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Most estimates place the number of stars in our galaxy at between 100 billion and 500 billion.

Observatories, like so much of astronomy, are truly ancient features of human culture; retracing the steps of the first person to stand on a sacred hillside and search the sky for answers is truly an impossible task. Across the globe, however, such sites -- devoted to deciphering the heavens -- are found in considerable numbers. Unlike modern mountaintop facilities that probe the entire spectrum of light with computerized equipment, ancient observatories were placed in positions for optimal viewing with the eyes -- on hilltops or atop structures such as the ziggurats of ancient Babylon or the rooftops of Mexico's Monte Alban.

Ancient observatories often employed reference markers to the seasonal procession of the natural calendar; consider, for example, the familiar stone pillars of Stonehenge or the spokes of Native American medicine wheels. Such places were of considerable spiritual significance; astronomers, able to predict the motions of the Sun, Moon, and planets, were considered to be interpreting the will of the gods as written on the tableau of the skies -- a considerable, and occasionally dangerous, responsibility.

The Milky Way's disk spans at least 100,000 light-years and probably contains several hundred billion stars, which makes our home galaxy a giant.

However, the Milky Way is surrounded by a "halo" of cold dark matter, which produces no detectable energy but that reveals its presence by exerting a gravitational pull on the visible matter around it. The dark matter probably consists of some type of subatomic particle created in the Big Bang. This halo probably spans 250,000 light-years, and perhaps a great deal more.

The Milky Way is the second-largest galaxy (but perhaps the most massive, or "heaviest") in what astronomers call the Local Group. The slightly larger Andromeda Galaxy and many smaller galaxies also populate the group, which spans about 6.5 million light-years.

Maps of the stars are nearly as ancient as human culture itself; inscriptions describing the positions of stars and constellations appear on Egyptian temples, Babylonian tablets, and Chinese calendars produced thousands of years ago. By around 500 BC, Greek philosophers had begun to approach the mapping of the heavens in an almost modern fashion, making careful and repeated observations of the positions and motions of the stars and planets in an attempt to rationally explain the movements of the universe.

Credit for modern maps of the cosmos is often given to Harlow Shapley, who in 1917 published a map of the distribution of globular clusters and correctly concluded that our solar system was located a considerable distance from the center of our galaxy. Not long after, in 1924, Edwin Hubble was able to clearly resolve individual stars in other galaxies, and so was able to determine the distances to these "island universes" -- paving the way for future astronomers who would map the universe.

Quite a few. Most estimates place the number of galaxies at around 100 billion. Most of those galaxies are little puffs of stars that are far smaller and less massive than our home galaxy, the Milky Way.

Many observatories around the world are at least partially accessible to the public, with some, such as McDonald Observatory in West Texas, even allowing regular public access to some of their research telescopes. Check out our list of publically accessible telescopes for more information.

The largest visible-light telescope currently in operation is at Gran Canarias Observatory, and features a 10.4-meter (34-foot) primary mirror.

The Hobby-Eberly Telescope at McDonald Observatory near Fort Davis, Texas, has the world's largest telescope mirror. Because of the way HET is designed, however, astronomers use only 9.2 meters of the 11-meter (36-foot) mirror at any one time, making HET the world’s fourth-largest telescope. Scheduled upgrades to the telescope, however, will improve its performance to that of a 10-meter telescope.

Two larger ground-based mirrors are in the planning stages: the Giant Magellan Telescope and the Thirty-Meter Telescope. The first would consist of eight individual mirrors working together, while the latter would consist of a large segmented mirror. Each would have an effective aperature of roughly 30 meters (100 feet), giving them as much surface area as a small office building.

Hubble Space Telescope looks at the nether regions of the universe with a 2.4-meter mirror. The James Webb Space Telescope, which NASA plans to launch as early as 2013, will have an eight-meter (25.6-foot) primary mirror.

The largest refracting telescope in the world is at Yerkes Observatory in Williams Bay, Wisconsin. Instead of a mirror, it gathers light with a 40-inch glass lens.

Astronomers also gather radio waves from space using dish-shaped antennas, the largest of which is the Arecibo Observatory in Puerto Rico. Featured in the movie “Contact,” Arecibo's dish is 1,000 feet in diameter.

Before using a research telescope, an astronomer submits a proposal detailing project goals and equipment needs for approval by a committee of other astronomers. In some cases, even when a project is approved, the astronomer never visits the observatory at all. Instead, telescope specialists operate the instruments and gather data for the astronomer.

When an astronomer does travel to an observatory, the engineers, electricians, opticians, computer scientists, cooks, and crew who live there prepare for the astronomer's "observing run," which typically lasts a few nights. The astronomer sleeps through the day, then spends a few hours before sunset preparing for the observations. After dinner, when night falls, the observations begin.

The astronomer spends the entire night pointing the telescope at distant objects -- planets, stars, nebulae, or galaxies -- and collecting the faint trickle of light from each object. A computer stores the data for later analysis. If clouds spoil the observations, the astronomer must submit a new proposal and hope for clear skies next time.

The astronomer never actually looks through the telescope, although most telescopes have a video system to display the area of the sky at which the telescope is aimed. Instead, the astronomer generally remains in the lighted, heated control room and monitors both the telescope and instruments as they collect and record data.

After completing an observing run, an astronomer may spend months or years analyzing and interpreting the results. Meanwhile, back at the observatory, the staff prepares for the next astronomer.

The true inventor of the first telescope is somewhat difficult to nail down. However, the first person to apply for a patent on a telescope was Hans Lippershey, a lensmaker in the Netherlands, in 1608.

The first astronomical use of the telescope is easier. After learning of the new device, the great Italian scientist Galileo Galilei designed and built his own. He turned his finest telescopes toward Jupiter, the Moon, and Venus in 1609 and 1610.

Telescopes gather light in one of two ways. Reflecting telescopes focus light with a series of mirrors, while refracting telescopes use lenses. For research purposes, reflecting telescopes have become the standard because of the relative ease of constructing and working with large mirrors. The lenses needed for refracting telescopes present endless engineering problems and must be extremely pure throughout their entire volumes, while mirrored surfaces require ultra-fine precision only on the surface.

Modern telescopes gather information from the electromagnetic spectrum far beyond the range of visible light. Telescopes that survey radio, x-ray, and gamma-ray wavelength have dramatically broadened our understanding of the universe. Radio telescopes -- huge wire-mesh dishes designed to focus radio signals from space -- have helped to map the spiral arms of our galaxy, while gamma-ray observatories high in Earth orbit have captured the high-energy signals of exotic objects such as black holes and gamma-ray bursts.

For the most part, the rise and fall of the tides is caused by the gravitational influence of the Moon. The Moon's gravity pulls the ocean surface upward, creating a bulge in the water -- high tide. Locations on Earth perpendicular to the Earth-Moon line experience low tide.

That's not the end of the story, however. The Sun also tugs on Earth's oceans, though to a lesser extent. When both Sun and Moon are lined up with Earth, as occurs during the full and new moons, their combined effects yield higher tides than normal, called spring tides. Conversely, when the Sun is at right angles to Earth and Moon, as at the first- and last-quarter phase, the Sun's gravitational influence works against the Moon, and we experience an unusually low, or neap tide.

While Hubble Space Telescope (HST) is certainly the most famous observatory in space, it is by no means the only one. There have been dozens of space-based telescopes, including past missions like the highly successful Compton Gamma-Ray Observatory, which helped astronomers unravel some of the mysteries of gamma-ray bursts, and the currently operating Chandra X-Ray Observatory, which is exploring the violent regions around black holes and other high-energy phenomena.

Another member of NASA's Great Observatories program is the Spitzer Space Telescope, which was launched in August 2003. It explores the infrared glow of stellar nurseries, planet-forming disks around newborn stars, and interstellar dust clouds.

The Kepler mission, launched in 2009, will spend three years looking for Earth-like planets in Earth-like orbits around Sun-like stars. It will scan 100,000 stars in the constellations Lyra and Cygnus in hopes of finding planetary transits, in which the star’s light dims slightly as a star passes across its disk.

NASA also has launched many smaller observatories through its Explorer program. These missions have probed the "afterglow" of the Big Bang (COBE and WMAP), the ultraviolet light from other galaxies (GALEX and EUVE), and the violent explosions known as gamma-ray bursts (SWIFT).

The farthest known object orbiting our Sun is a ball of ice and rock unofficially called Sedna, which lies about 10 billion miles away right now, although its highly elliptical orbit will carry it up to 84 billion miles from the Sun. Early measurements made from California's Palomar Observatory show that the object is probably 800 to 1,100 miles in diameter.

Another hop, skip, and a jump takes us to the heliopause, where the stream of particles emitted by the Sun collides with the galactic gases of interstellar space, forming a so-called "bow shock." The boundary between the Sun's influence and interstellar space may lie as much as 15 billion miles ahead of the Sun's path through the galaxy, and more than 30 billion miles behind it.

Farther still is the Oort Cloud, believed to be the source of extremely long-period comets (Hale-Bopp, for instance). This dark, incredibly cold region awaits interstellar travelers nearly six trillion miles away -- almost a quarter of the distance to the nearest star.

Procedures for operating satellite telescopes are somewhat different from traditional ground-based telescope observing runs. Telescopes orbiting in space are operated remotely from control stations on the ground, where specially trained staff point the telescope at the specific targets requested by the astronomers. This "queue-based" observing makes efficient use of the telescope's time -- several different projects can be done in a single day. As a result, many observatories have started using this approach for ground-based telescopes as well, such as the Hobby-Eberly Telescope at McDonald Observatory.

No. On August 23, 2006, the International Astronomical Union decided that the solar system contains only eight planets. Pluto, which since 1930 had been classified as the ninth planet, was dropped from the list.

This bright ring of light is quite common. It's related to the same process that creates rainbows: refraction. In this case, moonlight shines through a layer of ice crystals high in the atmosphere. The ice crystals act like prisms, splitting the light into a rainbow of colors that surrounds the Moon. If the effect is intense enough, the colors are visible to the unaided eye. If not, then the ring around the Moon looks milky white.

Incidentally, the ring has some practical value, too. The thin clouds that cause the rings often precede cold fronts or storm systems, so they may indicate that rainy weather is on the way.

Alignments of the fast-moving inner planets can occur as regularly as every few months or so, while groupings of the slower outer planets -- Jupiter, Saturn, Uranus, and Neptune -- occur far less often, but last longer when they do.

About every 100 years or so, six or more planets "line up" and appear together within a small area of the sky. A well-publicized conjunction of this type occurred May 5, 2000, when the Moon and all of the planets except Uranus, Neptune, and Pluto (which was still a planet then) lined up within 15 degrees or so of the Sun. Such gatherings have occurred tens of thousands of times in the past, with no observed physical consequences.

The stars aren't visible because they are too faint. The astronauts in their white spacesuits appear quite bright, so they must use short shutter speeds and large f/stops to not overexpose the pictures. With those camera settings, though, the stars don't show up.

The same thing happens if you try to take a picture of someone under a dark, starry sky. To get the person perfectly exposed, you have to use a flash or some other light source, and set your camera accordingly. When you do that, there is no way to see the stars in the background. To see the stars, you need long exposures and wide-open aperatures. But with those settings, the subject of the picture would appear dark and blurry.

Astronauts have taken many photographs of the stars from orbit (and many of them are available on NASA's web site, but, unfortunately, not with spacewalking astronauts in the foreground.

Because stars are so incredibly distant, to our eyes they appear strictly as points in the night sky. Irregularities in Earth's atmosphere cause starlight to dance around, and the minute changes in the path the starlight takes through the atmosphere results in apparent changes in color -- the familiar "twinkling" effect.

Planets, however, actually form a tiny but definite circle on the sky just large enough to counter the distorting effect of turbulence. Such extended objects only "twinkle" when their light passes through very large amounts of atmosphere, such as when they lie close to the horizon.

That is a tricky question, which astronomers didn’t answer correctly until fairly recently.

For a while, they thought the biggest moon was Titan, which orbits Saturn. But a deep atmosphere topped by orange smog surrounds Titan, so scientists couldn’t see its surface. Not until spacecraft began to visit Titan at close range could they measure its true size and find that it’s only the runner-up. The prize for biggest moon goes to (the envelope, please) ... Ganymede, a moon of Jupiter, at a diameter of 3,270 miles (5,262 km) — about 70 miles larger than Titan.

Earth's circumference was first accurately measured more than 2,000 years ago by the Greek astronomer Eratosthenes, who at the time lived in the Egyptian city of Alexandria. He had heard that in the nearby town of Syene midday sunlight shines straight down to the bottom of deep wells on the same day each year, indicating that the Sun was directly overhead in Syene. In Alexandria, however, sunlight on that date never reached the bottoms of wells, but instead fell upon the sides.

Eratosthenes reasoned that the difference in the angle of incoming sunlight was due to the curvature of Earth's surface, and so by measuring this angle, he related the distance between Alexandria and Syene to the total dimension of the globe.

On the day the Sun shone on the bottom of the wells in Syene, Eratosthenes measured the Sun's position in the sky over Alexandria. It was seven degrees away from the zenith, meaning Syene must be seven degrees away from Alexandria as measured on the circle that is Earth's circumference. Because seven degrees is about one 50th of a full circle (360 degrees), Eratosthenes simply multiplied the distance from Alexandria to Syene -- believed to have been about 515 miles (830 km) -- by 50. He calculated Earth's circumference at 26,000 miles (42,000 km), only five percent away from the modern accepted value of 24,901 miles (40,074 km).

Stars aren't visible during the sunlit hours of daytime because the light-scattering properties of our atmosphere spread sunlight across the sky. Seeing the dim light of a distant star in the blanket of photons from our Sun becomes as difficult as spotting a single snowflake in a blizzard.

The truth behind the demise of the dinosaurs may never be fully resolved, but a growing body of evidence has convinced many scientists that at least one of the culprits was a seven-mile-wide asteroid that hit Earth 65 million years ago near what is now the Yucatan peninsula. Exploding on impact, the monster rock produced a crater more than 180 miles wide, continent-drowning tsunamis, and winds far more powerful than any hurricane. In addition, dust kicked up by the explosion would have completely darkened daytime skies for months, devastating plant and animal life across the globe.

The prime piece of evidence supporting the theory is a thin layer of 65 million-year-old iridium-rich clay found in dozens of locations across the globe. (Iridium is extremely rare on Earth, but common in meteorites.) Furthermore, other rock samples from that era indicate an exposure to extreme heat and pressure, as would occur in an asteroid impact -- and there is that enormous 65 million-year-old crater in Mexico...

Still, the argument is far from over. Many paleontologists point out that the dinosaurs -- and many other life forms -- were already dying out before the massive extinctions at 65 million years ago. Global temperatures and sea level had been dropping for millions of years. The larger animal life of the time must have been feeling the effects. The rock layers from that era also show evidence of extensive volcanic activity, which could account for a deadly worldwide dust cloud as well as the elevated iridium levels.

So we may never know what really killed the dinosaurs, but research into the possibility that an asteroid caused such a major "extinction event" has at least opened our eyes to the real threat of such an impact -- and that puts us one up on the dinosaurs.


To begin with, Earth is rotating on its axis at the familiar rate of one revolution per day. For those of us living at Earth's midlatitudes -- including the United States, Europe, and Japan -- the rate is almost a thousand miles an hour. The rate is higher at the equator and lower at the poles. In addition to this daily rotation, Earth orbits the Sun at an average speed of 67,000 mph, or 18.5 miles a second.

Perhaps that seems a bit sluggish -- after all, Mars Pathfinder journeyed to Mars at nearly 75,000 miles per hour. Buckle your seat belts, friends. The Sun, Earth, and the entire solar system also are in motion, orbiting the center of the Milky Way at a blazing 140 miles a second. Even at this great speed, though, our planetary neighborhood still takes about 200 million years to make one complete orbit -- a testament to the vast size of our home galaxy.

Dizzy yet? Well hold on. The Milky Way itself is moving through the vastness of intergalactic space. Our galaxy belongs to a cluster of nearby galaxies, the Local Group, and together we are easing toward the center of our cluster at a leisurely 25 miles a second.

If all this isn't enough to make you feel you deserve an intergalactic speeding ticket, consider that we, along with our cousins in the Local Group, are hurtling at a truly astonishing 375 miles a second toward the Virgo Cluster, an enormous collection of galaxies some 45 million light-years away.

Almost all of the world's finest ground-based observatories are located on mountains, for a variety of reasons. First and foremost, starlight appears less distorted in the thin atmosphere on mountaintops. (Space-based telescopes such as Hubble and Spitzer Space Telescope circumvent the disturbing effects of the atmosphere by flying above it.)

At high altitudes, there is less atmosphere to absorb infrared energy, which reveals details about some of the coldest objects in the universe, such as clouds of gas and dust and the disks of dust that give birth to planets.

Mountaintops also have unobstructed views of the horizon in all directions. Lastly, most cities and towns -- with their accompanying light pollution -- are situated in valleys and plains, so remote mountaintops are among the last places on Earth to find the dark skies so sought after by astronomers.

Not much in our lifetimes -- perhaps 1 in 10,000 -- but over thousands or millions of years, major impacts become pretty likely. Ancient craters on Earth's surface prove that large objects have hit Earth in the past, and there's no reason to think this won't continue in the future.

The chance of an impact depends on the size of the object: the bigger the comet or asteroid, the smaller the chance, since there are many more small objects out there than large ones. Tons of debris -- much of it in pieces smaller than grains of sand -- strike Earth's atmosphere and burn up every day. These are the "shooting stars" commonly seen at night. Some larger rocks survive their fiery descent to the surface; you can see some of these "meteorites" displayed in museums. The truly dangerous objects, those large enough to cause regional or global catastrophe when they hit, may appear once every few hundred thousand years. Therefore, the chance that such an object will hit us in any given year is roughly 1 in 300,000 -- nothing to lose sleep over.

Many scientists believe an extremely large asteroid (about six miles in diameter) struck Earth 65 million years ago near the present-day Yucatan peninsula of Mexico. The impact caused catastrophic conditions across the entire planet, including thick clouds of dust and ash that caused global temperatures to plummet, causing the extinction of the dinosaurs and much of the rest of the life on Earth.

The path Earth follows in its orbit around the Sun is littered with untold pieces of debris. Unlike the dinosaurs, we have the means to find the largest of these "Near-Earth Objects" (NEOs) and calculate their orbits, to see if they might ever come close to us. Currently, several different telescopes routinely and automatically scan the sky for them.

The famous Northern and Southern Lights -- Aurora Borealis and Aurora Australis for those Latin lovers among us -- are caused by high-energy particles from the Sun cascading down on Earth. As they near our planet, they interact with Earth's magnetic field, which channels them toward the north and south magnetic poles.

There they are accelerated downward, and at altitudes ranging from 90 to 700 km (50 to 450 miles), the particles collide with atoms of our upper atmosphere, a process that results in a glowing field of excited gas. These sheets of lights can take on many beautiful colors, and often persist for hours on end, dancing gracefully in the polar skies.

Earth isn't the only planet that experiences aurorae; recent images from the Galileo probe, in orbit around Jupiter, clearly show massive auroral displays many times the size of Earth taking place high in the Jovian atmosphere. As we study our fellow planets in ever-increasing detail, it is all but certain that more such spectacular and eerily beautiful scenes will be discovered.

Astronomers have developed several techniques to indirectly measure the vast distances between Earth and the stars and galaxies. In many cases, these methods are mathematically complex and involve extensive computer modeling.

Parallax is the visual effect produced when, as an observer moves, nearby objects appear to shift position relative to more-distant objects. This common event is easily reproduced; hold your finger out at arm’s length, and look at your fingertip first with one eye closed, then the other. The "motion" of your fingertip as seen against background objects is caused by the change in your viewing position -- about three inches from one eye to the other.

As Earth orbits the Sun, astronomers invoke this same principle to determine the distance to nearby stars. Just like your fingertip, stars that are closer to us shift positions relative to more-distant stars, which appear fixed. By carefully measuring the angle through which the stars appear to move over the course of the year, and knowing how far Earth has moved, astronomers are able to use basic high-school geometry to calculate the star’s distance.

Parallax serves as the first "inch" on the yardstick with which astronomers measure distances to objects that are even farther.

For example, they use a class of variable known as Cepheids, which pulsate in and out like beating hearts. There is a direct relationship between the length of a Cepheid's pulsation and its true brightness. Measuring a Cepheid's apparent brightness -- how bright it looks from Earth -- allows astronomers to calculate its true brightness, which in turn reveals its distance. For this technique to work correctly, though, astronomers must first use the parallax method to get the distances to some of the closer Cepheids. This allows them to calibrate a Cepheid's true brightness, which then can be used to calculate its distance. Cepheids are especially bright stars, so they are visible in galaxies that are tens of millions of light-years away.

For more-distant galaxies, astronomers rely on the exploding stars known as supernovae. Like Cepheids, the rate at which a certain class of supernovae brighten and fade reveals their true brightness, which then can be used to calculate their distance. But this technique also requires good calibration using parallax and Cepheids. Without knowing the precise distances to a few supernovae, there is no way to determine their absolute brightness, so the technique would not work.

Perhaps a better question might be why isn't Earth covered with craters? Both Earth and Moon -- and the other inner planets -- were heavily bombarded by meteors and comets in the tumultuous days of the early solar system. Mercury and our Moon still bear the scars of the terrific pounding, while Earth, Venus and Mars show few signs of damage at all.

The relatively crater-free surfaces of Earth, Venus, and Mars can be explained by the existence on these three worlds of powerful surface-changing mechanisms, namely plate tectonics (Earth), the eroding effects of wind and water (Earth and Mars), and extensive volcanic activity (all three). These forces have helped smooth out the cratered landscapes.

While the Moon is known to have once been quite volcanically active -- great lava flows produced the dark "seas" on the surface -- it has long since quieted, and the extraordinarily tenuous atmosphere of the Moon is incapable of producing any erosion effects. Many of the craters we now see on the Moon remain almost exactly as they must have appeared hundreds of millions of years ago.

The "Moon illusion," in which the Moon appears larger than normal when close to the horizon, is not the result of magnification by the atmosphere or a change in Earth-Moon distance. Instead, the answer is, as Einstein might say, completely relative.

At most times we see the Moon high in the sky among thousands of stars. We develop our sense of how "big" the Moon ordinarily appears by comparing it with the vast panorama of outer space.

When the Moon is nestled along the horizon, however, we see it surrounded by a foreground of familiar Earth-bound objects -- trees, buildings, or distant landmarks. In comparison with these everyday features, the bright disk of the full Moon appears quite large indeed, and relative to our "normal" sense of the Moon's size, much bigger than we would expect.

According to Albert Einstein's Theory of General Relativity, it is feasible in areas of intensely strong gravity to create "wormholes" that connect two points that are widely separated in space or time (or both).

Unfortunately, the intense gravity needed to produce such an effect -- such as that of a black hole -- would collapse the wormhole before it could be used for a real traveler. In addition, while there is abundant evidence that black holes exist, there are no observations at all of any suspected wormhole.

While not all of the planets rotate on their individual axes in the same direction — Uranus and Venus both rotate opposite to the other six planets — the planets are in agreement as to which way to go. The shared motion is an artifact of the formation of the solar system from a giant rotating gas cloud 4.5 billion years ago.

As for the reversed, or retrograde, spin of Uranus and Venus, scientists are divided as to the cause. Most subscribe to the theory that early in their histories, these worlds were subjected to massive collisions with planetary-sized objects, so powerful they reversed the planets original direction of rotation — and in the case of Uranus, knocked it almost completely sideways.

While no direct link between sunspots and Earthly events, such as volatile stock markets, has ever been found, extremes in sunspot activity have been correlated in some cases with climate changes on Earth. For example, the "Maunder minimum," a period of exceptionally low sunspot numbers in the 16th century, does coincide with a time of unusually low temperatures around the world -- the so-called "Little Ice Age."

A more direct case can be made, however, for solar flares, which often cause quite a bit of terrestrial turmoil. These brief, intense explosions of high-energy particles and radiation from the Sun have been responsible at various times for frying the circuitry of artificial satellites, disrupting radio and television broadcasts, and bringing down the entire power grid of Quebec. Because of their proximity to the northern magnetic pole, which guides the particle streams down to Earth over the Arctic, Canada and the United States find themselves particularly susceptible to solar-flare disruptions. Flares also trigger brighter, more extensive aurorae that may be visible from the southern United States. A considerable amount of effort has been directed toward predicting and detecting these events.

For the person on the street, however, solar flares and sunspots generally have little impact. It's only the rare, extremely large flare that causes extensive electronic disruptions.

Space is indeed curved -- in four dimensions. Many people think the fourth dimension is simply time, and for some astronomical equations, it is. Einstein used time as a fourth dimension to describe a coordinate system called space-time. This is the stage on which planets, stars, galaxies -- all matter in the universe -- act their gravitational roles. In Einstein's Theories of Relativity, time helps us understand the three-dimensional experience of gravity.

But time is not a true spatial dimension like the three we're familiar with. Unlike width, length, and height, we only know time to move in one direction: past to present to future. In this way, time merely serves as a yardstick for our personal experiences and our laws of physics. Some theoretical equations incorporate many other "dimensions" -- velocity, temperature, or density, for example.

To assist you in imagining space curved in four dimensions, pay a visit to Flatland, a two-dimensional world full of square, triangular, and circular beings. If it helps, draw Flatland and its residents on a sheet of paper and place it flat on a table in front of you.

The laws of 2D space as understood by 2D beings would restrict light (and everything else) from moving up and down; in fact, there would be no up or down. So as a three-dimensional being, your actions would violate the Flatlanders' laws of physics. Place your fingertips on Flatland -- you appear to them in many places at once. Say "Hello" to Flatland -- your voice is also omnipresent. And while Flatlanders cannot see beyond the boundaries of their world, to you they are nothing more than hollow geometrical shapes.

Regardless of the number of dimensions that describe a space, the curvature of that space, by definition, implies the existence of at least one more. In other words, an additional dimension must exist into which the other dimensions can be curved. You can curve Flatland in your three-dimensional reality by rolling the piece of paper into a tube, folding it into a paper airplane, or just crumpling it into a wad. Doing so would not be immediately apparent to Flatlanders, because light still travels in straight lines within their 2D space. They might experience some interesting phenomena, though - triangular residents might find that their interior angles no longer add up to 180 degrees, and parallel lines everywhere might begin to intersect.

If a 4D being decides to "crumple" our 3D space, like the Flatlanders, you wouldn't notice anything right away, either. You might notice some unusual phenomena, like walking to your mailbox and ending up in Mongolia. Scientists believe, however, that our space is smoothly curved -- more like a ball than a wad of paper -- and thus the effects are only noticeable on cosmological scales. As 3D beings, we experience just the "surface" of the 4D ball, just as Flatlanders experience only two dimensions of their space that you can curve into three dimensions.

Of course, the nearest star to Earth is the one we see every day -- our dear old Sun. At a mere 93 million miles distant, it takes light from the Sun only eight minutes to arrive on Earth, shining through your bedroom window at dawn and on sunbathers on the French Riviera.

Several stars lie within a few light-years of the Sun. At a distance of just over four light-years, the three members of Alpha Centauri, including Proxima Centauri, are the nearest stars to Earth. The next three closest stars are red dwarfs: Barnard's Star is six light-years from Earth. Wolf 359 and Lalande 21185 are about eight light-years distant.

The brightest star in Earth's night sky is Sirius, 8.7 light-years away. The binary system called Procyon is a bit farther at 11.2 light-years.

The search for extrasolar planets might take us to Epsilon Eridani or Tau Ceti, Sun-like stars at 10.8 and 11.8 light-years away. Rounding out the stellar neighborhood at a distance of about 11 light-years are Epsilon Indi and the binary system 61 Cygni.

It depends on what exactly you think you see. If you believe you have discovered a new member of a known class of objects -- a new comet, asteroid, or even a supernova -- you should contact the International Astronomical Union as soon as possible; if your discovery is later verified, you could achieve a certain astronomical immortality, e.g. Messrs. Hale and Bopp!

The International Meteor Organization's Fireball Data Center tracks meteors and meteorite impacts. The National UFO Reporting Center collects information on sightings of other unusual phenomena.