Chasing Mercury

ON NOVEMBER 6, 1993, a relatively rare celestial phenomenon enraptured Indian skywatchers. From many parts of the country, at 8.36 am, they witnessed a black spot -- the planet Mercury -- graze past the Sun. This phenomenon, known as the transit of Mercury, last occurred on November 13, 1986, and will next take place on November 15, 1999.

With the number of highly accurate ephemerides -- astronomical almanacs that predict the positions of celestial bodies -- available in today's world, such observations seem commonplace, and one wonders that Mercury eluded humankind for almost 14 centuries. In a recent book, The Great Copernicus Chase, author Owen Gingerich says the quest for Mercury dates back to Claudius Ptolemy, the Alexandrian astronomer of circa AD 140, who miscalculated the planet's positions.

In 1629, German Renaissance astronomer Johannes Kepler was the first to predict a transit of Mercury on November 7, 1631, but he didn't live to see the event. Kepler's prediction helped Parisian astronomer Pierre Gassendi to become the first to observe the phenomenon (See box).

Difficult to observe
Of all the planets visible to the naked eye, Mercury is the most difficult to observe for two reasons. First, it appears very low on the Earth's horizon -- at an angle of less than 280 -- and only in the mornings and evenings when haze and mist are the thickest. Second, Mercury exhibits phases similar to those of the Moon: When it lies nearly between the Earth and the Sun, it appears as a thin crescent; when it is at its greatest distance from the Sun, the shape seen is semicircular; and when it is towards the side of the Sun away from the Earth, it can be seen fully.

Mercury is the planet closest to the Sun and orbits it in 88 Earth days. When it is closest to the Sun, Mercury is about 46,000,000 km away, and when furthest, 70,000,000 km away. Mercury's orbit is inclined at an angle of about 70 to the orbital plane of the Earth.

The planet has been known since the Sumerian times, about 5,000 years ago. In classical Greece, it was called Apollo when it appeared in the morning as happens in the months of October and November, and Hermes when it appeared in the evening in the months of March and April.

Ptolemy's Almagest, the greatest surviving astronomical work from antiquity, was the first to record a method of converting data on planetary positions into numerical parameters for models, which would help predict planetary orbits. With the help of the models, Ptolemy constructed ingenious tables from which planetary positions could be calculated for any given time.

He worked in a geocentric framework, which assumed that the Earth was at the centre of the solar system, a theory universally accepted in his day, but discarded in the 16th century by Polish astronomer Nicolaus Copernicus and other scientists. But more than any other book, the Almagest demonstrated that natural phenomena, though complex in appearance, could be described mathematically to express relatively simple regularities.

Ptolemy's model
Ptolemy introduced a model to explain planetary motion, which proved quite successfulsize and to incorporate this feature into his model, Ptolemy took recourse to a third device -- the equant, which is the locus of uniform angular motion within the deferent equal and opposite to the position of the Earth

Ptolemy's model had no problems in correctly predicting from planetary observations the motion of Mars, Jupiter, Saturn and Venus. Unfortunately, it wasn't easy to get adequate observations for Mercury, with the result that Ptolemy's model for the planet became too complicated and wrong.

For each of the planets, Ptolemy gave the minimum number of observations needed to determine its parameters. For Mercury, he wanted to explore the general shape of its orbit, but he was unable to observe Mercury in the evening in October or in the morning in April. From his incomplete data, he calculated an egg-shaped orbit for Mercury. But Mercury's orbit is actually so slightly elliptical that it is virtually a circle, or more precisely, an off-centre circle.

Gingerich shows the orbits of the Earth and Mercury as postulated by Copernicus and then discusses Ptolemy in heliocentric -- where the Sun is taken to be at the centre of the solar system -- terms. The off-centred nature of Mercury's heliocentric orbit cannot be accounted for in Ptolemy's model.

The discontinuity of observations played havoc with Ptolemy's work, for he mistakenly assumed that the unmeasured morning elongation -- distance of the planet from the Sun -- in April would have equalled the observed evening elongation. This confused bunch of observations made it difficult to pin down Mercury's orbital motion, which had to await Kepler and the generation of astronomers after him.

Mercury has also played an important role in the development and testing of theories of the nature of gravity because its elliptical, inclined orbit experiences the gravitational pull of the Sun and the other planets, resulting in a small motion (about 10' of an arc per century). This motion has been known for about two centuries. Newton's theory of gravity could explain only a part of this motion. The full explanation had to wait till 1915, when Albert Einstein came up with his general theory of relativity, which included a treatment of gravity.

Amit Mitra wrote this based on Owen Gingerich's The Great Copernicus Chase and other adventures in astronomical history, published by Cambridge University Press.