Astronomical motifs embellish humanity's most ancient artifacts, but until the winter of 1609 nobody depicted the lights in the sky as places we might visit. When Galileo turned his telescope toward the moon he saw its craters, valleys, plains and mountains. Squinting through his 10-power eyepiece, he sketched these features. He was not simply recording light and dark areas, rather, he was interpreting relief on a globe that had previously been assumed to be a smooth, ethereal orb. Arguably, Galileo became the first astronomical artist when he made the huge conceptual leap required to depict the moon as another world.
Once the planets were recognized as material bodies similar to the earth, people assumed they must be inhabited. The prescient Johannes Kepler had already written the first science fiction novel in 1608. Titled "Somnium" (Dream), it recounts a visit to the moon brought about by supernatural means. Mindful of the need for life to adapt to its environment (in itself a noteworthy insight), Kepler tried to imagine living conditions on that smaller, slowly-rotating world. Because of the lower gravity "everything...is monstrously large in size. Growth is very rapid." To avoid the two-week-long lunar night, the Moon's inhabitants "have no safe and secure established dwelling, but instead wander about their world in troops." Point by point, Kepler deduced a plausible scenario from the available facts. Astronomical artists do something similar when they interpret scientific data to create a painting.
Although artists created fanciful drawings of alien beings in the intervening centuries, realistic renderings of the landscapes of other worlds came surprisingly late in the 19th. James Nasmyth, a Scottish engineer and inventor, developed a passion for astronomy in his middle years and spent his nights sketching the moon. The illustrations in his 1885 book on the moon are remarkable. He created some of the first special effects shots by using a pinhole camera to photograph tabletop plaster models of lunar features. He retouched the photos to create a proper lunar environment. Because shadows in the moon's craters were inky black, he knew the lunar sky, bereft of light-scattering atmosphere, would be black also, even in daytime. The absence of air (and, consequently, weather) suggested lunar mountains might be more rugged and less eroded than their earthly counterparts. In this assumption he was partly mistaken, as the moon has indeed been eroded by billions of years of meteoritic battering. His dramatic alien peaks established a visual meme that persists even today in science fiction art.
Nasmyth's depiction of a lunar eclipse as seen from the moon's surface demonstrates well the process of composing a plausible astronomical painting. Earth, being 4 times larger than the moon, looks 4 times bigger in the lunar sky than the moon looks in ours. The distant sun looks the same as from earth. Computing the proper angular sizes requires only simple trigonometry. What is astounding about Nasmyth's painting is his amazingly accurate depiction of the ring of red light encircling the earth. This glow, from all the sunrises and sunsets refracting through earth's atmosphere, is what makes the moon turn dark red when it passes through earth's shadow. Not until the Apollo 13 astronauts returned from their near-disastrous voyage to the moon would humans see this 'diamond ring effect' around the earth. Nasmyth also depicted the glowing fan of luminosity known as the zodiacal light, which is visible only in the darkest skies. Nasmyth assumed the zodiacal light might be seen in the inky lunar sky during an eclipse.
Likely inspired by Nasmyth's work, the French astronomer-artist Lucien Rudaux (1874-1947) took astronomical illustration to the next level. A more accomplished artist better able to ignore preconceptions, Rudaux noted that lunar peaks, when seen in profile on the edge of the disk, were actually quite rounded, with gentle slopes unlike Nasmyth's plaster mountains. Rudaux's depictions of lunar landscapes were indeed the most accurate until space probes and astronauts photographed the moon. Rudaux was also one of the first to venture beyond the moon, depicting Jupiter and Saturn as they might appear from their satellites. He assiduously avoided what I've come to call "telescopism," a tendency to assume that astronomical objects, when viewed close up, will look the same as they do in a telescope, except bigger. His depiction of Jupiter from Io's surface shows the bowing of Jupiter's cloud bands toward the pole as would be observed from near the planet; in a telescope, the bands appear parallel.
Born in 1888, architect-turned-artist Chesley Bonestell is the grand master of astronomical art. His paintings have a near-photographic realism that elevated the idea of space travel from fantasy to possibility. His artwork in the 1947 book 'The Conquest of Space' inspired the scientists and engineers who made space travel a reality.
Today, many astronomical artists have traded paintbrushes for computer styluses, and 3D modeling software enables us to create images of breathtaking realism, but the challenge of interpreting data from telescopes and space probes remains the same. The faint, periodic dimming of stars hundreds of light years away appears to be evidence of a plethora of planets far beyond the sun's family of worlds. How big are they? How brightly do their suns shine in their skies? How might they differ from the planets we've explored? Space artists ponder these questions and voyage in their imaginations to seek answers.
Astronomical artists play a role similar to that of medical illustrators, in that they attempt to depict realistically aspects of nature beyond ordinary experience. The astronomical artist can show us the cataclysmic collision that formed the Moon and can transport us billions of years into the future to watch the sun swell into a red giant star. Even as space probes complete a preliminary reconnaissance of the worlds of the solar system, our imaginations can range farther, to the realm of exoplanets and black holes, to the frontier of the observable universe and to the dawn of time itself.