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Subject: [sci.astro] General (Astronomy Frequently Asked Questions) (2/9)
This article was archived around: 15 Aug 2007 16:36:47 -0400
Last-modified: $Date: 2004/01/27 00:00:01 $
Version: $Revision: 4.10 $
Posting-frequency: semi-monthly (Wednesday)
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
and it is on the World Wide Web at
<URL:http://www.faqs.org/faqs/astronomy/faq>. A partial list of
worldwide mirrors (both ftp and Web) is maintained at
<URL:http://sciastro.astronomy.net/mirrors.html>. (As a general note,
many other FAQs are also available from
Questions/comments/flames should be directed to the FAQ maintainer,
Joseph Lazio (email@example.com).
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
B.06 Is there scientific evidence for/against
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
B.10 Are the planets associated with days of the week?
B.11 Why does the Moon look so big when it's near the
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
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
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
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:http://www.usno.navy.mil/>. 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
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
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
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
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
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
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
Subject: B.02 What are the largest telescopes?
Author: Bill Arnett <firstname.lastname@example.org>,
William Keel <email@example.com>,
Joseph Lazio <firstname.lastname@example.org>,
Steve Willner <email@example.com>, 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:http://www.futureframe.de/astro/instr/index.html>, and a list of
large optical telescopes is at
A list of space-based observatories is at
(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)
6.5 Multiple Mirror Mt. Hopkins, Arizona
(6 mirrors, 1.8 m each; see also B.03)
6.0 BTA Nizhny Arkhyz, Russia
(Bolshoi Teleskop Azimutalnyi = Large Altazimuth Telescope)
5.0 Hale Palomar Mountain, California
4.2 William Herschel La Palma, Canary Islands
4.0 Victor Blanco Cerro Tololo, Chile
4.0 Mayall Kitt Peak, Arizona
3.9 Anglo-Australian Siding Spring, Australia
3.8 UK Infrared Mauna Kea, Hawaii
3.6 ESO Cerro La Silla, Chile
3.6 Canada-France-Hawaii Mauna Kea, Hawaii
3.5 New Technology Cerro La Silla, Chile
3.5 MPI-CAHA Calar Alto, Spain
3.5 ARC Apache Point, New Mexico (mostly remote control)
3.5 WIYN Kitt Peak, Arizona
3.5 Starfire Kirtland AFB, New Mexico
3.0 Shane Mount Hamilton, California
3.0 NASA IRTF Mauna Kea, Hawaii
Other telescopes of note:
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
Largest fully-steerable single dish radio telescope: Max Planck
Institut fuer Radioastronomie, 100 m, Effelsburg, Germany
Largest millimeter wave radio telescope: Nobeyama Radio Observatory,
Largest sub-millimeter radio telescope: James Clerk Maxwell Telescope
(Joint Astron. Center = UK, Canada, Netherlands), Mauna Kea, 15 m
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.
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).
[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.
HALCA (ISAS), 8 m dish, in Earth orbit
Infrared Space Observatory (ISO) (ESA)
Extreme Ultraviolet Explorer (EUVE) (NASA)
International Ultraviolet Explorer (IUE) [defunct] (NASA, PPARC and ESA)
Chandra, the Advanced X-ray Astrophysics Facility (NASA)
X-Ray Astronomy Satellite (SAX) (ESA)
X-Ray Timing Explorer (XTE) (NASA), 2 instruments: PCA & HEXTE
Roentgen Satellite (ROSAT) (MPE)
Einstein, the second High Energy Astronomy Observatory (HEAO-B) [defunct]
(NASA), 5 instruments: IPC, HRI, SSS, FPCS, & OGS
Fred Lawrence Whipple Gamma-Ray Observatory (SAO), a 10 m and 11 m
CANGAROO (U. Adelaide & Nippon), 4 4-m cameras
Compton Gamma-Ray Observatory (NASA) [space-based],
4 instruments: OSSE, EGRET, COMPTEL, & BATSE
The High Resolution Fly's Eye Cosmic Ray Detector HiRes
Subject: B.03 What new telescopes/instruments are being built?
Author: Bill Arnett <firstname.lastname@example.org>,
William Keel <email@example.com>,
Steve Willner <firstname.lastname@example.org>,
Joseph Lazio <email@example.com>,
with corrections and additions by many others
(These lists are undoubtedly incomplete. Additions and corrections
A list of astronomical instruments is also at
Optical/Infrared Telescopes (nighttime):
Now actually under construction:
16.4 Very Large Telescope Cerro Paranal, Chile
(quartet of 8.2-m telescopes)
11.0 Hobby-Eberly Telescope, Mt. Fowlkes, Texas
8.0 Gemini North Mauna Kea, Hawaii
8.0 Gemini South Cerro Pachon, Chile
8.2 Subaru (JNLT) Mauna Kea, Hawaii
6.5 MMT Mt. Hopkins, Arizona
(replace current six mirrors with single one; see B.01)
2.2 SOFIA NASA
(included because it will be an airborne observatory)
Others likely to start soon:
Large Binocular Telescope, (Italy; U. Arizona), pair of 8-m
telescopes, Mt. Graham, Arizona
Canary Islands Large Telescope Canary Islands, Spain, 10 m segmented mirror
Magellan (Carnegie Institution Observatories), 6.5 m, Las Campanas
Radio telescopes under construction in design stages:
Submillimeter Array, (Smithsonian Astrophysical Observatory), six 8-m
dishes at Mauna Kea
Millimeter Array (MMA) (NRAO)
Green Bank Telescope (NRAO)
High-Throughput X-Ray Spectroscopy Mission (ESA)
Antarctic Muon and Neutrino Detector Array (AMANDA)
Deep Undersea Muon and Neutrino Detection (DUMAND)
LIGO, (US), 4 km path
Virgo, (Italy), 3 km path
Subject: B.04 What is the resolution of a telescope?
Author: Steve Willner <firstname.lastname@example.org>
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 <email@example.com>
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
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 <firstname.lastname@example.org>
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
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.origins, 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 <email@example.com>,
Andrew Christy <firstname.lastname@example.org>
Mercury Mercurian mercurial
Venus Venerian venereal
Mars Martian martial
Jupiter Jovian jovial
Saturn Saturnian saturnine
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?
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
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
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.
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
Subject: B.11 Why does the Moon look so big when it's near the horizion?
Author: Carl J. Wenning <email@example.com>,
Steve Willner <firstname.lastname@example.org>
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
Much more extensive discussions are available in
* The Planetarian, Vol. 14, #4, December 1985, also available
at <URL:http://www.griffithobs.org/IPSMoonIllus.html>; 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 <email@example.com>,
Steve Willner <firstname.lastname@example.org>
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
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:http://sunearth.gsfc.nasa.gov/eclipse/>.
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 <email@example.com>
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
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
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 <firstname.lastname@example.org>
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
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
Author: Paul Schlyter <email@example.com>
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
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 <firstname.lastname@example.org>,
John Stockton <email@example.com>
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
Pictorially, we have something like this (not too scale!), with the
direction of revolution indicated for m:
\ orbit of m ^
L3 M L1 m L2 |
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
Additional diagrams for the L points is at the WMAP site,
Subject: B.17 Are humans affected psychologically and/or physically by
Author: Joseph Lazio <firstname.lastname@example.org>
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
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
Subject: B.17 How do I become an astronomer? What school should I
Author: Suzanne H. Jacoby <email@example.com>
This material is extracted from the National Optical Astronomy
Observatories' Being an Astronomer FAQ,
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:http://www.aas.org/education/career.html>, 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 <firstname.lastname@example.org>
[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
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
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
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:http://sciastro.net/portia/articles/thestar.htm>. 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 <KA0CZC@navix.net>
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:http://www.ksc.nasa.gov/history/apollo/apollo.html>, for more
exact locations and descriptions or
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.
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