notice that latitude and longitude do not tell us anything about height above the ground, or depth beneath it.

If you look at a celestial globe or at the maps of the sky in a star atlas, you will see a similar network of criss-crossing lines. But if you look up into the starry sky itself, you certainly do not see these lines running through the constellations. They are an imagined network for stating positions in the sky, and it is the job of meridian astronomy to make accurate measurements so that the positions of stars, planets and other objects in the sky can be plotted out on this framework.

In the sky, declination (like latitude on the Earth) is measured north and south, starting from 0° at the Celestial Equator which is overhead at the Earth's Equator. Declination increases to 90° at the celestial poles which are overhead at the Earth's poles. Corresponding to longitude, we have right ascension which is measured eastwards (never westwards) round the sky. The zero-point of right ascension, corresponding to Greenwich, is a point with the mysterious name of The First Point Of Aries. This is simply the point of the sky where the Sun appears to be as it crosses the celestial equator from south to north on about March 21 each year.

To sum up, meridian astronomy is that branch of astronomy concerned with making accurate measurements of positions (right ascension and declination) in the sky. Notice that these measurements do not include the distance of the object from the Earth; the measurement of distances is looked after by other branches of astronomy.

How is meridian astronomy done?

The meridian astronomer uses a special telescope called a transit circle. The special thing about the telescope is the way in which it is mounted. Most telescopes are pivoted in two directions so that they can be aimed at any part of the sky and can follow a star as it appears to move across the sky. Such telescopes usually live in round houses with domes that can turn so as to give the telescope a view in any direction. The transit circle is pivoted in one direction only, with its cross axis supported by two fixed piers, east and west of the telescope. This means that it can look south, overhead, north, straight down, or at some slanting angle north or south. It lives in a rectangular house, with shutters that can open in the north and south walls, and more shutters in the roof that can open along the north-south centre-line. It does not follow the moving stars, but just watches them go by.

While a star is passing by, the observer has to measure two things. He observes accurately the time at which the star passes due north or south, that is, the time at which it crosses his meridian. He also measures accurately the angle to which he has to tilt his telescope to see the star, that is, the elevation or altitude of the star as it crosses his meridian. From the observed time he can work out the star's right ascension and from the observed altitude angle he can work out the star's declination. But, before he can do this, he needs to know the time at which the First Point of Aries crosses his meridian. Unfortunately these zero lines and points are not visible, and so he cannot make direct observations of them. His next problem then, is to find out where they are.

One way of locating the celestial equator would be to observe some object which moves above and below the Equator, so that on the average, its declination is zero. The Sun could be used for this purpose, because every year it moves among the stars along a track called the Ecliptic, which lies equally above and below the Equator. The Moon and the planets also follow tracks near the Ecliptic, so they too will have an average declination of zero if we observe them for a long enough time.

Another way of finding the invisible equator is to use the fact that it is 90° from the celestial poles. These too are invisible, but can be located because they are the centres of the circles traced out by the stars each day and night. If we choose a star near the pole, it will trace out a fairly small circle. We can observe its altitude as it passes the meridian above the pole, and then, twelve hours later, observe its altitude as it re-crosses the meridian below the pole. The altitude of the pole, at the centre of the circle, is just mid-way between the two observed altitudes of the star.

In order to locate the First Point of Aries, all we have to do is to trace out the Ecliptic, the average path of the Sun, Moon and planets, and find the point where it crosses the equator. There are actually two such points, diametrically opposite to one another in the sky: the First Point of Aries and the First Point of Libra. We select the First Point of Aries as our zero.

Why not use photography?

Photographic telescopes are very good at showing the relative positions of many faint stars in a small region of the sky, but we cannot fit the photographs together like a jigsaw puzzle to cover the whole sky, and of course, the photographs do not show the invisible lines of right ascension and declination. In order to relate the faint-star positions to the invisible framework, we choose a limited number of reference stars spread over the sky and observe these stars with Transit Circles all over the world. The photographic telescopes can then relate the faint stars to the invisible framework via the reference stars.

The Carlsberg Meridian Circle

The Carlsberg Meridian Circle was made in 1952 to the same specification as the Cooke Reversible Transit Circle which was in operation at the RGO until 1982. It carried out observing programmes at Copenhagen University Observatory (CUO) from 1964 to 1976 when a major overhaul of the instrument was undertaken in order to automate the observing process. This removed the observer completely from the telescope thus eliminating such effects as heating of the instrument by the observer's proximity, mis-setting of the circle, mis-reading of the meteorological data and last, but not least, the inaccuracy of the observer's measurements.

In order to utilize the resultant increase in efficiency, it was decided to move the Carlsberg Automatic Meridian Circle, now known as the Carlsberg Meridian Telescope (CMT) to a better site. The telescope was taken out of operation in Denmark in March 1983 and refurbished before shipping to the Roque de Los Muchachos Observatory on La Palma in the Canary Islands in August 1983. It is operated through a tripartite collaboration between the Cambridge Astronomical Survey Unit, Institute of Astronomy, Cambridge, CUO and the Instituto y Observatorio de Marina, Spain.

Over the last 15 years the CMT has continued to operate very efficiently and in 1999 a CD-ROM containing the results of all the work carried out since 1984 was published. This contains measurements of some 180,000 positions, proper motions and magnitudes for stars down to magnitude 15 and over 25,000 positions and magnitudes of 180 Solar System objects ranging from the outer planets including Pluto to some of the many asteroids which orbit the Sun between Earth and Mars. In fact the CMT was the first meridian circle ever to measure the faint planet Pluto. Its observations have enabled us to calculate the orbit with greater precision than ever before and have largely removed the differences which led to speculation that there was another large planet beyond Pluto. Other achievements have included mapping stars in order to allow the Giotto spacecraft to navigate towards its encounter with Comet Halley.

In 1997 the telescope was made fully remote and is operated on a regular basis by astronomers at the three collaborating institutions. In 1998 a CCD camera was fitted allowing the number of stars to be observed each night to be increased by a factor of 20.

Why do it at all?

Three hundred years ago the Royal Observatory was founded at Greenwich, not to do fancy scientific research, but to solve a very practical problem that was of considerable importance to our life as a trading nation. That problem was the finding of the longitude of a ship at sea so that it could be navigated properly. This required using the stars as landmarks, or points of known position, and the observatory started making the necessary observations of star positions. A little later Sir Isaac Newton realised the scientific value of these observations, and encouraged the observatory to continue them in order to check his famous law of gravity as it applied to the Moon and planets. Later again, it became possible to combine the observations over a period of a century or so in order to show up the small individual motions of different stars. Nowadays it is possible to work out, from these motions, various theories of how old the stars are, and how the galaxy of stars evolved into its present state and shape.

The Carlsberg Meridian Telescope is now engaged on a survey of the sky between declinations +30° and -3° to a limiting magnitude of about 16.5. Although the star positions measured by Hipparcos are much more precise, they only go as far as magnitude 11 and cover the sky very sparsely (only 3 per square degree) so the CMT observations will help to extend the very accurate grid of star positions to much fainter magnitudes and much higher density. This will be of great use to the large telescopes now in operation which have very small fields of view.

Meridian astronomy may be a very old and classical part of astronomy, but it is still very much alive and is making an important contribution to our rapidly increasing knowledge of the universe.