by Woody Sullivan and Jim Bell
Reprinted from the January/February 2004 issue of The Planetary Report. Woody Sullivan, a professor of astronomy at the University of Washington in Seattle, works on astrobiology and the history of astronomy. He intends to make Seattle the sundial capital of North America. Jim Bell is a professor of astronomy at Cornell University in Ithaca, New York and is the lead scientist for the Pancam investigation. Most of his experience with sticks and shadows has been on a baseball diamond.
A closeup of one of the flight MarsDials shows the gnomon in the center; it will cast shadows on the face.
NASA’s Mars Exploration Rover (MER) mission will soon land two highly mobile robotic scientists on the Red Planet. Named Spirit and Opportunity, the rovers will explore the geologic and climatic history of two landing sites, chosen to provide new insights into the history of water and climate on Mars. Each rover carries a suite of sophisticated science instruments known as the Athena payload, as well as high-tech navigation, mobility, and communications systems. Unbeknownst to many, each rover also carries one of the simplest but most elegant scientific instruments ever devised: a sundial. Just as with sundials on Earth, these MarsDials will use the shadow of the Sun to determine the time of day on Mars. Students and space enthusiasts around the world will be able to read the Martian time by viewing images of the MarsDials on the Web.
The MER mission and Athena science payload have a long and tortuous history, with Athena having first been proposed in the mid-1990s in response to NASA’s early calls for “better, faster, cheaper” planetary exploration initiatives. At first, Athena was proposed for a rover mission, then it was selected by NASA for a stationary lander, then (after the Mars Polar Lander failure in 1999) it was reselected in 2000 for a rover mission and finally for two MER rover missions.
Pancam and Its Calibration Target
One of the key scientific instruments on the MER Athena payload is a multispectral panoramic stereo camera system known as Pancam. Pancam is a pair of charge-coupled device (CCD) cameras mounted atop the rover’s mast, about 1.5 meters above the surface, that provide high-resolution views of the landing site. Each camera has the equivalent of 20/20 human vision (about three times better resolution than the camera on the Mars Pathfinder lander), and the mast can move to allow Pancam to view the full 360 degrees in azimuth and ±90 degrees in elevation around each rover. Each camera also uses a small eight-position filter wheel to allow imaging in specific colors of the spectrum.
Pancam has a number of important scientific objectives. These include obtaining stereo color images of the surroundings of each MER landing site to study its geology, especially the spectra and light-reflecting properties of the minerals making up nearby rocks. These images will be vital for making decisions as to which rocks should be examined “up close and personal.” Pancam will also image the Martian sky to study its ever-present dust. Finally, Pancam has several important practical roles on MER, including helping the rover’s navigation by mapping out possible routes, as well as finding the Sun (using specially designed filters) to determine rover orientation.
The Pancams are just two of the nine cameras carried by each rover. Pancam’s filter wheels give these cameras a unique role on each rover, however, as they are the only cameras that can obtain color images. Having color imaging capability presents an additional problem to scientists working with Pancam images: How do we make sure the colors are correct? The MER team has taken a two-part approach to this problem. First, we calibrated the cameras before launch to determine how each filter will respond to sunlight reflected off Martian rocks and soils. Second, because we don’t know how or if the cameras’ response will change after the turmoil of launch and landing on Mars, we carry with us a calibration “target” that has known grayscale and color properties. By imaging the target and getting its color balance correct, we will be assured that subsequent images of the landing site will have their colors properly displayed.
This second approach is very much like the approach taken during the Viking and Mars Pathfinder missions, both of which also carried calibration targets. The MER target, in fact, was originally designed to be similar to the target for the imager used in 1997 on Mars Pathfinder. That target played a critical role in getting the colors right on Pathfinder’s marvelous pictures. The Pancam target initially also consisted of a small metal plate covered by silicone rubber materials pigmented to specific colors or shades of gray. We also placed a post in the center of the plate to cast a shadow across some of the materials; this allowed us to measure the contributions of direct sunlight and of the diffuse skylight that fills in the shadow region.
During a fateful airplane flight in 1998, one of us (Jim) noticed television writer and entertainer Bill Nye (the Science Guy) and struck up a conversation about Mars missions and Pancam in particular. Bill was intrigued by the mission and yearned for more details about the instruments themselves. When he learned about Pancam and its stick-casting-a-shadow calibration target, he had an epiphany: “It’s a sundial!” Bill’s eyes lit up as he foresaw an opportunity to merge science, education, his own personal interest in sundials (Bill’s father wrote a book on the sundials of Maryland and Virginia), and space exploration into an exciting new project. We could make that mundane little object into the first sundial on another planet! Wouldn’t it be great if we could tell time on Mars by reading the post’s shadow? Bill joined our team, and thus was born the Mars sundial project. Now, more than 5 years later, the two rovers that are about to land on the Red Planet are each outfitted with a MarsDial.
For millennia, humans have appreciated that the shadow of a post (also known as a gnomon) could be used to tell time as the Sun travels westward across the sky. In ancient Greece, sundials were fashioned out of large blocks of marble, with an iron rod casting a shadow onto a hemispherical bowl having lines for the hours of the day. The date can also be read from a sundial, for the path of the Sun through the sky is much higher in summer than winter, causing generally shorter shadows in summer.
By the time of the Renaissance, an amazing variety of sundials had been designed and were in common use. Some were on walls, with the gnomon protruding from the wall, while others were aligned with the Earth’s rotational axis and Equator (equatorial dials). Pocket sundials could be carried anywhere—some had built-in magnetic compasses, while others were self-aligning to north. Some dials were based on vertical gnomons, but these required the post to have a reference point (called a nodus) for reading the specific position of the shadow. There’s even a type of dial whereby a person stands upright and reads the time as indicated by his or her own shadow falling on markers on the ground.
Sundials reached their peak in the 18th century but then rapidly declined as clocks and watches became affordable and accurate (and with the distinct advantage of being able to work under clouds or at night!). Today we have remarkably accurate timepieces (based on exquisitely machined gears and springs or fast vibrations of quartz crystals or on energy transitions in atomic nuclei), and the minutes of our lives are dictated by the clock. So why is it that sundials are currently enjoying a revival? They are no longer practical for timekeeping, of course, but they are nonetheless still marvelous devices for educational, philosophical, and artistic purposes. A sundial today, when well designed for its users and location, makes one pause from the bustle of modern life and contemplate our history and our position in the cosmos.
Turning a Calibration Target into a Sundial
Once the decision was made by the Pancam team to make a MarsDial, an informal design team was assembled that “met” via e-mail over a period of more than 6 months. On the team, in addition to the present authors and Bill Nye, were Steve Squyres (Cornell), principal investigator for the MER Athena science instruments; Jon Lomberg, a well-known artist specializing in astronomical subjects; Louis Friedman, executive director of The Planetary Society; and Tyler Nordgren, an astronomer and artist then at the U.S. Naval Observatory and now at the University of Redlands. Larry Stark, scientific instrument maker at the University of Washington, was the key person for turning the design ideas into the reality of metal and silicone rubber.
The first problem we encountered was that the dial had to be fabricated long before the landing sites on Mars were to be selected, but a sundial’s hour lines very much depend on its latitude, whether you’re on Earth or on Mars. Thus, we couldn’t engrave hour and date lines onto the dial ahead of time. Instead, we decided to take advantage of the fact that everyone would be viewing images of the MarsDial and its shadow on the Web, so we could superimpose the needed lines later, by digital methods. Second, we realized that the dial’s gnomon needed a nodus, so we put a 2-centimeter (nearly 1-inch) diameter sphere on the top and a petal-like “daisy wheel” structure lower down. The sphere acts as our nodus when the Sun is nearly overhead (as will happen midday for the near-equatorial latitudes where the rovers will land), but when the shadow of the sphere falls off the 8-centimeter-square face of the dial, then the petal will step in to act as the nodus.
In the process of turning our esoteric little (all of 65 grams) calibration target into a piece of interplanetary art, other features—sundial designers call them “furniture”—were added. For example, a motto is traditional on sundials. After much discussion, and inspired by ideas from a number of schoolchildren (compiled by Sheri Klug of the Arizona State University Mars K-12 Education Program), we adopted “Two Worlds, One Sun” as ours. The name of Mars is written in 17 languages (more if you count all the other languages besides English for which the word for Mars is spelled “Mars”) around the edge of the dial face, as well as the common era year of the landings (2004). Two mirrored surfaces on the plate are designed to reflect the color of the Martian sky. A careful look will reveal that the post is actually off-center and that not all the rings are centered on the post. This is because we decided to make the middle ring represent Earth’s orbit (hence the blue dot) and the outer ring, Mars’ (the smaller red dot). Earth’s orbit is indeed close to a circle centered on the Sun, but Mars’ orbit is much more elliptical, with the Sun noticeably “off-center.” Finally, around all four sides of the plate we engraved a short text message and drawings, again inspired by those of schoolchildren, that tell the tale of the mission. How many years will it be before a human comes upon a half-buried rover, dusts it off, and first reads these words on Mars?
One final peculiar property of the MarsDial is that it sits on a moving vehicle, a strange fate indeed for a sundial, which always needs to know how it is aligned with respect to north. When originally designed, the calibration target was going to be on a fixed lander, but changes in the mission and spacecraft design meant first that the MarsDial was delayed by 2 years, then that it was riding along on a rover! Now we must figure out the orientation of the rover (and hence the MarsDial) before we can calculate the correct hour and date lines to superimpose on the Web image—and how does NASA determine the rover’s orientation and tilt? In a sweet bit of interplanetary irony, it’s by looking at the Sun with Pancam and knowing its position in the Martian sky for any given time!
Time and Calendars on Mars
As we contemplate the implications of reading a sundial on another planet, it is only natural to wonder what should be the units of time and the calendar system that would be natural for a denizen of Mars, whether robotic or human. Think like a Martian when considering the following facts: The Martian solar day, which has had the name sol since the Viking lander days in the 1970s, is 24 (Earth) hours, 39 minutes, 35.2 seconds long, or 1.02749 Earth days. But what should be the subunits of the sol? An unimaginative answer would be to decide that 24 Martian hours (perhaps called “mhours”?) make up a sol. The peculiar division of our day into 24 units goes back at least to the ancient Egyptians and has nothing to do with Mars, so why choose 24? Is 10 mhours in a sol better? As a Martian, you’d better first count the number of fingers on your hands! And how would you break the mhour up into smaller units?
A Martian calendar is even trickier. Mars’ axis of rotation has a similar tilt to that of Earth (25.2 degrees versus 23.4 degrees), which means that it similarly goes through four seasons (which need characteristically Martian names, please). But because Mars is much farther from the Sun, its year is a good bit longer, at 668.60 sols (686.98 Earth days). What should be the name of the Martian year? And how should it be divided? Here on Terra, we have 12 months in our year basically because our Moon happens to go around us roughly 12 times every time we orbit the Sun once. But Mars has two moons, Phobos and Deimos, that zip around every 7.6 and 30.3 hours, respectively, as seen from outside (the sidereal periods). You’re a Martian standing on a surface that is itself rotating once every 24.6 hours. The upshot is that speedy Phobos actually laps you about twice a day, rising in the west and setting in the east with an apparent period of 0.45 sols, while Deimos behaves more “normally” by rising in the east every 5.38 sols. Should there be a length of time called a phobe (or would a milliphobe be more useful?), or maybe a conjie for the interval of time between conjunctions of the two moons, when they pass each other in the sky and surely create an auspicious occasion for any Martian skygazer? Whatever the answers, Mars surely deserves a set of unique and distinctly Martian units of time!
Operations on Mars
When the two Rovers land on the 4th and 25th of January, Pancam and the other rover cameras will immediately start imaging the landscapes of Gusev Crater and Meridiani Planum, which of course means that the calibration target/MarsDial also will be imaged. The Pancam team plans to image the calibration target at least once per sol, and sometimes more frequently during times when additional calibration fidelity is required. Every sol, Student Astronauts sponsored by The Planetary Society will be in charge of getting these images onto a website where the public will be able to read the time and date on Mars, as well as view much more information about the mission’s instruments, goals, and results. We also plan to initiate a related project called EarthDial, which will link up images of similar sundials constructed by people from longitudes all around the Earth, each under the gaze of its own Webcam. (See www.planetary.org/mars/earthdial.html for how you can participate in this exciting project.)
It has been challenging but exciting to turn a simple instrument needed for critical color and grayscale calibration of the rover cameras into a fun, artistic, and educational piece of space art that will, we hope, inspire people on both Earth and (eventually) Mars to think beyond their everyday experiences. More than 2,000 years ago, the Egyptian astronomer Eratosthenes used a simple gnomon—a stick stuck in the ground casting a shadow—to estimate the diameter of the Earth. Today, in our world of digital computers and atomic clocks, it is fitting that we recall the wisdom of the ancients and the simple power of sticks and shadows to reveal the beauty and harmony of the world around us, whether that world is Earth, Mars, or some other place in our imaginations.
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