... the following morning de Sitter, while having breakfast in his rooms, received a message that Gill wished to speak with him in the laboratory. De Sitter did not possess at that time the fluent command of the English language which he afterwards acquired; Kapteyn was lecturing at the time and Mrs Kapteyn acted as interpreter at the interview. Gill invited de Sitter to come to the Cape as a computer and, as de Sitter afterwards stated in a letter to Gill, "thereby complete my astronomical education - or rather begin it, for up to that time I had never made a speciality of astronomy and intended to become a mathematician."

With the discussion of Gill's heliometer observations of Jupiter's satellites, de Sitter had entered upon the field which seemed so completely to suit his versatile mind: the talent, on one hand, for seeing clearly through the most intricate mathematical problems and dealing practically with them; on the other hand, the critical insight into the value and limitations of observations - an insight which sprang from a very active interest in observations as well as instruments.

Throughout the years at Groningen, as well as later, de Sitter showed an almost unbelievable activity. In looking through a number of his more important publications, one is struck with the rapidity with which discussions on different topics follow each other and by the amount of thorough critical thinking of which each of these brings evidence. Neither a serious illness from which he suffered during some years, nor the directorship of an observatory which he undertook to reorganise almost entirely, nor his constant activities in many matters connected with the University could slow up the flow of his scientific investigations.

If no matter exists other than the test body, does it have inertia.

... detracts from the symmetry and elegance of Einstein's original theory, one of whose chief attractions was that it explained so much without introducing any new hypothesis or empirical constant.

On various occasions I have found myself confronted with the question which would be the best value to adopt for one of the fundamental astronomical constants. We have, of course, the set of constants officially adopted in the national ephemerides ... Some of the adopted constants are inconsistent with each other. This the adopted length of the year and the constant of precession are contradictory, and similarly the mass of the earth and the solar parallax. Newcomb's great work is now more than thirty years old, but it still stands unsurpassed as an example of sound critical discussion. ... I have only been led by the considerations that on the one hand my results may perhaps be useful to others in similar circumstances, and on the other hand that a critical survey like the present may be helpful to guide the efforts of astronomers to those points where they are most needed.

The privilege accorded me involves a task of unusual difficulty, for the major part of de Sitter's work is in intricate, speculative, and mathematical fields dealing with the perturbations of the motions of bodies of the solar system according to classical celestial mechanics, and with the theory of relativity from a purely speculative point of view as well as in relation to perturbations observed, but not explained, and relativistic theoretical perturbations not perceived or as yet perceivable by observation. I, therefore, beg your indulgence if I fail in a clear presentation of the significance of some of his contributions to science. His intellectual abilities cover so wide a range and penetrate so deeply and so minutely into practical astronomy and the mathematical theories to explain what is observed, that only an intensive study of his brilliant work could do justice to the greatness of the man.

... for his theoretical investigations on the orbits of the satellites of Jupiter, and for his contribution to the Theory of Relativity.

In [de Sitter's] work we see the creative mathematician at his best. He is not a cold, dispassionate juggler of Greek letters, a balancer of equations, but rather an artist in whom wild flights of the imagination are restrained by the formalism of a symbolic language and the evidence of observation. Only the musician can fully grasp what it must have meant to de Sitter to see the cosmos shaping itself in new ways in his formulas. Like musical notes, strange symbols, standing for forces and masses that were divined rather than known, arranged themselves into a coherent message. And when the message came to be read a totally new universe was revealed. Here we have something of the direct personal experience of the outer world, of the significance of nature's wonders, that comes only to a Beethoven or a Milton. The expanding universe of de Sitter must be regarded as something more than an inexorable conclusion drawn from the strictest kind of logic with which the human mind is familiar. It is poetry of a new sort - the scientist's way of writing an epic.