Galileo originally intended to map the constellation of Orion, but he soon decided that it was far too complex a task, due to the vast number of stars he could see in the constellation through his telescope. He chose instead to include in Siderius Nuncius a map of the portion of Orion around the sword, in addition to a map of the Pleiades.
Galileo's map of a portion of Orion, showing about 80 stars in addition to the ones which make up Orion's belt and sword. The tiny blue dot inside the red circle represents the field of view (15 arc minutes) of Galileo's telescope. The tiny field of view shows the enormous amount of difficulty Galileo must have had in mapping large regions of the sky.
A modern map of the entire constellation of Orion. The yellow dot within the blue circle shows the field of view.
Looking at Galileo's map of a portion of Orion (above) it can be clearly seen that he has not drawn the Orion nebula, the "fuzzy" star in the middle of Orion's sword which measures a full degree in extent.
A photograph of the Orion nebula. The black circle shows the field of view.
Why did Galileo not include the nebula in his map? Many historians have offered theories to explain this unusual oversight in a skilled astronomical observer. Galileo may have failed to observe the nebula altogether because it has changed in appearance since his lifetime, a theory espoused by Thomas G. Harrison. He may, as suggested by Albert Van Helden, have seen the Orion nebula but chosen not to publish it in his map of Orion, assuming that, like the milky way, the nebula could be resolved into a collection of individual stars given a telescope with sufficient magnification. Tom Williams proposes that Galileo failed to observe the Orion nebula because the field of view of his telescope (15 arc minutes) was so small that he would have been unable to detect the subtle changes in brightness which make up the nebula. We hope to provide some insight into these possibilities through observations of the Orion nebula region using Galilean-type telescopes.
The following picture is a sketch of the sword region of the constellation Orion. The dots represent stars, with the size of the dot being a rough representation of the magnitude of the star. The circles overlayed on the star map represent the field of view of the 10x Galilean telescope used to make the observation. The group member who made the drawing was unable to see the Orion nebula, which should appear on top of the two adjacent stars between the two empty circles in the third field of view from the top.
Using a twenty-powered telescope with a large field of view (>60 arcminutes) in the relative darkness of Big Bend National Park, it was not difficult to see the Orion nebula. (See sketch below.)
Although we have attempted to do so, no group member has as yet been able to see the Orion nebula either within or outside of Houston using the ten-powered Galilean telescope. However, Tom Williams claims that, outside of Houston, he was able to detect a slight increase in brightness in the field of view when he oriented the telescope on the region where he knew the nebula to be.
The data so far seem to support the theory that Galileo was simply unable to see the Orion nebula due to the small field of view of his telescope. There are, however, several variables that remain to be investigated before a conclusive judgement can be made on this issue. Our failure to see the Orion nebula within and near Houston could be due to any one of three possibilities: The magnification (10x) may not have been sufficient, the field of view may have been too small to permit detection of the light gradient, or light pollution from the city may have obscured the light gradient. Furthur research must be performed to determine which of these variables is most likely. The effect of magnification can be tested by using Galilean telescopes of varying magnifications. Unfortunately, the field of view of a Galilean telescope decreases as its magnification increases, so this will be a difficult problem to investigate. The effect of field of view can be tested by itself by looking for the nebula using a set of non-Galilean telescopes of the same magnification but differing fields of view (ranging from, say, 5 to 100 arcminutes). In order to best emulate Galileo's viewing conditions, the problem of light pollution should be eliminated to as great an extent as possible. This could be done by visiting an isolated observatory.