For a good look at an Sb galaxy, check out our nearest spiral neighbor, M31 (NGC 224), the Andromeda Galaxy. You can detect its soft, elongated glow by following a line northwest from 2nd-magnitude Mirach (Beta [b] Andromedae) through 4th-magnitude Mu (m) And. In an 8-inch telescope, the most impressive feature may be the prominent jet-black dust lanes that skirt the galaxy’s core. But you don’t need a scope to enjoy this beauty. At 2.2 million light-years away, M31 is the most distant object visible to the naked eye.
Linger near M31 a bit and check out NGC 206, a huge cloud of young, massive stars located along the southwest periphery of the galaxy, about 40′ from the core. The combined light from these stars is faintly visible in a 6- to 8-inch scope. Users of 16-inch scopes and larger can challenge themselves by picking out the numerous 15th-magnitude globular clusters orbiting M31. They look like faint stars, so you’ll need a detailed finder chart to pinpoint them.
The brightest springtime Sb galaxy is M81 (NGC 3031) in Ursa Major. A small telescope shows it as a striking ellipse with a bright core. In a 17.5-inch scope, the view reveals a winding spiral arm on the south side of the galaxy and another arm just visible as a short extension hooking around the north end toward a 12th-magnitude star.
Just 38′ north of M81 lies M82 (NGC 3034), an eruptive starburst galaxy. In 6- to 8-inch telescopes at 100x, it draws your attention. Several dark lanes mottle the high surface spindle at an oblique angle to the major axis. Photographs reveal a tortured disk once thought to be exploding, but now explained by bursts of star formation in M82′s nucleus.
Finally, for a springtime view of an Sc spiral, visit M83 (NGC 5236) in Hydra. Long-exposure photographs and CCD images reveal massive spiral arms peppered with huge star clouds and HII regions. A moderate-sized scope under dark skies shows the bright central region’s elongated bar, which has subtle spiral arms that arc around the core forming a cosmic Greek letter Theta (q). Northern observers should not disregard this southerly galaxy, which ranks among the ten brightest in the sky.
Of all the galaxies in the night sky, spirals appear most abundant, but don’t overlook the giant ellipticals.
Sparse in numbers, giant elliptical galaxies often span great distances of sky. They can extend more than several hundred thousand light-years, considerably larger than the largest spiral. Older, reddish stars typically dominate their color, and their extreme mass often wreaks havoc on their smaller neighbors.
One of the closest giant ellipticals is NGC 5128, also known as Centaurus A. This 7th-magnitude system lies just 7[degree sign] north of the bright globular cluster Omega Centauri. The object’s most striking feature is the broad dust lane slashing through its center. This lane is actually the remnants of a spiral galaxy consumed by the massive elliptical about a billion years ago. Observers with large scopes can discern intricate structural details under moderate magnification.
Other hungry giants can be found near the core of the Virgo Cluster. Here, M84 (NGC 4374) and M86 (NGC 4406) form a pair of 10th-magnitude “eyes” in a galactic “smiley face.” The nearby edge-on spirals, NGC 4402 and NGC 4388, show evidence that they have lost some of their gas and dust to their massive neighbors. You can view both the big ellipticals and the smaller spirals in the same low-power field.
Much more spectacular is M87 (NGC 4486), an angry giant that has consumed a number of its brethren. Even the smallest instruments will resolve the galaxy’s bright core and slightly oval halo. For a challenge, use a moderate-sized scope with a CCD camera to see the optical jet discovered by H. D. Curtis in 1918. Look for the low- contrast, narrow wisp to the east-northeast of the core.
Even M87 pales in comparison with the largest of the ellipticals, the cD galaxy. Named for their immense, extended halos, these reign as the undisputed lords of the realm.
Perhaps the most famous is NGC 6166, a spectacular giant located in the constellation Hercules. It dominates the galaxy cluster Abell 2199 and has at least four nuclei, three of which are remnants of other galaxies. An 8- or 10-inch scope will reveal an oval, diffuse glow with a mottled central region, even though it lies almost 600 million light- years away. Larger scopes may reveal some of the nearly stellar nuclei plus a faint swarm of attendant galaxies.
Giant ellipticals are not the only ones producing catastrophe in the universe. Collisions often occur between spiral galaxies. The gravitational tug, called a tidal interaction, between spirals often results in some of the most fantastic phenomena in the cosmos. Well- known colliding galaxies include the Whirlpool Galaxy (M51), the Antennae, and the Mice. Other galaxies, such as the Cartwheel and the Helix, are famous remnants long after the merger has taken place.
Begin at two galaxies on the verge of colliding: NGC 4490, the Cocoon Galaxy, and NGC 4485 in Canes Venatici. Their cores lie a mere 4′ apart, a distance of only 30,000 light-years. An 8-inch scope shows the unusual shape of the tidally distorted tip of NGC 4490, the larger of the two galaxies. The smaller NGC 4485 has an irregular, oval glow with a brighter core. Large backyard scopes will show an intricate play of dusty motes and bright knots in the more massive spiral.
The spectacular Antennae, or NGC 4038 and NGC 4039 in Corvus, make up one of the most unusual sights in the night sky. Here, a pair of spirals dance in the earliest stages of merger. Tidal interactions have produced two curved tails spanning 20′, more than 550,000 light-years long. The resulting mass looks like a warped shrimp with faint condensations occurring in two regions. Recent images with the HST show these areas as hotbeds of star formation.
Observing with an 8- or 10-inch scope will resolve the shrimp, but amateurs equipped with 16 to 20 inchers should see the brightest HII regions and hints of the tidal tails. And better yet, some Dobsonian users have reported seeing the full extent of the galactic creature’s “antennae.”
The last two objects are examples of merged systems. The first is NGC 6240 in Ophiuchus, which, in HST images, looks like a lobster complete with claws. It’s one of the largest known spiral galaxies. Widespread star formation spans the entire disk of the galaxy, which is visible in a medium-sized instrument as an oval, mottled disk and core. Sixteen- inch scopes or larger should reveal some hints of the claws.
Far to the north lies NGC 2685, the Helix Galaxy in Ursa Major, a fine example of a polar ring galaxy. Here, the collision and subsequent merger have taken place between 2 and 5 billion years ago, leaving a belt of gas and dust strewn at right angles to the long axis of the galaxy. Moderate scopes should resolve the the 11th-magnitude spindle shape of the main galaxy. The delicate polar ring is probably not visible in any telescope but would make a nifty challenge for the CCD- equipped amateur.
While scanning the springtime sky for galaxies, pause for a moment in the direction of the Milky Way’s plane. Here, dust obscures and reddens objects, reducing visibility. Edwin Hubble called this band the zone of avoidance (ZOA). In recent years, however, infrared, radio, and x-ray studies have penetrated the ZOA. And scientists continue to learn more about the galaxies hidden behind the Milky Way’s veil.
Research has revealed, for example, that IC 10, an irregular dwarf member of the local group, has recently undergone a period of vigorous star formation, qualifying it as a classic “starburst” galaxy. Located just 3.3[degree sign] from the galactic plane in Cassiopeia, IC 10 is the closest starburst galaxy to the Milky Way. An 8-inch scope will reveal a hazy spot within a rich star field. In a 13-inch scope, the low surface brightness glow extends to 4′ in diameter, elongated northwest- southeast. The appearance is unusual, with no visible core and a superimposed 13th-magnitude star.
In 1967, Italian astronomer Paulo Maffei accidentally discovered a heavily obscured galaxy on a near infrared photograph of IC 1805, a huge star-formation complex in Cassiopeia. At a distance of only 12 million light-years, Maffei I had escaped previous detection – its luminosity reduced five magnitudes by intervening dust. If not situated toward the ZOA, this giant elliptical would be one of the showpiece galaxies in the sky. Ferret out Maffei I from the surrounding Milky Way star field using a 13-inch or larger scope. Maffei I’s core appears as a low-surface-brightness glow elongated east-west and sometimes mistaken for a small patch of galactic nebulosity.
If you enjoy observing galaxies, why not view them in bunches? Crammed into a mere 4′ is Stephan’s Quintet, a favorite deep-sky target located 30′ south-southwest of the bright galaxy NGC 7331. A 6- or 8-inch scope can resolve a couple of dim patches, but a clear view of the entire quintet will probably require a 12-inch scope and dark skies. The brightest member, NGC 7320, appears as a small, faint oval. Less than 2′ northwest lies NGC 7318, a pair of intertwined spirals containing two stellar nuclei within a common halo.
Backyard telescopes have access to two well-studied clusters: the Coma Cluster (Abell 1656) and the Hercules Cluster (Abell 2151). The Coma Cluster is a rich, concentrated cloud of hundreds of galaxies about 350 million light-years away. A pair of supergiant cD galaxies, NGC 4874 and NGC 4889, dominate the cluster’s core, attended by a retinue of dwarf ellipticals and lenticulars. Over time, a hot intracluster medium glowing with x rays has stripped the spirals of their gas. A few are visible in 6- to 8-inch scopes. But the Coma Cluster really comes into its own with a large scope. In a 16 inch, the field centered on NGC 4874 and NGC 4889 looks like a swarm of bees hovering around two hives with up to two dozen galaxies visible in a 20′ circle.
Of the two presiding giants, NGC 4889 is both slightly brighter and larger. In a 13-inch scope, the galaxy appears as a moderately bright oval oriented east-west with several diminutive companions huddled within a few arcminutes. NGC 4874 lies just 6′ south of a 7th-magnitude star. The roundish, 1.5′ glow is only weakly concentrated, but more impressive is the compact halo of many tiny, dim galaxies. Careful viewing can net at least 100 galaxies spread out over several fields in a 16-inch scope.
Hercules Galaxy Cluster
In contrast, the Hercules Galaxy Cluster is a remarkably diverse collection of spirals, ellipticals, irregulars, and distorted interacting pairs. This sprawling cluster lacks the high degree of organization and central concentration found in the Coma Cluster but maintains a high percentage of gas-rich spirals. At a distance of roughly 550 million light-years, the Hercules Cluster is a challenging visual target even in 12- to 16-inch scopes, so wait for a night of exceptional transparency and plan your attack carefully.
Our jumping-off point is the 7th-magnitude star SAO 101900. Just 10′ east-southeast you’ll find the interacting system, NGC 6040. The companion may require a 16-inch scope and is found dangling off the south edge. Nearby is a similar double system, NGC 6041 and NGC 6042.
Just 8′ away from NGC 6041, lies another dim trio with NGC 6045 as its centerpiece. This distorted edge-on spiral has NGC 6043 and NGC 6047 as its companions. The intertwined pair of spirals, NGC 6050 and IC 1179, swirl nearby, followed by a string of three IC galaxies oriented north- south. A keen observer with a 16-inch scope may glimpse up to three dozen members within 30′ of the central region.
Now that wasn’t so bad, was it? And this was just the edge of the celestial forest. A thicket of springtime galaxies awaits your discovery. All you have to do is walk in.
In striking contrast to the vague shadings sometimes reported by visual observers are the prominent markings clearly seen on ultraviolet photographs of Venus. They were discovered at Mount Wilson Observatory by Frank Ross, a pioneer in the photography of the planets through monochromatic filters. During a favorable eastern elongation of Venus in June and July of 1927, he obtained a series of photographs of the planet through the 60-inch and 100-inch reflectors in six regions of the visible spectrum and in infrared and ultraviolet light.
Now that’s a handsome planet!
Ross expected infrared to offer the greatest promise; it was already routinely used in aerial photography because of its ability to penetrate haze and give the clearest views of the Earth’s surface from aircraft at high altitudes. But, to Ross’s surprise, his infrared images of Venus proved to be every bit as bland as those taken in visible light.
However, the ultraviolet images showed distinct dark streaks and bands roughly perpendicular to the terminator, presumably owing to the presence of UV-absorbing materials in the planet’s upper cloud deck. These features also appeared on photographs taken in deep violet light (3800-4000 angstroms, at the threshold of perceptibility to the normal eye), but they showed much higher contrast on the ultraviolet plates (3400-3800 angstroms). On these they displayed about the same level of contrast as the dappled “seas” of the Moon show to the naked eye.
Ross, however, was unable to discern an obvious rotation period from his photographs. He ventured a very tentative estimate of about 30 days. His work was not immediately followed up, quite possibly because the notorious intractability of the problem made the subject unappealing.
Thirty years would pass before Ross’s discovery would be exploited – not by a professional astronomer but by a French amateur named Charles Boyer. Born at Toulouse in 1911, he experimented with wireless at an early age. This became the basis of his lifelong friendship with fellow ham radio enthusiast Henri Camichel, a professional astronomer at Pic du Midi Observatory in the French Pyrenees. Shortly before the Second World War, Camichel imparted to Boyer an interest in astronomy.
In France the gulf separating professional and amateur astronomers has not been as wide as in the United States. A number of great French planetary astronomers began their careers as amateurs, some never forsaking their amateur status. The great Eugene M. Antoniadi, for instance, always referred to himself simply as an “astronom voluntaire de l’Observatoire Meudon.” It was quite natural, therefore, that Camichel and Boyer continued their association by mail after the war, when Boyer embarked on a career in colonial Africa – first as Chief Magistrate at Dahomey (now Cotonu) and finally as President of the Bench at Brazzaville in the Congo, a position he held from 1955 until his retirement in 1963.
At Brazzaville, located only 4[degree sign] south of the equator, the planets were favorably placed for observation high in the sky, and the humid atmosphere was often exquisitely steady. Realizing his opportunity, Boyer constructed a 10-inch Newtonian reflector around a primary mirror made by renowned optician Jean Texereau. The resulting instrument was optically superb, but it rode atop a rather rudimentary altazimuth mounting. Boyer asked Camichel to suggest observing projects. It happened that Camichel was photographing Venus in the ultraviolet at the time, and he proposed that his friend also attempt to do so.
The advent of computer-controlled dual-axis drives for altazimuth mountings lay decades in the future, so Boyer’s telescope was ill suited to photographing the planets – a process requiring very steady tracking during exposures several seconds long. But the resourceful amateur cobbled together an ingenious device for moving his camera across the focal plane of his telescope at the correct speed using parts from a Meccano erector set.
In August and September of 1957, when the air was unusually dry at Brazzaville, he began to photograph Venus, which was then at evening elongation. He used Kodak Micro-File film, a high-contrast, relatively fine-grained emulsion that is painfully slow by today’s standards. Lacking a proper ultraviolet filter that blocked visible light, he made do with a blue-violet Wratten 34 filter that transmitted wavelengths shorter than 4500 angstroms.
Although the images in Boyer’s photographs were extremely small and aesthetically unappealing, he soon thought he could detect the return of the same dusky spot to the terminator at intervals of about four days. This feature made five returns between August 28th and September 16th. Alerted to the suspected four-day rotation period by Boyer, Camichel examined his own set of images. He, too, found evidence for a four-day period.
Boyer continued his observing campaign from Brazzaville until 1960. By this time he and Camichel had come to regard the four-day rotation of the upper atmosphere of Venus as “completely uncontestable.”
Boyer had taken the precaution of depositing a sealed envelope describing his discovery with the French Academy of Sciences in 1957. Not everyone was able to see the pattern in his tiny images, however. The eminent French planetary observer Audouin Dollfus recounted in 1992: “I examined the images carefully. They did not seem to me completely convincing at the time.” But with Camichel’s unflagging support, Boyer persisted. The four-day rotation became his idee fixe – in fact, he did no other astronomical work of importance before his death in 1989. His first published article, coauthored by Camichel, appeared in the popular magazine L’Astronomie in 1960, followed by papers in the prestigious journals Annales d’Astrophysique and the Comptes Rendus de l’Academie des Sciences. They failed to attract much attention or to elicit many comments.
Indeed, recognition of Boyer’s discovery was agonizingly slow. Even today, the details of the improbable story are little known outside of France. (A recent book in English makes the following bald statement about the prespacecraft era: “Strangely enough, the best results of all at that period came from visual work by the French observers, who recorded a characteristic Y-shaped dark feature centered on the equator.” This is, of course, untrue; they did not use visual means.)
In the meantime, radio astronomers in the United States and the Soviet Union bounced radar impulses off Venus’s solid surface in 1962. These revealed the very slow, 243-day retrograde (backward) rotation of the planet’s solid body. This finding seemed utterly incompatible with the four-day rotation rate of the planet’s upper atmosphere. How could the Venusian cloudtops rotate 60 times faster than the underlying surface? Shortly after the announcement of the radar results, Boyer and Camichel submitted a paper on the four-day rotation to Icarus, the leading international journal of planetary science. One of the journal’s referees, a young Harvard astronomer named Carl Sagan, rejected it on the grounds that “the four-day rotation is theoretically impossible, and shows how foolish the work of the inexperienced amateur can be.”
Following his retirement in 1963 Boyer himself frequently worked beside his professional colleagues at Pic du Midi. They were able to follow the motions of the UV markings continuously for periods of up to six hours. Among these were the now-famous Y- and y-shaped markings.
In 1964 yet another French astronomer, Bernard Guinot, employed a sensitive technique known as interference spectroscopy to measure Doppler shifts very precisely. From these he determined the radial velocities of various points on the limb of Venus. His data also suggested that the cloud canopy circulates every 4.3 days.
But final, irrefutable proof of Boyer’s discovery did not come until 1974. The Mariner 10 spacecraft, encountering Venus in February of that year, imaged the planet in the ultraviolet during its approach. When these images were combined into a movie sequence, the four-day retrograde rotation of the upper atmosphere was dramatically confirmed. Dollfus recalls that when he showed Boyer a copy of this film, Boyer reacted with la belle indifference; it contained no surprise, since he already knew the result.
So how did Boyer manage to solve one of planetary astronomy’s oldest and most enduring mysteries – a mystery that had defeated the best efforts of generations of astronomers, dating back to the time of the elder Cassini at Paris Observatory in the reign of Louis XIV?
Boyer was very clear in his purpose and methodical in his observing program, and he used an excellent telescope in a favorable climate. But even this does not completely explain his remarkable achievement, for three decades earlier Ross had employed far more powerful instruments that recorded far more detail. Here, ironically, even Boyer’s apparent deficits worked to his advantage. As Dollfus later explained: “Lack of resolution in this case helped, by making the true picture of what was happening clearer. On images of Venus taken with larger instruments, such as those by Ross and our own at Pic du Midi, there were simply too many details; the sheer plethora of markings confused matters.” In the end, Boyer the magistrate made exceptionally judicious use of the meager facts at his disposal.
When Dollfus was asked if he thought Boyer had been justified in divining the four-day rotation period from his 1950s images or if Boyer had merely been lucky, he hesitated a few moments before responding: “Difficult to reply – in between. As an amateur, he had more freedom, was not tied to the same high standards of rigor that he would have been as a professional.”
Three centuries of intense study had produced a legacy pathetically barren of results. The field had been all but abandoned when a persistent and single-minded amateur made one of the last fundamental discoveries in ground-based planetary astronomy. One of Boyer’s dearest friends, the renowned astrophotographer Jean Dragesco, summed it up: “This case is unique in the history of planetology.”
La Mitad del Mundo is a beauty.
Nowadays, a lot of tourists head for the more ballyhooed Equatorial Monument at La Mitad del Mundo outside Quito, where you’ll find a tall, massive obelisk topped by a huge globe. Here, at the heart of a tiny town square lined by whitewashed buildings, you can journey from one hemisphere to the other without the need of a travel agent. But there is a lesser-known monument – a nearby solar museum – that should be of interest to amateurs, archaeoastronomers, and anyone else who might enjoy a day of Sun worship.
The Museo Cientifico Solar (Scientific Solar Museum) is located in the village of San Antonio de Pichincha, 14 miles north of Quito and about 3 miles east of the Equatorial Monument. The small stone building sits on a roughly 1-acre plot among a proliferation of plant life. The unassuming museum (more like a brick-red crypt) has a peculiar T shape, with the long axis aligned with the sunrise-sunset points. Luciano Andrade Marin, the curator of the museum until his death in 1970, conceived and built it in 1950. It is a simple structure that speaks of the serene sensitivity and love Andrade Marin had for solar science and lore. This remarkable Ecuadorian geographer, university professor, and naturalist, who was featured on the cover of Sky & Telescope in October 1961, used the museum as his weekend sanctuary. Here he would entertain visitors with maps, models, and photographs and explain the motions of the Sun, seasonal changes, relations between the celestial and terrestrial equators, and the Sun lore of ancient peoples.
Aside from being a botanical wonder, the museum’s gardens, which contain plants from many parts of the world, demonstrate the seasonal motion of the Sun north and south of the equator quite effectively.
When the Sun is north of the equator, plants on the shaded south side of the building lie dormant for six months while those on the north side burst with blossoms, and vice versa. Visitors with any basic astronomical knowledge cannot be confused as to which side is the Northern or Southern Hemisphere, because on the north arm of the T is a stucco representation of a polar bear with the seven stars of the Big Dipper, over which is the inscription “Chincha” (“North” in the native Quichua language). And on the southern arm are a penguin and the four major stars of the Southern Cross, with “Colla” (“South”). The equator runs right through the center of the building.
If you follow a short stone path on the east side of the museum, you come to an elegant antique brass solar chronometer set on a pedestal. The chronometer was made in 1865 by A. Molteni, an Italian inventor.
Although three were built, it is the only one left in the world. Sunlight entering a tiny lens projects a pinpoint of light on a brass tongue on which an analemma is finely engraved. A careful study of the position of the point of light reveals the month, day, and time with great precision. Interestingly, the analemma’s figure-8 shape mimics the Quitu symbol for the Sun, which predates Incan astronomy by hundreds of years.
The museum’s entrance is on the west side of the T. Inside, the restricted and spartan quarters immediately create a monastic mood – a subtle insight into the nature of the museum’s creator. At the end of each arm is a small, round window peering out toward each pole. In the center of the room a narrow glass roof can be seen running along the length of the building. Hanging underneath this skylight is an old Earth globe lying in the horizontal position. Throughout the day of an equinox, the shadow cast by the globe is centered along the white equatorial stripe painted on the floor. The display effectively demonstrates, from a perspective in space, how the Sun shines down on Ecuador and the rest of the world. Other exhibits offer convincing evidence, by way of legends and artifacts, concerning the Quitus’ knowledge of the equator. There are also simple teaching aids, such as a scale model made from wire and bottle caps, that illustrate the relative positions of the Earth and Sun at the solstices and equinoxes.
Almost everything in the museum is still in its original condition, “including Dr. Andrade Marin’s spirit,” says the new curator, Oswaldo Munoz. In 1967 Munoz was working his way through school as a tour guide when he met Andrade Marin, who later became one of his most important mentors. After Marin’s death the museum would have been closed down if it hadn’t been for Munoz. His interest and perseverance in continuing to spread Andrade Marin’s teachings gave the museum a new lease on life. It boiled down to Munoz’s philosophy, “You love what you know and you protect what you love.”
A lean, bearded man with a neat, erudite look, Munoz directs the Ecuadorian Ecotourism Association. His eco-tour company, Nuevo Mundo, is a leading travel agency in the country. Like his predecessor, Munoz volunteers his time spreading knowledge and increasing interest in the natural sciences among the people of Ecuador and the tourists who visit the equator. Born in Quito in 1949, Munoz is a man of many talents. He is a teacher, consultant, and freelance writer and holds a degree in agronomy from Universidad Central del Ecuador.
“My interest,” he continues, “is not precisely in astronomy, but in what I would call ‘astronomical geography.’ When I was 111/42 my family migrated to the United States, settling in New York City. The climatic difference between Quito and New York captured my attention, especially when my American classmates were asking me if people in Ecuador were roasting to death as the country straddled the equator.”
Between 1951 and 1967 Munoz traveled a half dozen times between New York and Quito before finally returning to his native country for good. During this period he became aware of the seasonal variations at temperate latitudes and the lack of them at the equator. “A seasonless environment such as Ecuador,” he explains, “doesn’t allow Ecuadorians the advantage of keeping track of time as ‘four-season’ countries do. Here we have ringless trees and approximately 12 hours of day and 12 hours of night all year round. However, Ecuador is still pretty much Shangri-la in comparison to countries like the continental United States or most of Europe that have to cope with extreme heat and cold.”
So Munoz considers himself an “Earth native” who is privileged to live in a unique geographical setting. He feels it is important, then, to make more Ecuadorians aware of the advantages most take for granted. “People like Juan de Velasco, Alexander von Humboldt, Antonio de Ulloa, Jorge Juan, Pedro Vicente Maldonado, Luciano Andrade Marin, and other great geographers, naturalists, and humanists must be known by all Ecuadorians, as it is in their discoveries and contributions that part of our heritage lies.”
With some regret, Munoz points out that there is just one astronomy club on the verge of forming in Quito, one that will be affiliated with the country’s Natural History Museum. “Funny enough,” he says, “with the amount of UFO sightings in Quito and other parts of the country, more people are now stargazing. Astronomy is slowly making a comeback, though on shaky ground.”
But Munoz is optimistic about the future. Over the years there has been an increased interest in Ecuadorian heritage, he says, and he is confident that one day the solar museum and its creator will be given the credit and importance they deserve.
John Caldwell of York University in Toronto, Canada, got the ball rolling. He planned to use Hubble for taking spectra of Saturn’s moon Titan as well as the Galilean satellites of Jupiter. To better understand the spectra of sunlight reflected off these objects, however, he needed to test Hubble’s capabilities on another target. The sun was definitely out as a test subject; it shines far too brightly to have Hubble point anywhere near it. So Caldwell chose an object closer to home – the moon.
It’s safe to say that if an outside observer had suggested viewing the moon with Hubble, the proposal wouldn’t have stood much of a chance. But Caldwell had a built-in advantage: He belongs to that rare breed known as Guaranteed Time Observers. As the name implies, these people are assured access to the orbiting telescope to observe virtually whatever they want. It’s then up to the technicians at STScI to devise a way to accomplish the observing program, if it’s at all possible.
When Caldwell announced his plan in 1997, Tony Roman and Andy Lubenow of STScI got to work and figured out a way to track the moon as it moves across the sky. Hubble typically keeps an object centered in the field of view by using its Fine Guidance Sensors, which lock onto fixed guide stars and tell the telescope exactly where it is pointing. It’s reasonably straightforward to track objects outside the solar system because Hubble’s motion around Earth is small enough that they remain stationary relative to the stars.
It becomes trickier inside the solar system because planets, moons, asteroids, and comets are much closer and the spacecraft’s orbital motion requires an onboard correction. This can be done when the telescope’s Fine Guidance Sensors and gyros work in concert and the object’s changing position is known precisely. Tracking the fast-moving moon, however, raised this challenge to a whole new level because the onboard correction would overflow Hubble’s computer.
Images like this are possible with the Hubble telescope.
Caldwell chose as his target Mare Imbrium, the huge impact basin that spans some 780 miles (1,250 kilometers) and dominates the northwestern quadrant of the lunar nearside. When Storrs heard that the Space Telescope Imaging Spectrograph would be observing Imbrium, he suggested imaging the moon’s surface with the Wide-Field/Planetary Camera 2 at the same time. Observing with filters that let only a small fraction of the moon’s light pass through and keeping exposure times to less than a second, the team managed to diminish the moon’s glare enough to get the pictures shown here.
The team targeted an area near the southern margin of Mare Imbrium that stretches from the prominent crater Copernicus west to the smaller crater Kepler. Eight images were taken on November 6, 1998, just two days past full phase, and combined to produce the mosaic shown on the opposite page. (Hubble would have to take 130 images to create a mosaic of the entire lunar disk.) The image shows little topographic relief because the sun lay almost directly overhead at the time, so shadows were just about nonexistent. Still, the detail is extraordinary. Theoretically, Hubble can deliver images with a resolution of 280 feet (85 meters) – slightly better than that of the Clementine spacecraft, which orbited the moon in 1994. In practice, however, the ground controllers weren’t able to correct precisely for the moon’s changing position, so the resolution wasn’t quite that sharp.
The giant crater Copernicus spans 58 miles (93 km) and shows a bright rim and deep, terraced walls. This relatively young crater formed when an asteroid more than a mile wide slammed into the moon some 800 million years ago. A group of central peaks rises nearly 4,000 feet (1,200 m) above the crater’s flat floor, which digs down 12,300 feet (3,760 m) below the level of the surrounding terrain. Another relatively recent impact created the 20-mile-wide (32 km) crater Kepler, seen at the very bottom of the mosaic. The impacts that created Copernicus and Kepler blasted out tons of material, forming a pair of bright ray systems that adds to the craters’ prominence around the time of full moon.
The imaging team constructed a mineralogical map (below, left) of part of this region from exposures taken through three different filters. The map is centered near the crater Milichius, which lies about 185 miles (300 km) west of Copernicus and 155 miles (250 km) east-northeast of Kepler. (You can find the crater some 45 percent of the way along, and slightly above, a line from Kepler to Copernicus.) Milichius is much smaller than Copernicus and Kepler, only 8 miles (13 km) across, and significantly older. The bright ray systems of the younger craters interact here.
The yellow color in the mineralogical map represents aged ejecta from the crater as well as the surrounding mare material. The bright blue of the crater itself and its neighbor to the west, Milichius A, depicts fresher deposits exposed when landslides carried material down the walls of the craters. The centers of both craters remain yellow because the fresher material could not slide that far.