|GIOVANNI DOMENICO CASSINI|
|Genoa, Italy (1625 - 1712)|
Giovanni Domenico Cassini was an Italian mathematician, astronomer, engineer, and astrologer. Cassini was an astronomer at the Panzano Observatory, from 1648 to 1669, professor of astronomy at the University of Bologna and became, in 1671, director of the Paris Observatory. Along with Robert Hooke, Cassini is given credit for the discovery of the Great Red Spot on Jupiter (ca. 1665). Cassini was the first to observe four of Saturn's moons, which he called Sidera Lodoicea. Around 1690, Cassini was the first to observe differential rotation within Jupiter's atmosphere.
400 Years of the Telescope: A Journey of Science, Technology and Thought, traces the history of telescopic observation with simple, small lens-type telescopes (refractors) to todays technologically advanced and massive mirror-type telescopes (reflectors). This section describes the huge telescopes astronomers use today, and some of the modern techniques they employ to study our universe. We also describe advanced telescopes that study our universe in all regions of the electromagnetic spectrum that are now on the drawing board which will be put into operation in the not-too-distant future. Some might even help us find life on other planets. Further on in this section, take a look at the important roles, both past and present, that women in astronomy have played to improve our understanding of the cosmos:
NOTE: If you are not familiar with the basics of telescopes, we suggest you read our Introduction to Telescopes before you read this piece.
The larger the surface over which a telescope collects light, the fainter the objects in the sky that astronomers can see. As discussed in 400 Years of the Telescope, astronomers today are finding clever new ways to build even larger telescopes at lower cost, and to install them in observatories located at high, dry, and clear location in parts of our planet sometimes quite far from civilization. In the design of these new "monster" telescopes, scientists and engineers are pushing the envelope of what mirrors, motors, and computers are capable of.
When we think not just of the telescope itself, but of the support structure and the building that go with it, we must consider several issues. For example, telescopes and their buildings are securely attached to planet Earth (as they should be.) But this means they are operating on a moving platform. Every day, the Earth makes a complete turn on its axis. If your telescope is looking at a specific planet, star, or galaxy in the sky, the turning Earth will soon take your object out of the telescope's field of view, and you will be looking at or photographing something else. To keep the telescope pointed at the same object for any length of time, your telescope must turn in the exact opposite direction from the Earth, and must do so smoothly -- so it does not disturb a photo you might be taking for many minutes or hours.
Then when you are ready to observe a different object in the sky, you must be able to turn the telescope quickly and efficiently to the new location in the sky. All this requires a support structure and motors that increase in cost and complexity as the size of your telescope increases.
Furthermore, a modern telescope is much too delicate an instrument to leave unprotected from the weather and changes in temperature. Thus every large telescope is housed in a building, usually with a dome-shaped roof that allows a slit to open and view the right part of the sky. As the telescope turns, the dome must be able to move with it and always have the opening right above the telescope. Many times, the dome and support structure of a large telescope have cost as much or more as its mirror did.
Another key problem with large mirrors was that they became very heavy and tended to sag under their own enormous weight. This would cause them to change their shape and no longer focus all beams of light from the same star to a fine point. Such large mirrors took a long time to make the right shape to begin with and the process of making them was very expensive.
As the show explains, some novel solutions have been found to these problems. Roger Angel and his team at the University of Arizona have a new method of making light weight and less expensive mirrors by "spin-casting" them -- using the fast turning of the mirror mold while it is still molten to create the right shape. They also honey-comb the mirror material, so that it has large empty spaces within it, and is much less heavy than a solid mirror would be. Many of the world's largest telescopes have now been made by Angel's team, up to mirror size of 8.4 meters.
In addition, astronomers have discovered that they don't need to use a single mirror to get a large reflecting surface. Modern computer technology allows them to combine smaller mirror segments, working together under computer control, to provide a much larger light collecting area that can keep its overall shape with little thrusters in each corner. The two Keck telescopes atop the Mauna Kea volcanic peak in Hawaii use 36 mirror segments -- each in the shape of a precise hexagon -- working together to make a "light bucket" with a diameter of 10 meters (about 400 inches). A telescope in Texas, the Hobby-Eberly Telescope, uses 91 segments to make an 11-meter telescope.
The European Southern Observatory has built a telescope combination with the no-nonsense name of "The Very Large Telescope," which has four 8-meter wide mirrors some distance apart on the same high plateau in Chile. The four are being electronically connected to give the equivalent of a 16.4 meter reflector. Similarly, the Giant Magellan Telescope will use seven mirrors, each 8.4-meters wide (from the team at the University of Arizona) to make a combined collecting area of almost 25 meters. This too will be in Chile, whose high dry mountains provide a wonderful, if somewhat forbidding, site for astronomy. Plans for a 30 meter and a 42 meter telescope are now on the drawing board.
Computer control of telescopes is also allowing astronomers to move telescopes in more complex ways to simulate the turning of the Earth. The kinds of structures required to support the telescope when computers are able to move it at different rates in different directions turn out to be smaller and less expensive than the traditional ways telescopes were mounted in their domes. This allows cheaper for observatory buildings and for more of the money to be devoted to making the "light bucket" itself larger.
None of these telescopes are cheap, however, and funds must usually be pooled from governments, universities, private foundations, and wealthy individuals who might like to leave such a tool for exploration as their legacy. Just as our society invests in the creation of good music, the making and display of works of art, and the support of writers and poets who enrich our lives, so investment in the exploration of the universe is something that will continue to enlarge and change our view of ourselves -- much as Galileo's first tentative observations did 400 years ago.
Contributed by Andrew Fraknoi (Foothill College)
As 400 Years of the Telescope shows, the completion of the world's largest telescope in Southern California in 1918 made possible a revolution in our understanding of the large-scale structure of the universe.
At the beginning of the 20th century, astronomers were not sure whether the vast grouping of stars we call the Milky Way encompassed all of the known universe, or whether there were other "islands" of stars, beyond the confines of the Milky Way. Using the awesome light-gathering power of the 100-inch reflector on Mount Wilson, Hubble was for the first time able to demonstrate the existence of other galaxies of stars -- huge collections, many of them comparable to the Milky Way or even bigger, that lay far beyond the edges of our home galaxy.
Soon it was clear that there were such galaxies of stars in every direction, and the discovery of more and more of them awaited only the arrival of larger mirrors and better detectors of light. Having shown that other galaxies exist, astronomers could move on from simply finding them to trying to understand their properties.
Again, Edwin Hubble, an indefatigable observer, led the way. With his assistant (and later colleague) Milton Humason, Hubble began to measure the characteristics of other galaxies with remarkable precision. He (and others) found that galaxies came in a number of different shapes, that they were democratically distributed in every direction, and that they were made of the same basic chemical building blocks as our own Milky Way. These were heady times in astronomy, when we suddenly had the power to explore at distances Galileo and those that followed him had never dared to dream about.
One of the most powerful techniques astronomers have in their toolkit is the measurement of the motion of cosmic objects from the analysis of their light. Christian Doppler showed in the 1840's that when the source of light moves away from us, its colors shift slightly toward the red end of the color spectrum. Similarly, when such an object moves toward us, its colors change a bit in the direction of the blue (and violet) end of the palate of nature's colors. The amount of color change can tell us the speed with which a glowing object is moving, something we could not know otherwise.
When astronomers measure this "Doppler color shift" in the light of stars within the swirling pinwheel of the Milky Way Galaxy, we find that some stars show red-shifts (motion away), while others show blue shifts (motion towards us.) This makes a lot of sense, since the whole system of stars is turning over vast periods of time, and some stars will be moving toward us as it turns, while others will move away.
Hubble and Humason, using the power of the 100-inch telescope to spread the light of galaxies into their component colors, then spent years measuring the motion of galaxies. In this work, they built on the work of other astronomers, including Vesto Slipher, of the Lowell Observatory, who had measured such motions as early as 1912. To their utter amazement, Hubble and Humason found something completely unexpected. With the exception of the few nearest galaxies (with which we share group motion), every other galaxy showed a red shift. Not one galaxy outside our local group showed a blue shift. In other words, all the galaxies were moving away from us!
As the measurements accumulated, Hubble found that the speeds of the galaxies were not random. There was a pattern to them -- the further away from us a galaxy was, the faster it moved away. Further observations, by Hubble's team and other astronomers around the world, confirmed and broadened his initial discovery. Each galaxy for which astronomers have ever measured speed and distance is part of this pattern of behavior, no matter how far away it is located. Today, we call the pattern of galaxy motions "Hubble's Law" to honor the man who discovered it. And, since all the galaxies are moving away, we say that the entire universe must be expanding.
This seemed at first an astounding notion. Why would the galaxies be in motion in the first place? (Einstein, a decade before Hubble's work, was so frightened by the theoretical possibility of a universe in motion, he put a fudge factor into his equations to make the cosmos sit still.) And why would distance from the Milky Way, the seemingly average galaxy in which we happened to live, determine the speed of a galaxy so far away that light takes millions of years to reach us?
But the expanding universe idea works no matter who is asking about the speed of galaxies. If you were to relocate your perspective to a welcoming planet in another galaxy, and measure the speed of all your cosmic neighbors, you would also find that every galaxy is also moving away from you and that Hubble's Law applies for your home galaxy just as did for the Milky Way. Every galaxy in the expanding universe moves away from every other galaxy -- it is space itself which is stretching.
As Professor Alex Filippenko explains in 400 Years of the Telescope, you might image a little part of the universe of galaxies as a very stretchy rubber band, with galaxies attached to it at various points. Pretend you live on one of the galaxies and start stretching the rubber band evenly. You will see all the galaxies move away from you as the band itself stretches. And if each part of the rubber band stretches, then if two galaxies have more rubber between them, they will move apart faster. If you have twice as much rubber stretching, you will move apart twice as fast, for example. The same is true for space in the universe. If space stretches evenly, then galaxies separated by more space will stretch apart faster, explaining Hubble's Law.
Another useful way to picture the expanding universe is to think of the galaxies as little specks of glitter glued to the skin of a balloon. Now slowly blow up the balloon and watch what happens to the specks as the skin of the balloon stretches. (Kids can do this at home or in a classroom.) Note that two specks that are closer to each other move apart slowly, while two specks that are farther away to begin with separate with greater speed.
The stretching of space relieves the galaxies of the "responsibility" for their own motions. After all, galaxies do not have rocket motors attached, sending them off on a wild ride away from their annoying neighbors! In our modern view, the galaxies mind their own business, sitting (as everything does) in space. It is space which is the active agent, stretching everywhere and taking the galaxies in it along for the ride.
And WHY does space stretch in the first place? This question led astronomers naturally to the concept of the big bang, the mind-boggling explosion of space, time, matter, and energy with which the universe began. Evidence that the universe really was in such an extremely hot condensed state (14 billion years ago) has accumulated until the Big Bang Hypothesis has become the cornerstone of the understanding of the universe.
It's wonderful to think that such fantastic ideas can begin with the observations of small smudges of galaxy light -- seen with the descendants of the little telescope Galileo cobbled together 400 years ago.
Contributed by Andrew Fraknoi (Foothill College)
If you were to poll everyone who watches 400 Years of the Telescope and ask what discovery would be the most exciting for astronomers to make with the next generation of telescopes, I would bet that the discovery of intelligent beings around other stars would be near the top of that list. The search for cosmic company has been a major plot element in much science fiction and remains one of the favorite topics in every introductory astronomy class. Finding out whether we are alone in the vastness of the universe, or one among many intelligent civilizations, seems to be an irresistible quest.
Look to the Stars
As late as the 1950's, some people still held out hope that we might find intelligent life on some other planets nearby (such as Mars). However, the coming of the space age, and the surveys made by robot spacecraft dashed those hopes once and for all. If there is life in the hostile environments of our own solar system, it is likely to consist only of micro-organisms -- which are not as much fun to talk to as we like conversations to be. If we are to find "cousins" out in space, it's now clear that we have to focus our search among the stars.
The trouble is that the stars are very remote compared to the planets with which we share the solar system. If were to shrink our Sun to the size of a basketball, the Earth would be an apple seed, about 30 yards (30 meters) away from the ball. On that same reduced scale, how far away would the nearest other star be? The answer may astound you it would be over 4,000 miles from the basketball! And that analogy is only for the star closest to us all the other stars that shine in our sky would be even farther.
It is these awesome distances that make the task of finding intelligent life out there so difficult. Travel across the vast gulfs of space would be either extremely slow or fantastically expensive. But we don't have to travel to a distant place in order to set up communication. After all, today we are all used to getting news from distant places on planet Earth without having to travel there personally. Radio, television, email, fax, and mobile telephone service allow us to find out about people whom we don't have time or the funds to visit.
Similarly, astronomers have proposed for many decades that first contact with an intelligent civilization among the stars would most likely be made using remote communication rather than direct travel. One of the great discoveries of physical science is that nature regularly and easily produces waves that have the courtesy to travel at the fastest possible speed in the universe the speed of light. Because they come from charged particles within the atom and spread out in all directions, we call them electro-magnetic radiation. We've learned to produce these waves ourselves, and already use them to communicate quickly on Earth. While light is the best known example of such waves, radio transmissions, television broadcasts, and cell phones all use different forms of them.
We know how to code a great deal of information into these waves -- to send a television show (like 400 Years of the Telescope) to your home receiver in high definition, for example. And we know how to decode the information that the waves contain (whether from something natural or a human activity.) In the last century, we have gotten quite good at using these waves for a wide range of communications and information gathering.
So it naturally occurred to us to ask whether the use of these waves for communication across the Galaxy might also appeal to other intelligent beings (who, after all, will discover the same laws of nature that we do.) If they get interested in astronomy, as we did, they will surely know of the existence of waves like light. And they are likely to ask which of the waves in nature might be best for sending messages among the stars.
Their answer will, we think, be the same as ours the cheapest, most efficient, and most penetrating of these waves is the category will call radio waves. Radio waves get through the atmospheres of planets, they are not blocked by the gas and dust between the stars, they don't have to compete with the shining of stars (like light waves would), and they take the least energy to produce. (That's why we use them to get traffic reports to your car!)
Starting in the early 1960's, astronomer began to use giant radio dishes (like the satellite dishes many people and businesses now have on their roofs, but bigger) to search for possible messages from more advanced civilizations among the stars. Our hope is that, out there, civilizations that got started before we did might perhaps be broadcasting to their own outposts with so much energy that we can overhear them (sort of like overhearing the neighbors when they are shouting.) Or they might even in a charitable or missionary mood have set up a special beacon with simple "beginner" messages for younger civilizations like ours to discover.
This branch of astronomy is called SETI -- the Search for Extra Terrestrial Intelligence. SETI programs are being conducted at a number of observatories around the world. The key organization in the search is the SETI Institute, whose outreach scientist, Seth Shostak, is interviewed as part of 400 Years of the Telescope. They have just begun (with the University of California) to build the Allen Telescope Array, a network of radio dishes, which -- working together -- will perform the most sensitive search for alien radio signals ever undertaken.
Scanning the Dial
You yourself perform something like this search when you arrive in a new city and try to find your favorite kind of music on the radio. Since you don't know in advance how the stations are arranged on the dial, you must scan up and down until you find the kind of signal that appeals to your musical taste. But at least you have a radio set that is limited to the AM and FM bands. For communicating with other civilizations, we have no idea which part of the vast range of possible radio waves they might pick as their "channel," and so we must search through as many as possible.
The big step forward in this field has been the development of multi-channel analyzers -- software and hardware -- that can search through thousands or even millions of channels at the same time to see if any of them might contain an intelligent message. Astronomers point their telescopes toward a star like the Sun, and then the system scans the possible range of channels to see if there is anything besides static in any of them.
Of course, we don't know what star might harbor an intelligent civilization and we have no guarantee that our equipment can pick up whatever signal they might decide to send. (In the same way, when you are scanning the dial in a new city, you miss the stations that are too weak or far away for your car's antenna, as well as those that are now only using the web instead of broadcasting.) SETI's leading scientists, Jill Tarter (on whom the character that Jodi Foster planed in the film Contact is based) has compared the task of locating an alien message to searching for a needle in a haystack.
Indeed, it may take decades or centuries to hit upon any signal that might be out there. But if we don't search, we will never know whether or not messages are coming our way, from another civilization whose own Galileo may long ago have started them down the road of investigating the universe just as we do.
For more information, visit the SETI Institute web site: http://www.seti.org
Contributed by Andrew Fraknoi (Foothill College)
At the beginning of 400 Years of the Telescope, Neil Tyson discusses that cultures around the world have always incorporated the sky into their philosophy and religion. Today, as we in the modern world absorb the many things about the universe telescopes are teaching us into our self-view, we can also look back to see how other cultures have been shaped by their perception of the realms beyond Earth.
This brief resource guide is only an introduction to the study of astronomy of various civilizations around the world. Archaeoastronomy is one name we give to the study of the astronomical ideas and monuments of ancient cultures. Some of the books listed may go in and out of print, but you can often find them at larger libraries or through used book web sites.
Download The Astronomy of Many Cultures here: PDF
Contributed by Andrew Fraknoi (Foothill College)
A Chinese proverb says "Women hold up half the sky" and celebrating the contributions of women to astronomy can be a good way to give that proverb deeper meaning. For much of history, women with an interest in the universe were kept out of astronomy (as they were out of most professional fields) and they were restricted to helping their husbands or brothers in their scientific pursuits. But this has changed dramatically in the last century, and many of the most important posts in astronomy have been held by women and many key discoveries have been made by them.
400 Years of the Telescope features interviews with Catherine Cesarsky, the President of the International Astronomical Union (which is the U.N. of astronomers); Wendy Freedman, the astronomer who worked with the Hubble Space Telescope to pin down the age of the expanding universe; and Claire Max, who heads the Center for Adaptive Optics (helping astronomer get a clearer view of the sky).
For those who want to explore the contributions of women to astronomy in more detail, we list some general resources and then some articles and books that can help you understand the specific work of a few selected women astronomers. (Note that a number of books for younger readers are included in section 1.)
Download the Women in Astronomy resource guide here: PDF
Contributed by Andy Franknoi (Foothill College)