History of Astronomy From the Roman Empire to the Present, Part 3
Ole Roemer’s Blunder
Among the many ambitious spirits of that time, was one whose name is known only to a comparative few, nevertheless he has had a considerable influence both on astronomy and physics— Ole Roemer, best remembered for his observations of the Eclipses of Jupiter’s Satellites.
A study of the records which have been made during more than 3,000 years shows that eclipses repeat themselves with clock-work regularity, so that a given number of years, months, days and minutes elapse between every two eclipses of a given kind; but Ole Roemer observed that in the case of the eclipses of the satellites, or moons, of Jupiter, the period of time between them was not always the same, for they occurred 161 minutes later on some occasions than on others. He therefore tried to account for this slight difference in time, and was led to some strange conclusions.
These eclipses occur at different seasons of the year, so that sometimes they can be seen when the earth is at A (see dia. 3), and at other times when the earth is at B, on the opposite side of the sun and the orbit, (according to Copernican Astronomy).
So Ole Roemer reflected that when the observer is at B, he is further from Jupiter than he is when the earth is at A, by a distance as great as the diameter of the orbit; and that gave him a new idea, and a possible explanation.
He thought that although light appeared to be— for all ordinary pi^poses— instantaneous, it really must take an appreciable time to travel over the immense distance from Jupiter to the earth, just as a ship takes so long to travel a given distance at so many miles per hour. In that case the light from Jupiter’s satellites would take less time to reach the observer when the earth is at A than it would require to reach him at B, on the further side of the orbit; and as a result of these reflections he reached the conclusion that the 16 ½ minutes difference in time was to be accounted for in that way.
Following up this idea, he decided that if it took 16 ½ minutes longer for light to travel the increased distance from one side of the orbit to the other, it would require only half the time to travel half that distance, so that it would travel as far as from the sun to the earth in minutes. Therefore he gave it as his opinion that the distance to the sun was so tremendous that “ Light”— travelling with almost lightning rapidity— took minutes to cover the distance.
This ingenious hypothesis appealed strongly to the imagination of contemporary astronomers, so that they allowed it to pass without a sufficient examination, with the result that eventually it took its place among the many strange and ill-considered theories of astronomy. . . However, we ourselves will now do what should have been done in the days of Ole Roemer.
We will stand beside him, as it were, and study these eclipses of Jupiter’s satellites, just as he did, from the same viewpoint of Copernican astronomy; and then we shall find whether his deductions were justified or not.
The eclipses are to be seen on one occasion when the observer (or earth) is at A, and on another occasion when the observer (or earth) is at B, while the height of Jupiter’s satellites (or the image of the eclipse) is supposed to cross the orbit at one observation but not at the other. It is important to note that the observer at B will have to look in a direction toward the sun, and across the orbit; while the observer at A will see the eclipse outward from the orb it; in a direction opposite to the sun. . . Ole Roemer found that the observer at B saw the eclipse 16 ½ minutes later than he would have seen it from A, and he believed that this was because the image of the eclipse had a greater distance to come to meet his eye.
Let us now consider diagram 4, which shows two observers in the positions Ole Roemer supposed the earth to occupy at the respective observations.
We find that A would see the satellite in a state of eclipse while it would be hidden from B by the planet Jupiter; (triangle A, i, B). The planet and its satellite are both moving round the sun toward the east, as shown by the arrows, but the satellite is like a moon, travelling round Jupiter ; so that it moves faster than the planet. The satellite is eclipsed by Jupiter only when the two are together on the same line with the sun, (dotted lines), but, as time passes, the satellite moves to the eastward of that line ; it passes Jupiter ; and then it can be seen by the observer at B. (triangle B, 2, A).
Thus it is that B sees the eclipse a few mixtures later than A, and that is the very simple explanation which Ole Roemer overlooked. It would be possible to write a volume on this subject, and there are some who would want to debate it at interminable length, but in the end the explanation would prove to be just this ; which I prefer to leave in all its simplicity.
The 16 ½ minutes difference in time is due to a difference in the angles from which the eclipses are seen, and is not in any way connected with distance ; and so the speculations of Ole Roemer concerning the Velocity of Light and the probable distance to the sun amount to nothing.