“Where we know nothing we may speculate without fear of contradiction.” With these words, written in Scientific American in 1909, English ­astronomer F. W. Henkel, a Fellow of the Royal Astronomical Society, described without apparent embarrassment much of the culture of astronomy a century ago. It was an era when experts used very limited data to make all-encompassing claims about the formation and evolution of the solar system, the existence of a planet called Vulcan and the ­presence of life on other planets.

Captivating, but ultimately incorrect, ideas about our solar system are evident in many of the early Scientific American articles gathered in this special issue. More recent articles, we would like to think, present more robust claims, because they are based on something rather than nothing. Together this collection illustrates, wonderfully, how the science of astronomy has evolved over the past 150 years.

Today astronomers are learning to drink from a fire hose. The sheer volume of bits collected by modern telescopes floods our computers and overwhelms our ability to store and analyze the information. We have entered a golden age of astronomical information, and we are extremely confident about what we claim to know. The past, however, holds some lessons in humility.

A century ago our data-challenged predecessors made up for their deficit with guesswork disguised as sophisticated argument. In these pages in 1879 an unidentified astronomer wrote, “During the calmest night, it is almost impossible to succeed in making a passable drawing of a planet like Mars, the image seen in the reflector being wavy, tremulous, and confused.” A drawing! Today astronomers use giant segmented-mirror telescopes that continually sag under the force of gravity and are torqued back into shape several times a second by hundreds of computer-­controlled motors. Meanwhile other computers send laser beams skyward, where they reflect off the sodium layer in the stratosphere to create “guide stars” that enable modern telescopes to measure—and correct—image distortions caused by turbulence in Earth’s atmosphere.

When I try to tease one more discovery out of a data set full of ­discoveries not yet made, I am filled with enormous respect and ­ad­miration, even awe, for those who came before me, whose impressive discoveries were extremely hard earned, even if the conclusions they drew from their data were often dead wrong. The astronomers we meet in these pages were imaginative and extremely confident. Take, for example, Princeton ­University’s Charles A. Young, one of the foremost astronomers of his era; his prize pupil was Henry Norris Russell, one of the most important ­astronomers of all time. “Every now and then the papers announce the discovery of a new planet,” Young wrote in Scientific ­American in 1877. (Today we call these objects asteroids rather than planets, but that’s another story.) He then informs his readers: “At present the number of these bodies known is 172; the whole number is probably to be reckoned by thousands.” Thousands!

In 1928 none other than Russell, by then the dean of American astronomers, wrote his own article on asteroids. How much had scientists learned in the intervening half-century? “If a planet is defined as ­astronomers are wont to do, merely as a body pursuing an independent orbit about the Sun, the discovery of one more or of a dozen is hardly news at all,” Russell wrote. “More than a thousand of these little bodies are already listed … and it is probable that another thousand or more will yet be added before the tale comes to an end.” Today the International ­Astronomical Union’s Minor Planet Center catalogue lists more than one million asteroids, and astronomers add more than 50,000 new objects to this database every year. Unconstrained by data, both Young and Russell were guessing. Both were very wrong.

The articles in the pages that follow reveal other examples of ­misplaced certitude in the late 19th and early 20th centuries. The author of an unbylined 1879 article, “Another World Inhabited Like Our Own,” wrote that vegetation on Mars was responsible for the planet’s red tint and recommended making observations of Mars when its inhabitants “are enjoying fine weather.”

By the early 20th century the presence of life on Mars was less obvious. Expectations that life was abundant throughout the solar system remained high, but as a 1905 article entitled “Life on Other Worlds” reported, “We know of no other world suited for life outside the solar system.… our system appears to be absolutely unique in the known ­creation.” Today astronomers are on the verge of discovering Earth-size planets with Earth-like temperatures around sunlike stars. So much for absolute uniqueness.

Four years later Henkel observed, “Nothing seems to prevent the existence of totally different beings on every one of the planets.” Even the moons of Jupiter and Saturn should be populated, he reasoned: “There is no reason whatever, so far as we know, why some of their satellites, at least, should not be the abode of living beings.” As for Venus, Henkel wrote that it “turns round once on its own axis” every 23 hours and 21 minutes. Wrong. “Air, water, lands, continents, mountains, polar snows, etc., all seem to be present.” Wrong. “Thus, so far as our limited knowledge extends, the evidence for the existence of living beings [on Venus], of a character not so very dissimilar from those with which we are familiar, seems as complete as we can reasonably expect.” Wrong again.

What about Mars? Mars shows “green and purple patches.” Sorry, no. The atmosphere is “laden with clouds and mists,” and the surface is ­covered by “numerous narrow ‘seas.’ ” No. “Though some enthusiastic observers are convinced of the existence of rational beings, in an advanced state of civilization, inhabiting Mars, we may well pause before we arrive at this conclusion.” We should applaud Henkel’s final note of caution.

Another article, “The Red God of the Sky,” reveals that by 1909 progress had been made: an observing project under way at the 14,501-foot summit of Mount Whitney, Calif., revealed that “Mars has no more water about it than the moon … the polar areas cannot be ice, snow or hoarfrost; the most reasonable suggestion is that they are made of ­solidified carbon dioxide.” Finally, with good data in hand, astronomers concluded that Mars was an arid wasteland, with a very tenuous ­atmosphere composed of carbon dioxide gas lingering over “a dead world.” A full century ago the technologies of the new century were beginning to inhibit astronomers’ habit of speculating without fear of contradiction.

The progress made by the world’s community of astronomers as documented in these pages is reassuring. Science is self-correcting; our successors will toss our mistakes into the garbage pail we call history. Is the expanding universe accelerating because of dark energy? Maybe. Is 80 percent of the mass in the universe cold dark matter? Maybe. Does ­microscopic life exist underneath a rock near the Martian equator? Maybe. Do other universes exist in a 10-dimensional multiuniverse? Maybe.

We have petabytes of data, but we almost certainly are data poor in comparison with what future generations will have. We speculate with only a little fear of contradiction. The boundary between science and guessing is still blurry. If we’re honest with ourselves, we recognize that we know less than we claim to know.

This special issue of Scientific American opens a window into our scientific past, but it offers us much more than that. These articles reveal something exceedingly important about the scientific enterprise: as with our evolving solar system, knowledge itself changes over time. A backward glance provides a healthy reminder about how science works when it is done right.