How Fast Does Light Travel? | The Speed of Light How Fast Does Light Travel? | The Speed of Light By Nola Taylor Redd, Space.com Contributor | May 22, 2012 08:37pm ET MORE Einstein's theory of special relativity sets of the speed of light, 186,000 miles per second (300 million meters per second). But some scientists are exploring the possibility that this cosmic speed limit changes. Credit: Iscatel | Shutterstock The speed of light in a vacuum is 186,282 miles per second (299,792 kilometers per second), and in theory nothing can travel faster than light. In miles per hour, light speed is, well, a lot: about 670,616,629 mph. If you could travel at the speed of light, you could go around the Earth 7.5 times in one second. Early scientists, unable to perceive light’s motion, thought it must travel instantaneously. Over time, however, measurements of the motion of these wave-like particles became more and more precise.

Thanks to the work of Albert Einstein and others, we now understand light speed to be a theoretical limit: light speed — a constant called "c" — is thought to be not acheivable by anything with mass, for reasons explained below. That doesn’t stop sci-fi writers, and even some very serious scientists, from imagining alternative theories that would allow for some awfully fast trips around the universe. Speed of light: History of the theory The first known discourse on the speed of light comes from the ancient Greek philosopher Aristotle, who penned his disagreement with another Greek scientist, Empedocles. Empedocles argued that because light moved, it must take time to travel. Aristotle, believing light to travel instantaneously, disagreed. In 1667, the Italian astronomer Galileo stood two people on a hill at a distance of less than a mile, each holding a shielded lantern. One uncovered his lantern; when the second saw the flash, he uncovered his, as well. By observing how long it took for the light to be seen by the first lantern-holder (and factoring out reaction times), he thought he could calculate the speed of light.

Unfortunately, Galileo's distances were too small to see a difference, so he could only determine that light traveled at least ten times faster than sound. In the 1670s, Danish astronomer Ole Römer used eclipses of Jupiter 's moon, Io, as a chronometer for the speed of light. Over the course of several months, as Io passed behind the giant gas planet, Römer found that the eclipses came later than calculations anticipated, although over the course of several months, they drew closer to the predictions. He determined that light took time to travel from Io to Earth. The eclipses lagged the most when Jupiter and Earth were farthest apart, and were on schedule as they were closer. He concluded that light took ten to eleven minutes to travel from the sun to Earth, an overestimate since it in fact takes 8 minutes and 19 seconds. But at last scientists had a number to work with — his calculation presented a speed of 125,000 miles per second (200,000 km/s). In 1728, English physicist James Bradley based his calculations on the change in the apparent position of the stars due Earth's travels around the sun.

He put the speed of light at 185,000 miles per second (301,000 km/s), accurate to within about 1 percent. Two attempts in the mid-1800s brought the problem back to Earth. French physicist Hippolyte Fizeau set a beam of light on a rapidly-rotating toothed wheel, with a mirror set up 5 miles away to reflect it back to its source. Varying the speed of the wheel allowed Fizeau to calculate how long it took for the light to travel out of the hole, to the adjacent mirror, and back through the gap. Another French physicist, Leon Foucault, used a rotating mirror rather than a wheel. The two independent methods each came within about 1,000 miles per second of the speed of light measured today. Prussian-born Albert Michelson, who grew up in the United States, attempted to replicate Focault's method in 1879, but used a longer distance, as well as extremely high-quality mirrors and lenses.

His result of 186,355 miles per second (299,910 km/s) was accepted as the most accurate measurement of the speed of light for 40 years, when Michelson remeasured it. Einstein and special relativity In 1905, Albert Einstein wrote his first paper on special relativity . In it, he established that light travels at the same speed no matter how fast the observer moves. Even using the most precise measurements possible, the speed of light remains the same for an observer standing still on the face of the Earth as it does for one traveling in a supersonic jet above its surface. Similarly, even though the Earth is orbiting the sun, which is itself moving around the Milky Way, which is a galaxy traveling through space, the measured speed of light coming from our sun would be the same whether one stood inside or outside of the galaxy to calculate it. Similarly, Einstein calculated that the speed of light doesn't vary with time or place. What is a light-year? The speed of light can cause the universe to seem to host a time machine. The distance light travels in the course of a year is called a light-year.

A light-year is a measure of both time and distance. It is not as hard to understand as it seems. Think of it this way: Light travels from the moon to our eyes in about 1 second, which means the moon is about 1 light-second away. Sunlight take about 8 minutes to reach our eyes, so the sun is about 8 light-minutes away. Light from the nearest star system, Alpha Centauri , is requires roughly 4.3 years to get here, so that star system is said to be 4.3 light-years away. Stars and other objects beyond our solar system lie anywhere from a few light-years to a few billion light-years away. Thus, when astronomers study objects that lie a light-year away or more, they are seeing it as existed at the time that light left it, not as it would appear if they stood near its surface today. In this sense, everything we see in the distant universe is, literally, history . This principle allows astronomers to see how the universe as it looked after the Big Bang, which took place about 13.7 billion years ago.

Examining objects that are, say, 10 billion light-years away, we see them as they looked 10 billion years ago, relatively soon after the beginning of the universe, rather than how they appear today. Is the speed of light really constant? Light travels in waves, and, like sound, can be slowed depending on what it is traveling through. Nothing can outpace light in a vacuum. However, if a region contains any matter, even dust, light can bend when it comes in contact with the particles, which results in a decrease in speed. Light traveling through Earth's atmosphere moves almost as fast as light in a vacuum, while light passing through a diamond is slowed to less than half that speed. Still, it travels through the gem at over 277 million mph (almost 124,000 km/s) — not a speed to scoff at. Can we travel faster than light? Science fiction loves to speculate about this, because “warp speed,” as faster-than-light travel is popularly known, would allow us to travel between stars in time frames otherwise impossibly long. And while it has not be proven to be impossible, the practicality of traveling faster than light renders the idea pretty farfetched.

According to Einstein's general theory of relativity , as an object moves faster, its mass increases, while its length contracts. At the speed of light, such an object has an infinite mass, while its length is 0 — an impossibility. Thus, no object can reach the speed of light, the theory goes. That doesn’t stop theorists from proposing creative and competing theories. The idea of warp speed is not impossible , some say, and perhaps in future generations people will hop between stars the way we travel between cities nowadays. One proposal would involve a spaceship that could fold a space-time bubble around itself in order to exceed the speed of light. Sounds great, in theory. Further Reading: Numbers Big and Small (11 points) Let's get a little practice using scientific notation and converting units. (Refer to pg. 544 of your text for conversions between metric and English units.) Write the number 238,793.387 in scientific notation with only 3 significant digits. To write this number in scientific notation we need to figure out how many spaces to move the decimal point to the left until we have only one digit to the left of the decimal point. Each space we move the decimal is one power of ten.

So to obtain the number 2.38793387 we needed to move the decimal 5 times, or 5 powers of 10. This number to 3 significant digits is 2.39. We round the third digit up or down accordingly. So finally, the answer is: 2.39 x 105 Write the number 5.765 x 10-7 as a normal string of digits. In the first problem you can see that multiplying by a positive power of ten indicates how many decimal places to the right one must move the decimal point to obtain a normal string of digits (we went to the left to get the scientific notation, it's opposite the direction to go to obtain the normal string). So multiplying by a negative power of ten moves the decimal to the left by that power. And so we have 0.0000005765 What is (6 x 104)/(3 x 10-6)? Do not use a calculator. Show your work. What's the name of this number (i.e., how many million, billion, trillion, etc. is this)? First off 6/3 = 2.

That's easy enough. So now we are left with 104/10-6. Use the rules of multiplication and division of exponents. When dividing you subtract the exponents. So 4 - (-6) = 4 + 6 = 10. Thus the answer is 2 x 1010 or 20 x 109 = 20 Billion A light-year is the distance light travels in one year. If light travels at a speed of 186,000 miles/second how many miles are there in a light-year? How many meters is this? Give all answers in scientific notation and show all work. Let us remember the relationship between speed, distance, and time. distance = speed x time (1) In this problem we want to know the distance. We know the speed, and the time is 1 year. But the speed is not given in years, so we need to convert it into seconds. How many seconds are there in one year?

60 seconds/minute x 60 minutes/hour x 24 hours/day x 365.26 days/year = 3.16 x 107 seconds/year So now we plug this value into equation (1) with the speed of light given distance = (1.86 x 105 miles/second) x (3.16 x 107 seconds) = 5.88 x 1012 miles = 5.88 Trillion miles Now, to find out how many meters this is we need only do some unit conversions. From page 544 in the text you find that 1 km = 0.6214 miles and you know that there are 103 m/km. So we have (5.88 x 1012 miles) x (1 km/0.6214 miles) x (103 m/km) = 9.46 x 1015 m = 9.46 Quadrillion meters Scale Models (16 points) Suppose you wish to construct a scale model of the Universe.

The Sun has an actual diameter of about 1.5 x 106 km, and you represent it by a dot the size of a period (0.5 mm in diameter). The average distance between stars in our region of the Milky Way Galaxy is about 5 light-years. (1 light-year = 9.5 x 1012 km) What is the average distance between stars on the scale of your drawing? (Hint: use ratios, and keep track of your units! The whole point of this problem is to use ratios to get a sense of scale of space and time in the Universe. There's only one piece of math here and that's the ratio. In this scale model the Sun's diameter is 0.5 mm, let's put the numbers in meters for ease (0.5 mm x 1 m/103mm = 5 x 10-4 m). The Sun's actual size is (1.5 x 106 km) x (103 m/km) = 1.5 x 109 m.

So our ratio is: ratio = scale/actual = (5 x 10-4 m)/(1.5 x 109 m) = 3.33 x 10-13 So, in all the following calculations we will just multiply this number by the actual size to get the scale model size. For this part, the actual average distance between stars is about 5 light-years = 5 light-years x (9.5 x 1012 km/light-year) = 4.75 x 1013 km. So let's multiply it by our ratio to find out what it is in the scale model. scale size = ratio x actual = (3.33 x 10-13) x (4.75 x 1013 km) = 1.58 x 101 km = 15.8 km Think about it, if the Sun were the size of a period located in Berkeley, the nearest star would be in San Francisco! The Galaxy is about 105 light-years in diameter. How large is this on your scale? Let's put this number in km again.

105 light-years x (9.5 x 1012 km/light-year) = 9.5 x 1017 km. Now multiply by the ratio. scale size = ratio x actual = (3.33 x 10-13) x (9.5 x 1017 km) = 3.16 x 105 km That's just a little less than the average distance between the Earth and the Moon! On this same scale, what is the distance to the Andromeda galaxy, about 2 million light-years away? (This is the nearest large galaxy.) Again, put the number in km. 2 x 106 light-years x (9.5 x 1012 km/light-year) = 1.9 x 1019 km. Now multiply by the ratio.

scale size = ratio x actual = (3.33 x 10-13) x (1.9 x 1019 km) = 6.3 x 106 km On this same scale, what is the distance to the most distant objects we have seen (quasars and very distant galaxies), about 13 billion light-years away? 13 x 109 light-years x (9.5 x 1012 km/light-year) = 1.2 x 1023 km. Now, multiply by the ratio. scale size = ratio x actual = (3.33 x 10-13) x (1.2 x 1023 km) = 4.1 x 1010 km = 41 Billion kilometers Compare your scaled answer in part (d) with the true diameter of the Solar System (as defined by Pluto's orbit), about 12 billion km. Which is larger, and by roughly what factor? Since the scale is 41 Billion kilometers from the Sun to the most distant reaches it is clearly bigger than the diameter of Pluto's orbit (12 Billion km).

The factor by which it is bigger is just (41 billion km)/(12 billion km) = 3.4 times Now the distance to the farthest reaches is actually a radius not a diameter and so to do this completely correct you must compare the diameter of the scale model Universe (2 x 41 billion = 82 billion) to the diameter of Pluto's orbit. So that's (82 billion km)/(12 billion km) = 6.8 times Either answer will get you full credit. Now let's make a scaled down cosmic calendar. Evidence suggests that Earth is about 4.6 Billion years old. Let's scale this to one day (24 hours). This will be the same as above but with a different ratio. Let's put both times in seconds so that we can properly compare them. 24 hours x 60 minutes/hour x 60 seconds/minute = 8.64 x 104 seconds 4.6 x 109 years x (3.16 x 107 seconds/year) = 1.45 x 1017 seconds So, this gives us the ratio...

ratio = scale time/actual time = (8.64 x 104 seconds)/(1.45 x 1017 seconds) = 5.96 x 10-13 How many seconds ago did the USA achieve independence (1776 A.D.)? This event occurred 2000 - 1776 = 224 years ago. Let's put that in seconds. It's 224 years x (3.16 x 107 seconds/year) = 7.08 x 109 seconds ago. Now multiply by the ratio: scaled time = ratio x actual time = (5.96 x 10-13) x 7.08 x 109 seconds ago = 4.2 x 10-3 seconds ago How long ago did Rome fall (around 476 A.D.)? This occurred 2000 - 476 = 1,524 years ago.

Put that in seconds: 1.524 x 103 years x (3.16 x 107 seconds/year) = 4.82 x 1010 seconds. Now multiply by the ratio... scaled time = ratio x actual time = (5.96 x 10-13) x 4.82 x 1010 seconds = 2.87 x 10-2 seconds ago How long ago was the first city, Jericho, built (about 10,000 years ago)? 104 years ago x (3.16 x 107 seconds/year) = 3.16 x 1011 seconds. Now multiply by the ratio... scaled time = ratio x actual time = (5.96 x 10-13) x (3.16 x 1011 seconds) = 0.188 seconds ago How long ago did the first modern Homo Sapiens show up in the fossil record (about 100,000 years ago)?

105 years ago x (3.16 x 107 seconds/year) = 3.16 x 1012 seconds. Now multiply by the ratio... scaled time = ratio x actual time = (5.96 x 10-13) x (3.16 x 1012 seconds) = 1.88 seconds ago How long ago did the Dinosaurs go extinct (65 Million years ago)? 65 x 106 years ago x (3.16 x 107 seconds/year) = 2.05 x 1015 seconds. Now multiply by the ratio... scaled time = ratio x actual time = (5.96 x 10-13) x (2.05 x 1015 seconds) = 1.224 x 103 seconds ago 1.224 x 103 seconds x (1 minute/60 seconds) = 20.4 minutes ago How long ago did the first plants appear on dry land (250 Million years ago)?

250 x 106 years ago x (3.16 x 107 seconds/year) = 7.9 x 1015 seconds. Now multiply by the ratio... scaled time = ratio x actual time = (5.96 x 10-13) x (7.9 x 1015 seconds) = 4.71 x 103 seconds ago 4.71 x 103 seconds x (1 minute/60 seconds) x (1 hour/60 minutes) = 1.3 hours ago How long ago was the origin of life on Earth (about 3.8 Billion years ago)? 3.8 x 109 years ago x (3.16 x 107 seconds/year) = 1.2 x 1017 seconds. Now multiply by the ratio...

scaled time = ratio x actual time = (5.96 x 10-13) x (1.2 x 1017 seconds) = 7.16 x 104 seconds ago 7.16 x 104 seconds x (1 minute/60 seconds) x (1 hour/60 minutes) = 19.9 hours ago How Far is the Horizon? (13 points) As we discussed in class the horizon is the place where the Earth meets the sky, and the horizon is farther away the higher you get above the surface of Earth. The mean radius of Earth (sea-level) is 6,371 km. Let's say you are standing on a small (10-foot) boat out in the ocean. Your feet are essentially at sea-level. Estimate or measure the height of your eyes above your feet while standing. Calculate the distance to the horizon. In class I drew a figure similar to the one above and showed you that to do this calculation you needed only to recognize that the horizon is where your line of sight runs tangent to Earth's surface.

This is of course a line that is perpendicular to the radius of the Earth, R , and thus we can identify a right triangle with legs, dH (the distance we seek), and R . Also the triangle has a hypotenuse equal to the sum of the Earth's radius and your height above it, h. Using the Pythagorean theorem for right triangles we may then write dH2 + R dH = SQRT[(6,371 km + 1.83 x 10-3 km)2 - (6,371 km)2] dH = 4.8 km x (0.6214 miles/km) = 3.0 miles Now you hike up to Volmer peak (elevation 1,913 ft). Neglecting your own height, how far away is the horizon?

For this part h = 1,913 ft x (1 mile/5280 ft) x (1 km/ 0.6214 miles) = 0.583 km, and thus we have dH = SQRT[(6,371 km + 0.583 km)2 - (6,371 km)2] dH = 86.2 km x (0.6214 miles/km) = 53.6 miles Finally, you are flying out east to see a friend. During the flight the captain tells you that you are flying at a height of 35,000 ft. A young child is sitting in the window seat next to you looking out and wondering how far she can see from way up here. What's the answer?

For this part h = 35,000 ft x (1 mile/5280 ft) x (1 km/ 0.6214 miles) = 10.7 km, and thus we have dH = SQRT[(6,371 km + 10.7 km)2 - (6,371 km)2] dH = 369 km x (0.6214 miles/km) = 229 miles National Lampoon's: Vacation on the Moon (12 points) Let's say that you took a trip to the Moon. After checking in to your hotel room you walk out onto your balcony to check out the view. It's rather spectacular. Earth is up in the Southwest sky and it's a waxing gibbous, nearly full. You want to call your friends back in Berkeley on Earth to tell them about the awesome view. What phase will they see the Moon to be in? Recall from class that we discussed how the Earth appears from the Moon. In that discussion I told you that Earth exhibited phases as seen from the Moon. That could be readily seen during the demonstration with the globe and the lamp.

Also, recall that I told you that the phases were exactly opposite the ones people on Earth would be seeing for the Moon. When the Moon is New, Earth is Full, etc.. So, if you see Earth to be a waxing gibbous from your window, then the Moon is a waning crescent. Think about it. If the Earth is a waxing gibbous then that means it will soon be full. If it will soon be full then the Moon must be viewing it from the position in its orbit where it is between the Earth and Sun. That is when Earth will see the Moon as New. The phase just previous to New is the waning crescent. The phone company will only allow you to make the call when the Moon is directly overhead in the location you are calling on Earth. What general time of day will that be in Berkeley? (i.e. noon, midnight, dawn, dusk, or sometime between those) Since the Moon is a waning crescent it is between 90° and 0° West of the Sun as seen from Earth (Berkeley included). Thus it rises after midnight and before sunrise. It is therefore overhead in the morning sometime before noon and after sunrise.

You're kinda tired and would like to take a nap, but the Earth is pretty darn bright and is shining a lot of light into the room. You have difficulty sleeping with that much light in the room. How long will you have to wait until Earth sets over the horizon? Forever! Recall in class how I explained that the Moon always presents the same face to Earth. It spins only once per orbit about the Earth. Its solar day is the same as the synodic month. That means that if you are in a location where the Earth is in the sky, it is always there! It never rises, sets, or moves at all. The only change it goes through is the phases. So pull the shades if you want that nap. Eclipse (11 points) During an annular eclipse of the Sun, the observer sees a ring (Latin annulus) of light, which is the rim of the Sun's photosphere. In other words, the Moon's angular size isn't quite as large as that of the Sun, so one gets a special kind of partial eclipse.

When in Earth's elliptical orbit around the Sun and the Moon's elliptical orbit around Earth would this phenomenon reach an extreme (i.e., Sun's angular size is as large as it gets at the same time the Moon's angular size is as small as it gets)? Explain. Since the orbits are elliptical there are times when the Moon is at it's closest approach to Earth (perigee) and places where it is farthest from Earth (apogee). In these places the Moon is at its largest and smallest apparent angular sizes, respectively. Likewise the Earth's orbit brings it to a point of closest approach (perihelion) and farthest distance (aphelion). In these places the Sun is at its largest and smallest apparent angular sizes, respectively. The angular size changes in an inverse way with distance: smaller the greater the distance, bigger the smaller the distance. So when the Moon is far enough away in its orbit that its angular size is less than that of the Sun we get an annular eclipse. This is most extreme if the eclipse were to occur when the Moon was at apogee and Earth at perihelion.

(NOTE: you need not have mentioned the words apogee and perihelion specifically) When people in New York see a total lunar eclipse, what type of eclipse do people one-fourth of the way around Earth see? (Assume that the Moon is visible in both skies.) They will also see a total lunar eclipse. This is true for two reasons. One is that since it's stated that the Moon is visible in both skies at the same time both places are seeing the same Moon and they will both see it completely covered by Earth's shadow. This was mentioned specifically in class. Secondly, since Earth's shadow is huge it takes the Moon longer than 12 hours to move all the way through it. Thus everyone on Earth will be able to witness at least part of the eclipse, but not the total eclipse (that only lasts for a maximum of 1 hour 40 minutes). Where in the World is Gilligan? (11 points) In a forgotten episode of Gilligan's Island the Professor finds a magical goat that can communicate via radio signals. They use the goat to make contact with civilization. They are saved! But first they must give someone their coordinates so that they can be found.

Not much survived the Minnow's demise, but they do have a watch that is still running on Pacific Standard Time. Plus they have a sextant and can measure angles in the sky with high accuracy. First they wait for nightfall and locate the Big Dipper and use the pointer stars Merak and Dubhe to locate Polaris. They know that Polaris is about 1° from the North Celestial Pole (I should say the professor knows that, the magical goat might also). They measure the angle of the North Celestial Pole above the horizon to be 28°. What is their latitude? As mentioned numerous times in class, the angle of Polaris above the horizon is equal to your latitude. Take a look at the following figure: In the figure, angle l is your latitude and angle p is the angle above the horizon that the North Celestial Pole makes. l + b = 90° also l + a = 90° and a + p = 90° as well. Putting those three equations together tells you that l = p. So Gilligan and company are at a latitude of 28° North.

On the next day they wait for the Sun to cross the local meridian, this happens at 3:04 PM PST, by their watch. Recall that Pacific Standard Time is the Mean Solar Time at a longitude of 120° West of the Prime Meridian. What is their longitude? For this calculation you need to recall that the Sun moves about the sky, covering 360° every 24 hours, or 15°/hr. Also recall that at a time of 12:00 noon PST the Sun is crossing the meridian as seen from the longitude of 120° West. Since the Sun does not cross Gilligan's meridian until 3 hours later that means that the Sun was East of the meridian at noon and hence they are West of 120° West Longitude. Since it was 3:04 PM when the Sun transited for them then they are 3.0667 hrs x 15°/hr = 46° West of 120° West Longitude in other words they are located at 166° West Longitude Hey Baby, What's Your Sign?

(12 points) As we discussed in class the Sun signs of Astrology are out of date. The precession of Earth's axis has altered the constellation that the Sun is in during any given month. Additionally, the International Astronomical Union recognizes 13 constellations in the Zodiac. Using an online star map generator, or the planetarium software provided with your text or in class, determine what constellation the Sun was in at the time of your birth. Is this the same as the Sun sign given for you in the newspaper (check out any newspaper and site the source)? If you took note of your actual sign when I put up the overheads in class that's fine as well. Some of you may have found that you still had the same sign whereas others would have shifted over one sign. For example, My birthdate is September 18th, 1973. That put me just at the cusp of Virgo in the old astrologers' horoscopes and just at the other cusp in the modern constellation recognized by astronomers. But I remain a Virgo. Read the Horoscope of your real Sun sign from at least three different newspapers or websites. If you are an Ophiuchus (Nov.

30th - Dec. 17) just read the one for the date of your birth printed in the paper (either Sagittarius or Scorpius). Make a copy of the three horoscopes and turn them in. How well do they agree? Do they actually make any predictions? If so, did they come true? You will likely have found that the predictions did not agree well, at times even being contradictory. Most of these so-called predictions were also likely just advice and not a prediction at all. Any predictions that were made were likely so vague as to have any interpretation to make them appear accurate in retrospect. Here is one example of a horoscope for Virgo on Tuesday, May 30th: VIRGO (Aug. 23 -Sept. 22) Tackling major household projects could be your main area of focus today as things have been put off for as long as you, or the spouse. are willing to put up with. even though you are as industrious as the busiest beaver don't forget to take some time out to rest during the heat of the day. Well that was not true at all since I didn't spend much of that day at home, and the last bit is just advice not a prediction.

Here's another for 05/30/2000: Actions speak louder than words, so get busy. Don't wait for approval--go ahead even if the light's red. Hmmm, I don't even know what that's supposed to mean. Certainly didn't apply to anything I did that day. Here's one last horoscope from the New York Post for 5/30/2000: VIRGO (Aug. 24-Sept. 23): Some think your ambitions are too lofty. Some think you are aiming too high. But according to the planets, you have what it takes to reach the top and stay there. Don't listen to those who counsel caution, because caution never got anyone anywhere. Take a few risks - they will pay off. That pretty much can apply to anyone doing anything. It's basically just advice. Not really a prediction. Going Retro (14 points) In words and in pictures show how both Ptolemy's model of the Universe and Copernicus' model of the Universe explained the retrograde motion of the planets.

In Ptolemy's model the planets are attached to circles that are attached to circles that both rotate at the same time, the larger one is rotating about the Earth. This causes a real loop-the-loop motion in space that would appear as retrograde motion to observes on Earth. See figure 3. In Copernicus' model the planets (including Earth) all revolve about the Sun in perfect circles. The retrograde motion is just an illusion of perspective when one planet is "lapped" by another. See figure 4 In Copernicus' time measurements of the Planets' positions were not sufficient to be able to differentiate one model from the other as being more accurate. Explain why, using modern scientific reasoning, we would have to go with Copernicus' model as the best explanation of nature if we lived then. In modern scientific research we use a philosophy called Occam's razor. It states that when presented with two hypotheses that explain and predict a phenomenon equally well, we should go with the more simple. There's absolutely no reason why we should prefer the more complex explanation if the simpler does just as well at explaining it.

Thus when it was seen that Copernicus' model did just as well at predicting the positions of the planets we should have chosen it for its simplicity, even though it went against our most fundamental beliefs and prejudices. Return to Main Homework Page Science on the Brain - How far is a lightyear? How far is a lightyear? In the video, you have a chance to see how far a lightyear is. It really is an incredibly long distance -- a lightyear is roughly 6 trillion miles or nearly 10 trillion kilometers. Here's how you calculate it. You start with the speed of light, which is about 186,000 miles per second. Then you multiply that by 3,600 seconds in an hour. Then you multiply that by 24 hours in a day. Then you multiply that by 365 days in a year. And you get: 186,000 * 3,600 * 24 * 365 = about 6 trillion miles. Scientists use lightyears to measure the really long distances between stars and galaxies. For example, a spiral galaxy close to our Milky Way galaxy is called the Andromeda galaxy.

It is 2,200,000 lightyears away. One advantage of using the lightyear as a unit of distance is that you automatically know how long it took the light from the Andromeda galaxy to reach earth. We know that when we look at the Andromeda galaxy today, we are seeing light that was created 2,200,000 years ago! There are some galaxies that are billions of lightyears away. Because 6 trillion miles is such a huge number, we really need to break it down to understand just how far it is. So let's start by looking at a light second. Since light moves at 186,000 miles per second, a lightsecond is 186,000 miles. The moon is about 1.3 lightseconds away. Since the circumference of the earth is about 25,000 miles, light could travel all the way around the earth 7 and a half times in one second. A millisecond is a thouandth of a second. So a lightmillisecond is 186 miles. This is a distance we all can find easier to understand.

In a thousandth of a second, light travels 186 miles. A microsecond is a millionth of a second. So a light microsecond is about 0.186 miles, or roughly 1,000 feet. A nanosecond is a billionth of a second. So a light nanosecond is about one foot, or 30 centimeters. Every nanosecond, light travels one foot. This is actually important in the computer industry, because lots of things inside modern computers can happen in a nanosecond or less. The maximum distance that light can travel in a nanosecond controls how big and fast computers can be. What if we go the other direction? There is the lightminute. It is 60 lightseconds, or about 11 million miles. And a lighthour is 60 lightminutes, or about 670 million miles. Our solar system is about 8 lighthours across. In other words, it takes light roughly four hours to get from the sun to Neptune. You can really impress your friends with this information.

The next time someone says, "Hey, I just rode my bike 10 miles!", you can say, "that's about 53 lightmicroseconds. It would take light 53 microseconds to go 10 miles." This table is a quick summary: lightnanosecond How far ? How big ? How many ? What is the Sun's diameter ? 110 times the Earth's diameter. What is the Sun's mass ? 330 000 times the mass of the Earth. The Sun seen in ultraviolet light by the SOHO spacecraft. (NASA/ESA). Jupiter to scale Earth to scale How big is the solar system (Sun and planets) ? 10 light hours from one side to the other. Are there planets around other stars ? At least 80 had been found by January 2002. They were found by detecting very slight wobbles in the motion of the parent stars. One was confirmed by measuring the dimming the light of the parent star as the planet passed in front of the star. The Stars How far is the nearest star (after the Sun) ? Proxima Centauri is 4.3 light years away.

It is a dim red star orbiting the bright double star, Alpha Centauri. These nearest stars are 270 000 times further from the solar system than the Sun is from the Earth. What are the smallest stars ? Neutron stars collapse to about 15 kilometers in diameter. We observe them as pulsars. What are the biggest stars ? Red Giant stars swell up to 500 times the diameter of the Sun. Betelguese, Antares and Aldebaran are well known red giant stars. Our Galaxy, the Milky Way How far is the Sun from the middle of our galaxy, the Milky Way ? 30 000 light years. How big is the Milky Way ? 100 000 light years across. This is 100 million times the diameter of Pluto's orbit. How many stars are there in the Milky Way ? About 100 billion (= 1011). Stars and dust clouds towards the center of the Milky Way. Stars form half the detectable mass of the Milky Way. The other half is in the form of gas and dust. The Universe How far away is the nearest galaxy ?

The newly discovered dwarf galaxy in Sagittarius, only 60 000 light years away, is spiralling into the Milky Way. The nearest large spiral galaxy like the Milky Way is the Andromeda galaxy, about 2.7 million light years away. How many galaxies are there in the visible universe ? Over 2 million have been counted, but there could be about 100 000 million. How big is the visible universe ? About 15 000 million light years. Distant galaxies seen by the Hubble Space Telescope (STSCI/NASA). how long would it take us to travel to alpha centauri [Ask An Astronomer] Night Sky and Telescopes If light travels at 186,000 miles per hour and a star is 75,000 light years away how long would it take for the light from the star to reach us. And how long would it take for a flight to it. Also could you provide formula to your solution. I'll give you some hints to your problem, but I won't solve your exact problem. I hope that's okay.

By the way, light travels at 186,000 miles per SECOND, not per hour. First of all, a light-year is the distance light travels in one year. The star Alpha Centauri is about 4 light-years away, so light from Alpha Centauri takes 4 years to get to the Earth. Now, if we wanted to travel to Alpha Centauri, we would have to know how fast our spaceship is travelling. The fastest rockets move at about 20,000 mph. Suppose we launch one at 20,000 miles per hour toward Alpha Centauri. How long would it take to get there? Well, distance travelled equals speed times time, so the time it takes to travel a distance is that distance divided by the speed, or 4 light-years / (20,000 miles / hour) Now this isn't terribly illuminating. We need to convert units so that we can do the division. There are 6 trillion miles in a light-year (approximately), so the distance we need to go is 6 trillion miles / light-year times 4 light-years, or 24 trillion miles.

So, this trip would take 1.2 billion hours. There are 24 hours a day and 365.25 days per year, so this time in years is 137 thousand years. That's a long time! Now you can start to see while it is virtually impossible to visit other stars - even the closest star would take hundreds of thousands of years to reach! Thanks for writing, Kurtis Williams

Question: Approximately how many kilometres does light travel in an hour?
Answer: A billion