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[sci.astro] General (Astronomy Frequently Asked Questions) (2/9)

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Subject: Introduction sci.astro is a newsgroup devoted to the discussion of the science of astronomy. As such its content ranges from the Earth to the farthest reaches of the Universe. However, certain questions tend to appear fairly regularly. This document attempts to summarize answers to these questions. This document is posted on the first and third Wednesdays of each month to the newsgroup sci.astro. It is available via anonymous ftp from <URL:>, and it is on the World Wide Web at <URL:> and <URL:>. A partial list of worldwide mirrors (both ftp and Web) is maintained at <URL:>. (As a general note, many other FAQs are also available from <URL:>.) Questions/comments/flames should be directed to the FAQ maintainer, Joseph Lazio (
Subject: B.00 General [Dates in brackets are last edit.] B.01 What good is astronomy? [1997-08-06] B.02 What are the largest telescopes? [2000-04-04] B.03 What new telescopes/instruments are being built? [2000-01-01] B.04 What is the resolution of a telescope? [1995-08-23] B.05 What's the difference between astronomy and astrology? [1995-08-23] B.06 Is there scientific evidence for/against astrology? [1995-08-23] B.07 What about God and the creation? [1995-08-27] B.08 What kind of telescope should I buy? [2001-01-17] B.09 What are the possessive adjectives for the planets? [1995-12-05] B.10 Are the planets associated with days of the week? [2000-11-12] B.11 Why does the Moon look so big when it's near the horizon? [1997-01-21] B.12 Is it O.K. to look at the Sun or solar eclipses using exposed film? CDs? [1996-11-20] B.13 Can stars be seen in the daytime from the bottom of a tall chimney, a deep well, or deep mine shaft? [1996-06-14] B.14 Why do eggs balance on the equinox? [1996-06-14] B.15 Is the Earth's sky blue because its atmosphere is nitrogen and oxygen? Or could other planets also have blue skies? [1998-02-06] B.16 What are the Lagrange (L) points? [2003-10-18] B.17 Are humans affected psychologically and/or physically by lunar cycles? [2000-06-03] B.18 How do I become an astronomer? What school should I attend? [1996-07-03] B.19 What was the Star of Bethlehem? [2002-05-04] B.20 Is it possible to see the Moon landing sites? [2003-09-18]
Subject: B.01 What good is astronomy anyway? What has it contributed to society? Author: many This question typically arises during debates regarding whether a government should spend money on astronomy. There are both pratical and philosophical reasons that the study of astronomy is important. On the practical side... Astronomical theories and observations test our fundamental theories, on which our technology is based. Astronomy makes it possible for us to study phenomena at scales of size, mass, distance, density, temperature, etc., and especially on TIME scales that are not possible to reproduce in the laboratory. Sometimes the most stringent tests of those theories can only come from astronomical phenomena. It must be understood that these theories influence us even if they don't tell us that we can invent new things, because they can tell us that we can't do certain things. Effort spent on astronomy can prevent effort wasted trying to come up with antigravity, for instance. Astronomy provided the fundamental standard of time until it was superseded by atomic clocks in 1967. Even today, astronomical techniques are needed to determine the orientation of the Earth in space, e.g., <URL:>. This has military applications but is also needed by anyone who uses the Global Positioning System (GPS). Furthermore, it may be that millisecond pulsars can provide an even more stable clock over longer time scales than can atomic clocks. Closely related is navigation. Until relatively recently (post-WW II) celestial navigation was the ONLY way in which ships and aircraft could determine their position at sea. Indeed, the existence of navigation satellite systems today depends heavily on the lessons learned from aspects of astronomy such as celestial mechanics and geodesy. Even today, in the UK, RAF crews and RN officers need to learn the rudiments of celestial navigation for emergency purposes; until the late 1990s so did US Naval officers. Astronomical phenomena have been important in Earth's history. Asteroid impacts have had major effects on the history of life, in particular contributing to the extinction of the dinosaurs and setting the stage for mammals. The Tunguska impact in 1908 would have had a far greater effect if it had occurred over London or Paris as opposed to Siberia. The debate over the magnitude, effect, and cost of greenhouse warming is motivated, in part, by research on Venus. Astronomy has prompted study of the Earth's climate in other ways as well. The study of the atmospheres of other planets has helped to test and refine models of the Earth's atmosphere. The Sun was fainter in the past, an important constraint on the history of the climate and life. Understanding how the Earth's climate responded to a fainter Sun is important for evolution and for the progress of climate modelling. More generally, there is weak evidence that solar activity influences climate changes (e.g., variations in sunspot cycle, the Maunder minimum, and the Little Ice Age) and therefore is important in the greenhouse warming debate. (This is by no means proven by current evidence but *may* prove to be important.) The element helium was discovered (in a real sense) and named, not by chemists, but by astronomers. In addition to making many birthday parties more festive, liquid helium is useful for many low-temperature applications. Solar activity affects power-grids and communications (and space travel). Prediction is therefore important, indeed is funded by the U.S. Air Force. Many advances in medical imaging are due to astronomy. Even the simple technique that astronomers used for decades, of baking or otherwise sensitizing photographic materials, was slow to catch on in medical circles until astronomers pointed out that it could reduce the required x-ray dose by more than a factor of 2. Many of those now involved in some of the most advanced developments of medical imaging and imaging in forensics were trained as astronomers where they learned the basic techniques and saw ways to apply them. More recently, image reconstruction of the flawwed Hubble images led to earlier detection of tumors in mammograms (see back issues of Physics Today). While we don't yet have a good method for predicting earthquakes, the techniques of Very Long Baseline Interferometry are used routinely to measure ground motion. Interferometry has also led to the development of Synthetic Aperture Radar. Today SAR is used for earth remote sensing. Applications include mapping sea ice (safety of ships, weather forecasting) and ocean waves (ditto), resource location, agricultural development and status checks. Jules Verne would never have written "From the Earth to the Moon" without astronomy. Astronomy helped spawn science fiction, now an important component of many publishing houses and film studio productions. There has been a complex interplay between scientific, military, and civil users, but astronomy has played an important role in the development of such things as security X-ray systems (like those at airports), electro-optics sensors (security cameras, consumer video cameras, CCDs, etc.), and military surveillance technology (like spy satellites). On the philosophical side... Perhaps the most important aspect of being human is our ability to acquire knowledge about the Universe. Astronomy provides the best measure of our place in the Universe. In this century, the ability of astronomy to test General Relativity led directly to Karl Popper's distinction between science and pseudo-science and from there to the way intellectuals (at least) look at science. Astronomy's support of modern physics (such as quantum mechanics) in this century had have important influences on general philosphical and intellectual trends. The "Earthrise" photo, of the Earth rising over the Moon's horizon, from an Apollo mission is often credited as being partially responsible for driving environmental and "save the planet" impulses. In previous centuries, astronomy led to Copernicanism and subsequent "Principle of Mediocrity" developments---that the Earth, and by extension, humans, is not at the center of the Universe. Eliminating geo- and human-centred perspectives was a major philosophical leap. Astronomy's support of a mechanistic universe in the 19th century had important influences on general philosphical and intellectual trends. In general, but certainly more vaguely, the last century of astronomy has provided many supports to the view that the scientific method is capable of answering many questions and that naturalistic thinking can explain the world. Thus, scientists can answer many creation questions (e.g., where metals come from, why the Sun shines, why there are planets).
Subject: B.02 What are the largest telescopes? Author: Bill Arnett <>, William Keel <>, Joseph Lazio <>, Steve Willner <>, Jennifer Imamura The "largest" telescope is a bit difficult to determine. One can obtain many different answers, depending upon the adjectives placed in front of "largest." Nonetheless, what follows is one such list. A list of astronomical instruments is also at <URL:>, and a list of large optical telescopes is at <URL:>. A list of space-based observatories is at <URL:>. (Optical/Infrared telescopes, nighttime) The list below gives the largest optical telescopes operating today. For complicated pupil shapes, the effective aperture diameter is given. Location is geographic; we omit most organizational details, amusing and intricate as they may be. The list has been truncated at 3 m because there are so many telescopes of that size or smaller. URL's are given where known. Aperture Name Location 10.0 Keck I Mauna Kea, Hawaii (mirror composed of 36 segments) <URL:> 6.5 Multiple Mirror Mt. Hopkins, Arizona (6 mirrors, 1.8 m each; see also B.03) <URL:> 6.0 BTA Nizhny Arkhyz, Russia (Bolshoi Teleskop Azimutalnyi = Large Altazimuth Telescope) <URL:> 5.0 Hale Palomar Mountain, California <URL:> 4.2 William Herschel La Palma, Canary Islands <URL:> 4.0 Victor Blanco Cerro Tololo, Chile <URL:> 4.0 Mayall Kitt Peak, Arizona <URL:> 3.9 Anglo-Australian Siding Spring, Australia <URL:> 3.8 UK Infrared Mauna Kea, Hawaii <URL:> 3.6 ESO Cerro La Silla, Chile <URL:> 3.6 Canada-France-Hawaii Mauna Kea, Hawaii <URL:> 3.5 New Technology Cerro La Silla, Chile <URL:> 3.5 MPI-CAHA Calar Alto, Spain <URL:> 3.5 ARC Apache Point, New Mexico (mostly remote control) <URL:> 3.5 WIYN Kitt Peak, Arizona <URL:> 3.5 Starfire Kirtland AFB, New Mexico <URL:> 3.0 Shane Mount Hamilton, California <URL: > 3.0 NASA IRTF Mauna Kea, Hawaii <URL:> Other telescopes of note: Solar Telescope: Global Oscillation Network Group (GONG), six sites around the world for velocity imaging Largest single dish radio telescope: Arecibo Observatory (Nat. Astron. & Ionosphere Center, Cornell U.) 305-m, Puerto Rico <URL:> Largest fully-steerable single dish radio telescope: Max Planck Institut fuer Radioastronomie, 100 m, Effelsburg, Germany <URL:> Largest millimeter wave radio telescope: Nobeyama Radio Observatory, 45m, Japan <URL:> Largest sub-millimeter radio telescope: James Clerk Maxwell Telescope (Joint Astron. Center = UK, Canada, Netherlands), Mauna Kea, 15 m <URL:> Largest (connected-element) radio interferometric arrays: Very Large Array (NRAO, New Mexico), 27 dishes, each 26.4 m effective diameter The maximum separation between antennas is ~35 km. <URL:> MERLIN (NRAL, University of Manchester, UK) up to 8 dishes, various specifications. The maximum separation between antennae is 217 km (between the Cambridge and Knockin dishes). <URL:> [MERLIN actually uses radio links between the antenna elements, so maybe it should go into a separate category.] Longest-baseline (dedicated) radio interferometric array: Very Long Baseline Array (NRAO), 10 dishes, each 26.4 m effective diameter, United States. The maximum separation between antennas is ~8600 km, between the islands of St. Croix and Hawaii. <URL:> HALCA (ISAS), 8 m dish, in Earth orbit <URL:> Infrared: Infrared Space Observatory (ISO) (ESA) <URL:> Ultraviolet: Extreme Ultraviolet Explorer (EUVE) (NASA) <URL:> International Ultraviolet Explorer (IUE) [defunct] (NASA, PPARC and ESA) <URL:> X-ray: Chandra, the Advanced X-ray Astrophysics Facility (NASA) <URL:> X-Ray Astronomy Satellite (SAX) (ESA) <URL:> X-Ray Timing Explorer (XTE) (NASA), 2 instruments: PCA & HEXTE <URL:> ASCA/ASTRO-D (ISAS) <URL:> Roentgen Satellite (ROSAT) (MPE) <URL:> Einstein, the second High Energy Astronomy Observatory (HEAO-B) [defunct] (NASA), 5 instruments: IPC, HRI, SSS, FPCS, & OGS <URL:> Gamma-ray: Fred Lawrence Whipple Gamma-Ray Observatory (SAO), a 10 m and 11 m instrument <URL:> CANGAROO (U. Adelaide & Nippon), 4 4-m cameras <URL:> Compton Gamma-Ray Observatory (NASA) [space-based], 4 instruments: OSSE, EGRET, COMPTEL, & BATSE <URL:> Cosmic ray: The High Resolution Fly's Eye Cosmic Ray Detector HiRes <URL:>
Subject: B.03 What new telescopes/instruments are being built? Author: Bill Arnett <>, William Keel <>, Steve Willner <>, Joseph Lazio <>, Jennifer Imamura with corrections and additions by many others (These lists are undoubtedly incomplete. Additions and corrections welcome!) A list of astronomical instruments is also at <URL:>. Optical/Infrared Telescopes (nighttime): Now actually under construction: 16.4 Very Large Telescope Cerro Paranal, Chile (quartet of 8.2-m telescopes) <URL:> 11.0 Hobby-Eberly Telescope, Mt. Fowlkes, Texas (spectroscopy only) <URL:> <URL:> 8.0 Gemini North Mauna Kea, Hawaii 8.0 Gemini South Cerro Pachon, Chile <URL:> 8.2 Subaru (JNLT) Mauna Kea, Hawaii <URL:> 6.5 MMT Mt. Hopkins, Arizona (replace current six mirrors with single one; see B.01) <URL:> 2.2 SOFIA NASA (included because it will be an airborne observatory) <URL:> Others likely to start soon: Large Binocular Telescope, (Italy; U. Arizona), pair of 8-m telescopes, Mt. Graham, Arizona <URL:> Canary Islands Large Telescope Canary Islands, Spain, 10 m segmented mirror <URL:http//> Magellan (Carnegie Institution Observatories), 6.5 m, Las Campanas <URL:http//> Radio telescopes under construction in design stages: Submillimeter Array, (Smithsonian Astrophysical Observatory), six 8-m dishes at Mauna Kea <URL:http//> Millimeter Array (MMA) (NRAO) <URL:http// Green Bank Telescope (NRAO) <URL:http//> X-ray: Astro-E (ISAS) <URL:http//> High-Throughput X-Ray Spectroscopy Mission (ESA) <URL:http//> Gamma-ray: INTEGRAL (ESA) <URL: > Neutrino: Antarctic Muon and Neutrino Detector Array (AMANDA) <URL:http//> Deep Undersea Muon and Neutrino Detection (DUMAND) <URL:http//> Gravitational Waves: LIGO, (US), 4 km path <URL:http//> Virgo, (Italy), 3 km path <URL:http//>
Subject: B.04 What is the resolution of a telescope? Author: Steve Willner <> The _limiting_ resolution of a telescope can be no better than a size set by its aperture, but there are many things that can degrade the resolution below the theoretical limit. Obvious examples are manufacturing defects and the Earth's atmosphere. Another interesting one is the addition of a central obstruction (e.g., secondary mirror) which degrades the resolution for most practical purposes even though it _shrinks_ the size of the central diffraction disk. The problem is that even though the disk diameter decreases, the central disk contains a smaller fraction of the incident light (and the rings contain more). This is why modest sized refractors often outperform reflectors of the same size. Giving a precise value for the resolution of an optical system depends on having a precise definition for the term "resolution." That isn't so easily done; the most general definition must be based on something called "modulation transfer function." If you don't want to be bothered with that, it's enough to note that in all but pathological cases, the diameter (full width at half maximum in radians) of the central diffraction disk will be very close to the wavelength in use divided by the diameter of the entrance pupil. (The often seen factor of 1.22 refers to the radius to the first null for an _unobstructed_ aperture, but a different factor will be needed if there is a central obstruction.) In practical units, if the wavelength (w) is given in microns and the aperture diameter (D) in meters, the resolution in arcseconds will be: R = 0.21 w/D .
Subject: B.05 What's the difference between astronomy and astrology? Author: Phillippe Brieu <> Although astronomy and astrology are historically related and many individuals were interested in both, there is today no connection between the two. Hence two different USENET newsgroups exist: sci.astro (for the former) and alt.astrology (for the latter). DO NOT CONFUSE THEM. Astronomy is based on the laws of physics (and therefore mathematics) and aims at describing what is happening to the universe based on what we observe today. Because the laws of physics are constant (as far as we can tell), astronomy can also explain how the universe behaved in the past and can propose a limited number of possible scenarios for its future (see FAQ entry about Big Bang). Everyday life applications of astronomy include calculations/predictions of sunrise/sunset times, moon phases, tides, eclipse locations, comet visibility, encounters between various celestial bodies (e.g., SL9 comet crash onto Jupiter in 1994), spacecraft trajectories, etc. Astrology on the other hand claims it can predict what will happen to individuals (or guess what is happening to them), or to mankind, based on such things as solar system configurations and birth dates. Common applications include horoscopes and such. Regardless of whether there is scientific support for astrology, its goal and methods are clearly distinct from those of astronomy.
Subject: B.06 Is there scientific evidence for/against astrology? Yes, but this question should be discussed in alt.astrology and/or sci.skeptic, not in sci.astro.
Subject: B.07 What about God and the creation? Author: Joseph Lazio <> Astronomy is silent on the matter of God and the creation. Astronomy is based on applying the laws of physics to the Universe. These laws of physics attempt to describe the natural world and are based on experiments here on Earth and our observations of the rest of the Universe. The key words are "natural world." It is obvious that the existence of a supernatural being(s) is outside the realm of the natural laws. It should be noted that people do use the results of astronomy to attempt to deduce the existence of God (or gods). Unfortunately, one can reach two, equally valid conclusions: * Many atheists (including some astronomers) argue that the regularity of the natural world, combined with our apparent lack of distinction in it (the Earth is just one planet, around one star, in one galaxy, etc.), are compelling reasons not to believe in any god. * Many theists (including ordained ministers and priests who are also astronomers) find the study of the natural world another means of understanding God. The beauty, order, and sheer scope of the natural world are profound clues to the power and intelligence which created it all. Since sci.astro is devoted to science of astronomy (i.e., the natural world), sci.astro is not the appropriate forum for such a religious debate. If you would like to discuss such things, you should go to, talk.religion.*, or maybe soc.religion.*
Subject: B.08 What kind of telescope should I buy? See the Purchasing Amateur Telescopes FAQ, posted regularly to sci.astro.amateur, or at your favorite FAQ location.
Subject: B.09 What are the possessive adjectives for the planets? Author: Steve Willner <>, Andrew Christy <> Mercury Mercurian mercurial Venus Venerian venereal Venusian Cytherean Earth Terrestrial Telluric Mars Martian martial Arean Jupiter Jovian jovial Saturn Saturnian saturnine Uranus Uranian Neptune Neptunian Pluto Plutonian The first form(s) refers to the planet as an object (e.g., "Saturnian rings"). The second form refers to human characteristics historically associated with the planet's astrological influence or with the god or goddess represented by the planet (e.g., "a jovial individual").
Subject: B.10 Are the planets associated with days of the week? Author: many Surprisingly, yes. This comes from the historical association of the "planets" with gods and goddesses. In ancient times, the word "planets" was from the Greek for "wanderers" and referred to objects in the sky that were not fixed like the stars. Some of these associations are clearer in English, especially if we compare with names of Norse or Old English gods/goddesses, while others are clearer from comparing French/Spanish with the Roman gods and goddesses. We have: Sun Moon Mars Mercury Jupiter Venus Saturn Roman Luna Mars Mercury Jupiter Venus Saturn Norse Tiw Woden Thor Freya French dimanche lundi mardi mercredi jeudi vendredi samedi Spanish domingo lunes martes miercoles jueves viernes sabado Italian Domenica Lunedi Martedi Mercoledi Giovedi Venerdi Sabato English Sunday Monday Tuesday Wednesday Thursday Friday Saturday German Sonntag Montag Dienstag Mittwoch Donnerstag Freitag Samstag Notes: 1. Sun: Dimanche and domingo are from the Latin for "Day of the Lord." 2. Saturn: Sabado is from "Sabbath." 3. German and English use Teutonic, not Scandinavian forms of the God names, e.g., "Woden" in "Wednesday," not "Odin," which is the Norse equivalent. The God of Tuesday was Tiw. 4. Russian numbers three days (Tuesday = 2nd, Thursday = 4th, and Friday= 5th) and does not use God/Planet names for the rest. In Sanskrit (an Indo-European language), we also find ("vaar" means day) Sun Ravivaar Ravi Sunday Moon Somvaar Som Monday Mars Mangalvaar Mangal Tuesday Mercury Budhvaar Budh Wednesday Jupiter Brihaspativaar Brihaspati Thursday Venus Shukravaar Shukr Friday Saturn Shanivaar Shani Saturday This association between planets and days of the week holds in at least some non-European languages as well. In Japanese the days Tuesday through Saturday (and the associated planets) are named after the five Asian elements, rather than gods. Japanese days planets Sun nichiyoubi hi (same kanji as nichi) Moon getsuyoubi tsuki (same kanji as getsu) Mars kayoubi kasei Mercury suiyoubi suisei Jupiter mokuyoubi mokusei Venus kinyoubi kinsei Saturn doyoubi dosei For additional reading, particularly about Eastern day naming, see <URL:>.
Subject: B.11 Why does the Moon look so big when it's near the horizion? Author: Carl J. Wenning <>, Steve Willner <> The effect is an optical illusion. You can verify this for yourself by comparing the size of the Moon when it's on the horizon to that of a coin held at arm's length. Repeat the measurement when the Moon is overhead. You will find the angular size unchanged within the accuracy of the measurement. In fact two effects contribute to making the Moon slightly *smaller* on the horizon than overhead. Atmospheric refraction compresses the apparent vertical diameter of the Moon slightly. A really precise measurement will reveal that the horizontal diameter is about 1.7% smaller when the Moon is on the horizon because you are farther from it by approximately one Earth radius. The Sun, incidentally, shows the much same effects as the Moon, though it's a *really* BAD idea to look directly at the Sun without proper eye protection (NOT ordinary sunglasses). The change in apparent angular diameter is, of course, less than 0.01% instead of 1.7% because the Sun is farther away. (See the next entry.) The probable explanation for this illusion is that the "background" influences our perception of "foreground" objects. If you've seen the "Railroad Track Illusion"---in which two blocks of the same size placed between parallel lines will appear to be different sizes---you're familiar with the effect. The Moon illusion is simply the railroad track illusion upside-down. For some reason, the sky nearer the horizon appears much more distant than the point directly overhead. The explanation for this apparent difference in distance is not known, but an informal survey by one of the authors (CJW) indicates that all people see this distance difference. The explanation for the Moon illusion is then that when we see the moon "against" a more "distant" horizon it appears larger than when we see it "against" a much "closer" one. Additional evidence in support of this idea is the behavior of "afterimages." An afterimage of a constant size can be impressed upon the human eye by staring at a light bulb for a few minutes. By projecting the afterimage on a sheet of white paper, the size of the afterimage can be varied by changing the eye-to-paper distance. A similar effect is seen with the night sky---an afterimage projected toward the horizon appears larger than one projected toward the zenith. Much more extensive discussions are available in * The Planetarian, Vol. 14, #4, December 1985, also available at <URL:>; and * Quarterly Journal of the Royal Astronomical Society, vol. 27, p. 205, 1986.
Subject: B.12 Is it O.K. to look at the Sun or solar eclipses using exposed film? CDs? Author: Joseph Lazio <>, Steve Willner <> This question appears most frequently near the time of solar eclipses. The short answer is no! The unobscured surface of the sun is as bright as ever during a partial eclipse and just as capable of causing injury. The injured area on the retina may be a bit smaller, of course, but that's no reason to risk damage. Moreover, there are no nerve endings in the retina, so one can do permanent damage without being aware of it. People have proposed a host of methods for viewing the Sun, including exposed film and CDs. These home-grown methods typically suffer from two flaws. First, they do not cut out enough visible light. Second, they provide little protection against ultraviolet or infrared light. The only safe method for viewing the Sun directly is using No. 14 arc-welder filter or a metallicized glass or Mylar filter. A local hardware store or construction supply store should carry or know where to obtain arc-welder filters. Many astronomy magazines carry ads for solar filters. Whatever filter you use, inspect it to make sure it has not been damaged. Even a pinhole can let through enough light to cause injury. If you use a filter over a telescope or binocular, make sure the filter is firmly attached and cannot come off accidentally! Never use an eyepiece filter, which can overheat and crack. Any filter should cover the entire entrance aperture (or more precisely, any part of the entrance aperture that isn't covered by something completely opaque). If using only one side of a binocular, cover the other side. An alternative way to view the sun is in projection. You can use a pinhole camera or a telescope, eyepiece, and screen. Many observing handbooks illustrate suitable arrangements. This method is not only safe, it can give a magnified image and make it easier to see details. If you are lucky enough (or put in the advance planning) to see a total solar eclipse, the total phase can be enjoyed with no eye protection whatsoever. In fact, experienced eclipse-goers often cover one eye with a patch for several minutes before totality so the eye will be dark-adapted during totality. Just be sure to look away (or through your filter again) the instant totality is over. Additional information on the safe viewing of solar eclipses is at the Eclipse Home Page, <URL:>.
Subject: B.13 Can stars be seen in the daytime from the bottom of a tall chimney, a deep well, or deep mine shaft? Author: Michael Dworetsky <> The short answer is no (well, almost no). The long answer is given by David Hughes in the Quarterly Journal of the Royal Astron. Soc., 1983, vol. 24, pp 246-257. This mistaken notion was first mentioned by Aristotle and other ancient sources, and was widely assumed to be correct by many literary sources of the 19th century, and even believed by some astronomers. But every astronomer who has ever tested this by experiment came away convinced it was impossible. If you want to try an interesting experiment to see why it is believed that whatever people see up chimneys cannot be stars, try the experiment at night, as I have done, using a cardboard tube centre from a paper towel roll (mine had an opening of 25 square degrees). You will see that, at random, you will seldom include one visible star, rarely two, and virtually never more than two, in the field. Separate experiments to attempt to see Vega and Pollux through tall chimneys were performed by J. A. Hynek and A. N. Winsor. They were unable to detect the stars under near perfect conditions, even with binoculars. The daytime sky is simply too bright to allow us to see even the brightest stars (although Sirius can sometimes be glimpsed just after the Sun rises if you know exactly where to look.) Venus can be seen as a tiny white speck but again, you have to be looking exactly at the right spot. The most likely explanation for the old legend is that stray bits of rubbish get caught in the updraft and catch the sunlight as they emerge from the chimney. It is possible to see stars in the daytime with a good telescope, as long as it has been prefocused and can be accurately pointed at a target.
Subject: B.14 Why do eggs balance on the equinox? Author: Bob Riddle <> Luck. In short, there's no validity to the idea that eggs can only be balanced on the equinox. This question often arises during March and September, when it is not unusual to hear, see, or read news reports about the equinox occurring during that month. It is also not unusual to hear news reports being able to balance an egg on the equinox day. In fact many times these reports will highlight a classroom wherein the students are shown trying to balance eggs. Naturally some eggs will balance and others will not---one time, then perhaps do differently the next time. The focus in these reports, however, seems to be on the eggs that do balance rather than the observations from the experiment that not all eggs balanced the first time tried, nor did all eggs always balance, or perform the same way every time. There are a number of problems with the idea of balancing an egg: 1. Typically, explanations about the balancing act involve gravity. One explanation that I've heard suggested that gravity is "balanced" when the sun is over the earth's equator. Another gravity-based explanation is that the sun exerts a greater gravitational attraction on the earth on these two days. If gravity is involved in balancing the egg shouldn't other objects balance as well? Or is gravity selective such that only an egg is affected on this particular day? 2. The equinox is a certain day, while the sun is actually at the equinox point for an instant (0 degrees on the celestial equator and 12 hours within the constellation Virgo). Therefore, shouldn't the egg only be balanced at the specific time that the sun reaches that position? 3. If the Sun's gravity is involved, shouldn't latitude have an effect? For example I live at 40 degrees north. Shouldn't the egg lean at an angle pointing towards the sun where I live---and if so, then it should only be standing straight up at the equator? You can of course conduct your own experiment. Issues to consider when designing your experiment include, Would the same egg balance on any other day(s) during the year? What would be the results of standing the same egg under the same physical conditions and at the same time each day throughout the year?
Subject: B.15 Is the Earth's sky blue because its atmosphere is nitrogen and oxygen? Or could other planets also have blue skies? Author: Paul Schlyter <> The Earth's sky is blue because the air molecules (largely nitrogen and oxygen) are much smaller than the wavelength of light. When light encounters particles much smaller than its wavelength, the scattered intensity is inversely proportional to the 4'th power of the wavelength. This is called "Rayleigh scattering," and it means that half the wavelength is scattered with 2**4 = 16 times more intensity. That's why the sky appears blue: the blue light is scattered some 16 times more strongly than the red light. Rayleigh scattering is also the reason why the setting Sun appears red: the blue light has been scattered away from the direct sunlight. Thus, if the atmosphere of another planet is composed of a transparent gas or gases whose molecules are much smaller than the wavelength of light, we would, in general, also expect the sky on that planet to have a blue color. If you want another color of the sky, you need bigger particles in the air. You need something bigger than molecules in the air---dust. Dust particles can be many times larger than air molecules but still small enough to not fall out to the ground. If the dust particles are much larger than the wavelength of light, the scattered light will be neutral in color (i.e., white or gray)---this also happens in clouds here on Earth, which consist of water droplets. If the dust particles are of approximately the same size as the wavelength of light, the situation gets complex, and all sorts of interesting scattering phenomena may happen. This happens here on Earth from time to time, particularly in desert areas, where the sky may appear white, brown, or some other color. Dust is also responsible for the pinkish sky on Mars, as seen in the photographs returned from the Viking landers. If the atmosphere contains lots of dust, the direct light from the Sun or Moon may occasionally get some quite unusual color. Sometimes, green and blue moons have been reported. These phenomena are quite rare though---they happen only "once in a blue moon...." :) The dust responsible for these unusual color phenomena is most often volcanic in origin. When El Chicon erupted in 1982, this caused unusually strongly colored sunsets in equatorial areas for more than one year. The much bigger volcanic explosion at Krakatoa, some 110 years ago, caused green and blue moons worldwide for a few years. (See also Section 3 of the FAQ, Question C.08, on the meaning of the term "blue moon.") One possible exception to the above discussion is if the clouds on the planet are composed of a strongly colored chemical. This might occur on Jupiter, where the clouds are thought to contain sulfur, phosphorus, and/or various organic chemicals. It's also worth pointing out that the light of the planet's primary is quite insignificant. Our eyes are highly adaptable to the dominating illumination and perceive it as "white," within a quite wide range of possible colors. During daytime, we perceive the light from the Sun (6000 K) as white, and at night we perceive the light from our incandescent lamps (2800 K, like a late, cool M star) as white. Only if we put these two lights side-by-side, at comparable intensities, will we perceive a clear color difference. If the Sun was a hot star (say of spectral type B), it's likely we still would perceive its light as "white" and the sky's color as blue. Additional discussion of the color of the sky on planets and moons in the solar system is in Chapter 10 of _Pale Blue Dot_ by Carl Sagan.
Subject: B.16 What are the Lagrange (L) points? Author: Joseph Lazio <>, John Stockton <> The Lagrange points occur in a three-body system. Take a system consisting of a large mass M, orbited by a smaller mass m, and a third mass u, where M >> m >> u. There are five points where u can be and have the same orbital period as m. Three lie on the line connecting M and m. One (L1) lies between M and m, one (L2) lies outside the orbit of m, and one (L3) lies on the other side of M from m. Two are in the orbit of m, 60 degrees ahead (L4) and 60 degrees behind it (L5). Pictorially, we have something like this (not too scale!), with the direction of revolution indicated for m: L4 \ \ orbit of m ^ \ | L3 M L1 m L2 | / | / / L5 The Lagrangian points are often considered as places where objects, such as satellites can be "parked" for long periods. For instance, the SOHO satellite sits at the Sun-Earth L1 point in order to have a continuous, unobstructed view of the Sun, and the Wilkinson Microwave Anisotropy Probe observed from the L2 point. There is a group of asteroids, known as Trojans, which occupy the Sun-Jupiter L4 and L5 points. There are also various groups advocating human colonization of space which support putting a colony at the Earth-Moon L5 point. In fact, the L1, L2, and L3 points are "unstable equilibria." That is, an object placed there will slowly drift away if there are any other gravitational tugs on it (which there always will be due to other objects in the solar system). Thus, placing a spacecraft at the Sun-Earth L1 or L2 point requires regular "course corrections" so that it doesn't move too far from the L1 or L2 point. The L4 and L5 points are generally stable so that one should be able to remain at them indefinitely. Additional diagrams for the L points is at the WMAP site, <URL:>.
Subject: B.17 Are humans affected psychologically and/or physically by lunar cycles? Author: Joseph Lazio <> I contend that the answer is yes and no. Some people will travel hundreds, even thousands of kilometers to watch a total solar eclipse in which the Moon passes in front of the Sun. Professional astronomers routinely ask for "dark time," i.e., time during the new Moon, for their observations. (The reason is that the light from the Moon can make it more difficult to see faint objects. Compare the difference in the brightness of the sky between new and full Moon some month.) Clearly these are examples in which the phase of the Moon affects people's behavior. However, when people talk about the effect of the Moon, they are typically referring to the idea that X increases during the full Moon, where X is "crime," "births," or some other aspect of human behavior. (The word "lunacy" is derived from "luna," the Latin word for Moon.) I am aware of almost no evidence to support this belief, despite ardent support for it from police officers and emergency room and OB/GYN nurses. For instance, the late astronomer George Abell examined the birth records from LA hospitals for over 10,000 natural births (i.e., no C-sections). He could find no correlation between the number of births and the phase of the Moon. The accepted explanation for this perceived effect is a human tendency to find order where there is none. After a particularly busy shift one night, a police officer or nurse will notice a full or nearly full Moon. The full Moon can be such a brilliant sight that it is easy to see how one might think there would be an association. Humans also have a tendency to forget contrary evidence. Thus, the police officer or nurse will not remember the last busy night that was during a new Moon (after all it is difficult to see the new Moon!). From this start, it doesn't take long for one to become convinced that the full Moon might have an effect on humans. This belief might also become self-fulfilling. For instance, a police officer might become less tolerant of minor offenses during the full Moon (and the additional light provided by the full Moon might help him/her see more). Another contributing factor might be people's inability to tell when the full Moon actually occurs. When I was teaching astronomy, I had a student tell me that the first-quarter Moon was "full." I've also been told by a futures trader that recommended practice is to buy during one phase and sell during another. Although he thought it was a result of the phase of the Moon influencing the buying and selling, I think a more simple explanation is that this practice is apparently what they are taught (perhaps resulting from the same kind of misconception that produces the crime and birth myths). (I'm not picking on police officers or nurses. I've just heard this belief expressed most strongly from them, and their professions can require them to be up late at night, when the full Moon is most likely to be noticed.) Another common belief is that the human female's menstrual cycle is influenced by the phase of the Moon. There are two problems with this belief. First, the average woman's menstrual cycle is 28 days, which is close to the orbital period of the Moon, but is not exactly equal to it. The range of menstrual cycle lengths, though, is quite large. I've heard of women having cycles as short as 21 days and as long as 52 days. If the Moon controlled or influenced the length of the cycle, it is not clear why the range would be so large. Second, other major mammals do not have a cycle close to 28 days. In particular, the length of the cycle for chimpanzees, our closest relative species, is 35 days.
Subject: B.17 How do I become an astronomer? What school should I attend? Author: Suzanne H. Jacoby <> This material is extracted from the National Optical Astronomy Observatories' Being an Astronomer FAQ, <URL:>. Astronomers are typically good at math, very analytical, logical, and capable of sound reasoning (about science, anyway). Computer literacy is a necessity. While not all astronomers are skilled computer programmers, all should be comfortable using a computer for editing files, transferring data across networks, and analyzing their astronomical data and images. Other valuable traits are patience and determination for sticking to a difficult problem or theory until you've seen it through---which can take years. The final product of scientific research is the dissemination of the knowledge gained, so don't overlook the importance of communication skills like effective public speaking at professional meetings and the ability to publish well written articles in scientific journals. Many of these skills are developed during one's education and training. In the U.S., a typical astronomer will obtain a Bachelor of Science (B.S.) degree in a physical science or mathematics, then attend graduate school for 5--7 years to obtain a Ph.D. After earning a Ph.D., it is common to take a postdoctoral position, a temporary appointment which allows an astronomer to concentrate on his or her own research for about two to three years. These days, most people take a second postdoc or even a third before they are able to land a faculty or scientific staff position. If you want to become an astronomer, a general principle is to obtain as broad and versatile an education as possible while concentrating in mathematics, physics, and computing. It is not critical that your Bachelor's degree be in astronomy. Students with a strong core of physics classes in addition to some astronomy research experience are most likely to be accepted to graduate programs in astronomy. Additional information on astronomy as a career can be obtained from the American Astronomical Society, <URL:>, and the Harvard-Smithsonian Center for Astrophysics (contact their Publications Department, MS-28, 60 Garden Street, Cambridge, MA 01238, USA, or call 617-495-7461, ask for the brochure "Space for Women").
Subject: B.19 What was the Star of Bethlehem? Author: Mike Dworetsky <> [This question is most popular around Christmas time.] It is first and most important to stress that the Bible is a religious book. The Star of Bethlehem is mentioned only briefly in the book of Matthew. As such Matthew's description of it may have been religious rather than scientific. Indeed, it has also been pointed out that the Star story is similar to a Jewish Midrash, or moral tale illustrating a religious point, which does not necessarily have to have any basis in fact. Furthermore, at the time the Bible was written the word "star" could be used to indicate essentially anything in the sky. The Star of Bethlehem was almost certainly not what we understand today a star to be (namely a ball of gas shining by interior thermonuclear fusion). Nearly any spectacular sky phenomenon (comet, supernova, nova, etc.) has been identified as the Star of Bethlehem at one time or another, but recent interest has focussed on conjunctions of various planets, possibly in auspicious constellations. Two examples are the following: Michael Molnar has found that there was an double occultation of Jupiter in March and April of 6 BC in Aries that would have been calculable even by the means available to astrologers (which the Magi were) and that would have been of high significance in magian astrology (which differed somewhat from astrology of the modern era). However it would have been invisible, taking place in daylight. Thus there is a perfectly good explanation as to why Herod's courtiers had not seen it, but "wise men from the East" knew all about it. The occultation also provided a neat explanation of why the star was seen over Bethlehem---from Jerusalem, the second occultation's azimuth was close to the direction of the town. Molnar also points out that the Romans regarded the horoscope of Jesus as a royal one. And for a small commentary on one of Molnar's points, see my paper with Steve Fossey in The Observatory in 1998 or at <URL:>. On 3 May 19 BC, the planets Saturn and Mercury were in close conjunction, within 40 minutes of arc of each other. Then Saturn moved eastward to meet with Venus on 3 June 12 BC. During this conjunction the two were only 7.2 minutes of arc apart. Following this conjunction, on 3 August 12 BC, Jupiter and Venus came into close conjunction just before sunrise, coming within 4.2 minutes of arc from each other as viewed from earth, and appearing as a very bright morning star. This conjunction took place in the constellation Cancer, the "end" sign of the Zodiac. Ten months later, on 2 June 17 BC, Venus and Jupiter joined again, this time in the constellation Leo. The two planets were at best 6 seconds of arc apart; some calculations indicate that they actually overlapped each other. This conjunction occurred during the evening and would have appeared as one very bright star. Even if they were 6 seconds of arc apart, it would have required the sharpest of eyes to split the two, because of their brightness. (Some of this information is adapted from a longer article at <URL:>. There is also other pertinent information at this site regarding the astronomy during that time.)
Subject: B.20 Is it possible to see the Moon landing sites? Author: David W. Knisely <> It is possible to locate and observe the Apollo landing "sites," but it is *not* possible with current equipment to see the hardware left there, since their sizes are far too small to be resolved successfully. For example, a common backyard 6 inch aperture telescope can only resolve craters on the moon which are about 1.5 miles or so across. Even telescopes with a resolution comparable to that of the Hubble Space Telescope can only resolve details about 100 meters across (the size of a football or soccer field). Lasers fired from Earth are bounced off special retro-reflectors left at these sites by the astronauts, and the faint return pulse is then detected by Earth-based telescopes equipped with special instruments to measure the Earth-moon distance, but otherwise, we can't see any man-made equipment left at the landing sites. If you wish to see the sites through a telescope for yourself, here are the approximate locations of the Apollo landing sites (see the Project Apollo Web site, <URL:>, for more exact locations and descriptions or <URL:> for set of images of the landing sites at increasingly higher resolution): APOLLO 11: 0.67 deg. N, 23.49 deg. E, near southwest edge of Mare Tranquillatis a little northwest of the 6-mile wide crater Moltke. APOLLO 12: 3.20 deg. S, 23.38 deg. W, in Oceanus Procellarum southeast of the crater Lansberg (also the landing site of Surveyor 3). APOLLO 14: 3.67 deg. S, 17.47 deg. W., in Fra Mauro highlands just north of northwestern rim of large shallow Fra Mauro crater. APOLLO 15: 26.10 deg.N., 3.65 deg. E., Next to Hadley Rille and southwest of Mt. Hadley in the lunar Apennine Mountains. APOLLO 16: 8.99 deg. S., 15.52 deg. E., higlands north of the ruined crater Descartes and southeast of the double crater Dolland B/C. APOLLO 17: 20.16 deg. N., 30.77 deg. E., in the southwestern Taurus Mountains roughly between the craters Littrow and Vitruvius.
Subject: Copyright This document, as a collection, is Copyright 1995--2000 by T. Joseph W. Lazio ( The individual articles are copyright by the individual authors listed. All rights are reserved. Permission to use, copy and distribute this unmodified document by any means and for any purpose EXCEPT PROFIT PURPOSES is hereby granted, provided that both the above Copyright notice and this permission notice appear in all copies of the FAQ itself. Reproducing this FAQ by any means, included, but not limited to, printing, copying existing prints, publishing by electronic or other means, implies full agreement to the above non-profit-use clause, unless upon prior written permission of the authors. This FAQ is provided by the authors "as is," with all its faults. Any express or implied warranties, including, but not limited to, any implied warranties of merchantability, accuracy, or fitness for any particular purpose, are disclaimed. If you use the information in this document, in any way, you do so at your own risk.

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