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Name: ________________________________ Date:___________________ Meteor Shower Observations “Now slides the silent meteor on, and leaves A shining furrow, as thy thoughts in me.” Tennyson: The Princess VII INSTRUCTIONS: Read the laboratory exercise carefully before attempting to do the laboratory. Be sure to complete the pre-observation questions hours before attempting to do the laboratory exercise. When you do your observations, remember that it is often a much lower temperature in the pre-dawn hours of the day than it is at sunset. Be sure to plan to dress accordingly. You will need your red filtered flashlight. BACKGROUND INFORMATION: Since ancient times, sky watchers have seen flashes of light in the sky that looked like extremely fast moving stars. These flashes of light in layman terms are often called shooting stars or falling stars. Carefully observing the stars, though, astronomers found that no stars were missing. Furthermore, widely separated observers would often see the shooting star at slightly different locations in the sky, indicating that the phenomenon was located high in the atmosphere, rather than among the distant stars. Astronomers, thus, believed that the shooting star was an atmospheric event, similar to lightning. The term meteor was applied to the sudden flash of light. It wasn’t until the Nineteenth Century that astronomers realized that the meteors were in fact extraterrestrial in origin. They were caused by something hitting the atmosphere moving at very high speed many miles overhead. The Solar System is composed not just of the Sun, planets, and moons. There are a large number of small objects orbiting the Sun. These objects range in size from microscopic particles of dust to larger bodies hundreds of miles across. The larger objects are called asteroids if they are rocky or iron rich in nature and comets if they are composed of significant amounts of ice and frozen gasses. Most comets are believed to be a mixture of ices and rocky material. Smaller objects are called meteoroids. There is not an exact size distinction between a meteoroid and an asteroid, so many of the larger meteoroids are often referred to as small asteroids. Very small meteoroids are sometimes called micrometeoroids. Most of the meteoroids orbiting the Sun are very small, typically the size of sand grains or rice grains. Orbital motions around the Sun are very fast, with the Earth moving at 29.8km/s around the Sun. Many meteoroids can move faster than this. Thus, when they encounter the Earth, they can be moving very fast. The highest speeds come from meteoroids moving quickly towards the Earth and hitting it head on as the Earth orbits the Sun. As a meteoroid strikes Earth’s atmosphere, friction with the atmosphere heats up both the meteoroid and the atmosphere through which it is passing. Small meteoroids often do not survive the heating intact, and they burn up. As they pass through the atmosphere, however, the meteoroid is called a meteor. Passing through the atmosphere, the meteoroid ionizes and heats the air through which it is passing. This leaves a tube of hot ionized air that briefly glows behind the meteor. It is 1 this trail of light that observers from the ground usually see as a shooting star. In addition to be used to refer to the object passing through the atmosphere, the term meteor is also sometimes used to refer to the flash of light generated in the atmosphere by the passage of the meteoroid. Should the meteoroid be large enough when it strikes the atmosphere, or be going slow enough to avoid completely burning up, then it can survive the passage through the atmosphere to strike the ground. A meteoroid that strikes the ground is called a meteorite. Not all meteoroids are the same size. When a somewhat larger one hits the atmosphere, it is often much brighter than the typical meteor. Such a very bright meteor is often called a fireball or a bolide. The term bolide is the more technical term. Occasionally, a bolide can result when a meteoroid is large enough to survive the trip to the ground and become a meteorite. Sometimes bolides are associated with a popping or crackling sound as it passes by at supersonic velocities. Most of the meteors that survive long enough to pass low enough into the atmosphere to be heard from the ground yield meteorite falls. With bolides, and sometimes with smaller meteors, a train can remain after the passage of the meteor. This train appears as glowing trail through the sky marking the path of the meteor. The train is due to air ionized by the passage of the meteor. Usually the train lasts just seconds after the passage of a meteor, but sometimes it can last several minutes. Such a long lasting train is called a persistent train. The likelihood of forming a train depends upon the speed of the original meteoroid, angle of impact with the atmosphere, and size and composition of the meteoroid. Most meteors are simply random bits of material orbiting the Sun that run into the Earth. These are called sporadic meteors. As the Earth goes around the Sun, more meteors hit the front of the Earth in it orbit, just as more bugs hit the front of a car driving down the highway. The front of the Earth is defined as the side of the Earth that is facing the direction that the Earth is moving around the Sun. An observer is on the front side of the Earth from local midnight until local noon. From a dark, clear location, about five or six such sporadic meteors per hour can be seen on any given night between local midnight and dawn, with the most meteors generally just before dawn. Prior to midnight, fewer sporadic meteors are seen. In general, slightly more sporadic meteors tend to be seen during the summer than during winter months. Beginning in 1833, astronomers noticed that sometimes there are periods on greater than average meteor activity. Such periods of time are called meteor showers, and are caused when Earth passes through a swarm of meteoroids. The meteoroid swarms are generally believed to be remnants of material ejected from comets. During a meteor shower, meteors strike Earth’s atmosphere from the same direction. Due to our perspective on Earth, the meteors appear to radiate away from one point in space. This point is called the radiant of the meteor shower. Not all meteoroid swarms are evenly distributed. Most, in fact are rather clumpy, meaning that sometimes there are more meteors in a shower than other times. If the Earth crosses the orbit of the meteoroid swarm every year, then the shower is called an annual meteor shower. 2 Meteor showers were not recognized until about 1833 when a meteor shower was observed with meteors radiating away from the constellation Leo in mid November. By 1837, astronomers had also observed that there was a meteor shower in mid August with a radiant in the constellation Perseus. Over the years, astronomers have observed numerous meteor showers. To designate which meteor shower is which, the meteor shower is generally designated by the location of its radiant. For example, the meteor shower with a radiant in Leo is called the Leonid meteor shower. The individual meteors are called Leonids. The meteor shower with a radiant in Perseus is called the Perseid meteor shower. If more than one meteor shower had a radiant in a constellation, then the meteor shower is often named for the nearest star. For example, there are two meteor showers with radiants in Aquarius: the Eta Aquarid shower (radiant near η Aqr), and the Delta Aquarid shower (radiant near δ Aqr). One meteor shower, the Quadrantid meteor shower that occurs in January, is named for a constellation that is no longer recognized by astronomers. Just as sporadic meteors tend to be more numerous in the predawn hours, so do shower meteors. A few meteoroid swarms, however, catch Earth from behind. Most notably the Arietid Meteor shower catches Earth from behind in June of each year. This results in evening meteors rather day predawn meteors. This is the exception, though. Not all meteoroid streams are alike. Some are dense and some are fairly thinly distributed. Thus, not all meteor showers display the same number of meteors. The most common way to describe the activity of a meteor shower would be to describe the number of meteors that an observer would see under a dark clear sky if the radiant were directly overhead. This is called the zenithal hourly rate (ZHR). Typical major meteor showers have at ZHR of about 30 to 40 meteors per hours. Some are more impressive with a ZHR of nearly 100 meteors per hour. Note that the ZHR is a calculated value, and does not really represent the actual number of meteors that an observer would expect to see, unless the radiant were near the zenith and the shower were being observed from a dark sky. Thus, for a typical observer, with some light pollution, and the radiant near the horizon, a meteor shower with a ZHR of 80 might only produce a visible meteor every couple of minutes. Minor meteor showers have a ZHR of only 5 to 10. This is roughly the same number of meteors from the shower during optimal viewing conditions that one would expect from sporadic meteors! On extremely rare occasions, the Earth passes through a particularly dense meteoroid stream, and the ZHR of the resulting meteor shower can climb to values in excess of 1000 meteors per hour (about one every few seconds). In this case, the meteor shower is generally called a meteor storm. Only a handful of meteor storms have been recorded in history. Meteoroid streams come in a variety of sizes and distributions of particles. Some meteoroid streams are very broad, and it takes the Earth several days to cross the stream. In such cases, observers may see meteor activity from the shower for several nights in a row. In other cases, the meteoroid streams are very narrow. In such a case, the meteor shower may show peak activity for just one night, or even for as short a period of time as just one hour. Some meteoroid streams have meteoroids that have dispersed throughout the stream’s orbit. In this case, there will be a consistent meteor shower every year as the Earth crosses the meteoroid stream orbit. In other cases, the meteoroids are bunched together into swarms. There may be a minor meteor shower each year as the Earth crosses the stream’s orbit, but a major shower happens only when Earth is crossing the 3 orbit at the same time as the swarm happens to be in that part of its orbit. The Leonid meteor shower is an example of a meteor shower that is normally a very minor shower, but exhibits major activity about every 33 years or so when Earth passes through denser portions of the meteoroid stream. Different meteoroids have slightly different compositions and strike the Earth’s atmosphere at different speeds. Not all produce trains. Furthermore, not all meteors are simply white, but some meteors exhibit color. A few are orange or yellow. Others appear red, green, or blue. A great many do, in fact appear as simply white. The colors are due in part to the composition of the meteor, but also due to the speed at which they enter the atmosphere. In addition to color, some meteoroids, particularly the slightly larger ones, can break apart as they pass through the atmosphere. This is seen as a meteor sometimes breaking into two or more pieces that then diverge in their paths across the sky. Sometimes, bits and pieces of a meteoroid break off as it flies through the sky, giving an impression of a shower of sparks coming off of the meteor. This is particularly common in bolides. OBSERVING METEORS: Observing meteors is one of the easiest things in astronomy. You just look up at the sky! Any night of the year, at any time of the night, there will be occasional sporadic meteors. If you happen to be looking in the right part of the sky at the right time, and if the sky is dark enough, then you will see the meteor. Your chances of seeing meteors improves if you happen to be looking up after local midnight. If your goal is to observe meteors, it is generally not very productive to try to observe prior to local midnight. Also, as indicated earlier, there are slightly more meteors in the summer months than in the winter months. Winter nights are longer, though, giving you more time to look. Winter nights are also often clearer than summer nights, especially after a hard cold front drives the temperature below freezing, helping to drive moisture out of the lower atmosphere. Clearer, darker nights tend to allow you to see dimmer meteors, the greater number of which offsets the fewer total meteors seen in the summer. Obviously, if you happen to go out on the night of a meteor shower, then your odds of seeing a meteor greatly improve! During the meteor shower, you will see not only the meteors of the shower, but the usual complement of sporadic meteors, too. If you wish to look at meteors from a particular meteor shower, they you should look on a star chart, star wheel, or appropriate computer program to see when the radiant of the meteor shower rises. You are unlikely to see many meteors when the radiant is below the horizon. You should then situate yourself where you can observe the sky. Ideally, you wish to look at a portion of the sky 45 to 60 degrees away from the radiant. If you look right at the radiant, any meteors that you see will be moving straight at you. They will appear as brief flashes like stars. These are called point meteors, and are generally much less impressive than the ones that shoot across the sky, sometimes leaving trains. Meteor astronomy differs a bit from many other forms of astronomy in that you do not need a telescope in order to observe meteors. While telescopes or binoculars would allow you to see dimmer meteors, such instruments restrict your view of the sky to very small areas, thus you miss most of the brighter meteors. Most recognized meteor showers result from swarms of meteoroids that produce fairly bright meteors, so you 4 don’t gain much by being able to see dimmer ones. Thus, the best way to observe meteors is with the naked eye. Additionally, since the normal human reaction is to have one’s attention drawn to motion, just looking into a region of the sky allows you to see meteors not just where you are concentrating, but in nearby areas, too. As long as your peripheral vision encompasses a portion of the sky, your attention will be drawn to the meteor. Thus, the best way to look is to look upwards at a steep enough angle that your entire field of vision is on the sky. This is best accomplished by laying either on the ground or on a blanket or lawn chair inclined at 45 degrees or more to the vertical. Don’t worry about looking directly towards the radiant. The best views are generally some distance in the sky from the radiant. The best thing is to look overhead. Though most recognized meteor showers tend to have fairly bright meteors, this statement means fairly bright compared to stars, not streetlights. The more light pollution, the fewer meteors that you will be able to see. The brighter the moon light, the fewer meteors that you will see, too. If possible, observe when the moon is not up. Also, if possible, find a location with as little light pollution as possible. Even from fairly light polluted areas, though, you may be able to see several meteors per hour during a meteor shower, so don’t completely give up if you can not find a dark location, especially if a major meteor shower is predicted. Finally, since meteor observing is best done in the predawn hours, lots of sleep the night before, or at least something to drink such as coffee, cola, or hot chocolate to keep the observer awake is often useful. It is also very useful to have a friend or family member observing with you. Conversation (as long as you don’t look away from the sky) helps to keep you awake. Also, the predawn hours are the coldest part of the day. Be prepared to dress as warmly as possible. It can be MUCH colder just before dawn than it is in the afternoon, especially in the winter. Since meteor observing does not involve much activity, then you tend to lose heat very readily. In the winter, you should dress like you are going on an Arctic expedition. If you wear too much, you can always take something off. If your dress too lightly, then you are just cold and uncomfortable. Even in the summer, it can become somewhat chilly just before dawn, especially if there is a light wind. Some Significant Meteor Showers Quadrantids Lyrids Eta Aquarids Arietids Zeta Perseids (Early January) (Late April) (Early May) (Mid June) (Mid June) South Delta Aquarids Perseids Orionids Leonids Geminids 5 (Late July) (Mid August) (Late October) (Mid November) (Mid December) METEOR OBSERVATION LABORATORY Equipment Needed: Red Filtered Flashlight Clipboard (or other way of writing) Pen or Pencil Correctly Set Timepiece (You can set watch to the US Naval Observatory Website Clock.) Prior to doing the laboratory, some preliminary work is needed. It is important that you do this in order to do the best job with doing the laboratory. 1) Determine what meteor shower you are observing, and what constellation the radiant will be coming from. Meteor Shower: ______________________ 2) 3) 4) Radiant Constellation:___________ Use a star chart or star wheel to determine several easily identifiable stars visible at the time that you will be observing the meteor shower. Use a star chart, almanac, computer software, or other means to determine the approximate magnitudes of these stars. ___________________________ ___________________________ ___________________________ ___________________________ ___________________________ ___________________________ Next, you will observe the meteors. Record your observations on the enclosed observing sheet. If you should fill up the sheet, then use your own paper to continue the observations. For each meteor that you observe, note the time of observation, the direction that the meteor was moving (eg: southwest), and you must estimate the magnitude of the meteor by comparing to the stars that you have previously identified. You may write the name of the star that is most nearly the same magnitude as the star, and later look up the magnitude, if you wish, or if you know the magnitude, then write down the magnitude directly. During a meteor shower, you will likely see the usual sporadic meteors as well as the meteors of the shower. Note on your observation sheet whether the meteor is part of the shower or is a sporadic meteor (shower meteors will be the ones coming from the radiant). Also, you need to note the color of the meteor. Finally, note any interesting things that you might observe, such as a train or “sparkling” effect. Finally, you wish to determine the hourly rate of the meteor shower. To do this, pick two reference times some time apart (preferably over an hour apart). Then count how many shower meteors occurred between the reference times. Divide the number of meteors by the time in hours to get the average hourly rate. Reference Time One: ________________ Reference Time Two: ________________ Time Difference ___________ hours Number of Shower Meteors: ______________ Hourly Rate :_____________ 6 METEOR OBSERVATIONS Time Shower/ Sporadic? Direction Color 7 Mag. Comments METEOR OBSERVATIONS Time Shower/ Sporadic? Direction Color 8 Mag. Comments