We start our stellar journey tonight with a tool that
you might like to grab:
From Rhodes University Astronomy Society
[ http://astrosoc.soc.ru.ac.za/ ]
their Software Archive:
[ http://astrosoc.soc.ru.ac.za/archive/software/ ]
Grab the program called Skyglobe v3.6:
[ http://astrosoc.soc.ru.ac.za/archive/software/Skyglo36-Win32.exe ]
== alternate download location ==
[ http://www.maa.mhn.de/Tools/skyglb36.zip ]
Here are the directions for SkyGlobe:
[ http://astrosoc.soc.ru.ac.za/archive/docs/skyglobe_course.html ]
I've had many astronomy programs since I've had a PC
at my disposal, I always come back to SkyGlobe as a
learning tool, because it's easy to use, it's small,
and you don't "install it". Using it will answer a
lot of other questions you might have.
Here is a picture to get us started:
[ http://aa.usno.navy.mil/graphics/Moon_phases.jpg ]
Copyright Antonio Cidadao [ http://www.astrosurf.com/cidadao/ ]
[ mailto:firstname.lastname@example.org ]
While I was doing some research for this question
I found a site (hadn't seen this one before) that
has Java applets for astronomy visualizations,
Juergen Giesen's GeoAstro Applet Collection
[ http://www.jgiesen.de/GeoAstro/GeoAstro.htm ]
I will be referring to some of those applets.
For simplification, our solar system at this point
will consist ONLY of our Sun, the Earth and the
Moon. Like any good physics experiment, we will
ignore reality :) and just leave the math.
In our solar system we have a whirling dance
occurring between the Moon and the Earth. You
can imagine the Sun at this point to be a
Disco-Ball at the center of a roller-skating rink.
It is at a fixed, unmoving point but rotating.
Quickly rotating in place, our Disco-Ball Sun
is casting light on the only things enjoying
it's shine... the Earth and Moon. The Earth
is a single person holding a volleyball (the
Moon) with outstretched arms. It is held tight,
the same side of the volleyball always visible,
never changing. It is 'tidally locked'
[ http://en.wikipedia.org/wiki/Tidal_locking ]
with the person, it will never rotate freely
You can imagine now, that the person enjoying
the skating rink is a retired Olympic skater,
training and keeping fit. As part of the
elaborate routine, they are spinning around
and around like a top letting go of the volleyball
with each spin, and catching it again. When the
ball is caught each time though, it has moved
slightly from where it was released. Still
tidally locked to the skater, the volleyball
has made a retrograde motion to the skater's
[ http://dictionary.reference.com/search?q=retrograde ]
All of this is happening as they make this
oval orbit around the Disco-Ball
Sun. Ignoring the dizzying effects of continually
spinning; the Olympic star notices the abnormally
bright Disco-Ball is illuminating the volleyball with
the crescents and gibbous arcs. Only occasionally
does the Moon shine full & bright while typically
[ http://aa.usno.navy.mil/graphics/Moon_phases.jpg ]
only the shadowy side is visible to the spinning
Earth. Briefly, the Moon disappears completely to
the Earth. At this point, no Sunlight is hitting
the side of the Moon that is always seen. That is
when they are all in line, they are in conjunction.
This of course was for visual sake. It is
far from the celestial mechanics involving
mass and gravity that keeps our Earth and Moon
moving around the Sun.
Gravity takes the place of the skater's arms,
it is also what would force the skater to go
around the rink. Gravity holds the Earth in
place around the Sun.
Gravity has made the Moon rotate in sync with the
Earth. (IF there was no rotation at all in the
Moon, we would see different faces of the Moon).
This hopefully sets a foundation for your
couple of questions about why we see the
Moon at different positions each nite and why
a "new" crescent is seen at a different time
and part of the sky as an "old" crescent.
An excerpt of this site: Observing the Moon FAQ
[ http://www.madison.k12.wi.us/planetarium/Mooncal/Moonfaq.htm ]
"Q: Why don't the phases of the Moon happen on the
same day each month?
Answer: The phases of the Moon do not happen on
the same days each month because the Moon's
orbit around the Earth does not take exactly one
month. Actually, there is no such thing as
"exactly" a month because our months are
different lengths, some with 30 days, some with
31, and even one with 28 or 29 days (depending
on whether it's a leap year or not). Calendar
makers many years ago originally tried to make
our months relate exactly to the phases of the
Moon, but quickly became frustrated because they
would then not be able to have 12 equal months
for one year. Unfortunately, 12 full Moons do
not equal one 365 day year. It's not even close.
Calendar makers eventually decided to stick with
a solar year of 365 days (with a leap year every
fourth year) and to make the months vary between
30, 31, and 28 days. The cycle of the phases,
like from full Moon to full Moon, takes 29 and
1/2 days. Because of this, Moon phases can
happen anywhere during the months."
I guess a good summary of why the Moon moves
backwards across the sky from night to night
would be, lazy math skills :) Had we chosen
to use the complicated math associated with
the exact cycle of the Moon, we would have
severely messed up calendars but a Moon that
would be at the same point in the sky at the
same time each night.
For clarity I will post this excerpt as well:
[ http://www.madison.k12.wi.us/planetarium/Mooncal/Moonfaq.htm ]
"Q: So exactly how long does it take the Moon to
orbit the Earth?
Answer: This explanation can be a bit
confusing, so have patience. The Moon takes 27
1/3 days to orbit the Earth once, but the
number of days between one full Moon and the
next full Moon is 29 1/2 days. Why are these
two "months" not the same? The answer has to
do with the Earth and Moon relationship to the
Sun. If you chart the Moon's position every
night for a full month, you could compare its
position with a bright star, and then wait to
see how long it takes for the Moon to come back
to its position relative to that star. The
answer would be about 27 1/3 days. But during
this time, the Earth has moved nearly 1/12 of
the way in its orbit around the Sun. So to get
from one full Moon to the next would take about
two days longer, 29 1/2 days, because the Moon
would have to travel a little further around
the Earth to get to 'full' again."
So you understand the shadow on the Moon:
"Q: Wouldn't the Earth block the light from the
Sun to the Moon?
Answer: Many people grow up with the idea that
the phases of the Moon are caused by the shadow
of the Earth. The Earth does cast a shadow (as
does any object in Sunlight), but the Moon
usually passes above or below it. When the Moon
does pass through the Earth's shadow, it is
called a lunar eclipse (see illustration at
right). When the Moon passes into the shadow of
the Earth, it can at first glance mimic some of
the phases, but the effect is in fact very
different. A lunar eclipse, when the Moon IS in
the Earth's shadow, can only happen when the
Moon is full (on the opposite side of the Earth
from the Sun) Lunar Eclipse Animation"
[ http://www.madison.k12.wi.us/planetarium/Mooncal/leclipse.gif ]
With reference to the position of the Moon at
new and old crescents:
"Q: Why isn't the Moon visible every night (or every
Answer: The Moon is only visible during part of each
month. Whether it's visible during the day or night
depends on what day it is, and how "far" the phase
is from full or new. The Moon orbits the Earth once
(approximately) each month. As a result, sometimes
the Moon appears very close to the Sun in the sky,
and sometimes it is far away from the Sun in the
sky. When the Moon is opposite the Sun in the sky
(which is when full Moon happens), it will rise as
the Sun sets and set as the Sun rises. Therefore, at
this time, it will be up all night long. The further
before or after full Moon (in days), the more the
Moon will be visible during daytime hours (when the
Sun is in the sky). However, as the Moon gets very
close to new Moon (when the Moon and the Sun are
closest together), it is very difficult or
impossible to see in the daytime sky."
"... Since most people observe the crescent Moon in
the evening just after Sunset, this is sometimes
called "the new Moon in the old Moon's arms"
(the crescent is the young or nearly "new" Moon,
the Earthshine illuminated part is the "old" part)."
Which takes us to the conjunction of the Moon,
or the astronomical 'new Moon'.
"First Visibility of the Lunar Crescent"
by John A. R. Caldwell and C. David Laney
[ http://www.saao.ac.za/sky/fvlc.html ]
A rather long article describing the accuracy
and timing of new crescents. They have
tables associated with the timing.
"Crescent Visibility Criteria"
Since antiquity, astronomers and crescent
observers have tried to find simple parameters
which can be used to predict crescent visibility,
usually by looking for a clear separation between
occasions when the Moon was visible and when it
was not. A totally clear separation, however, is
impossible even with an ideal parameter set:
observers and conditions are both highly variable
"Observers are by no means equally likely to look
at the right spot at the right time, with the same
visual acuity and properly aimed and focused
equipment. Assuming good, properly corrected,
eyesight, there are still factors like
preparedness, experience, and having got various
"teething troubles" out of the way beforehand,
that can make a difference."
"It is also clear that one must subdivide the
visibility criteria into subcases for naked-eye
and optically-aided viewing, since magnifying the
crescent enhances its visibility. This is
supported by the record ages for young crescents
at the time of sighting: 15.4 hours with
naked-eye, 12.7 hours with binoculars, and 12.2
hours with a telescope."
Lunar Crescent Visibility
[ http://www.saao.ac.za/sky/vishome.html ]
Let's get back to our skater. They have made
quite a few orbits around the rink at this point
and are getting dizzy. Skating upright isn't
easy anymore and with shaky knees as if
intoxicated they slowly TILT to the side. Now
with a noticeable lean as they spin around,
they catch glimpses of the Disco-Ball Sun
coming over their left shoulder crossing to
the right... but depending on what part of
the rink they are at, sometimes the Sun is
high up... sometimes it is down low.
But the Sun hasn't changed its position at all.
Our point of observation has. As we orbit
around, our noticeable tilt impacts the
angle that the Sun strikes the Earth.
Our Earth has a nice tilt to it. For whichever
cosmic reason (collision with another object
or a formation process) The Earth is tilted 23.5deg.
[ http://inkido.indiana.edu/a100/celestialsphere4.html ]
The Sun rising at a different location on the
horizon from summer to winter is a result of
This tilt gives us our 4 seasons:
"The Seasons and Axis Tilt" - EnchantedLearning.com
[ http://www.enchantedlearning.com/subjects/astronomy/planets/Earth/Seasons.shtml ]
"The Earth's seasons are not caused by the
differences in the distance from the Sun
throughout the year (these differences are
extremely small). The seasons are the result of
the tilt of the Earth's axis."
"The Earth's axis is tilted from perpendicular
to the plane of the ecliptic by 23.45°. This
tilting is what gives us the four seasons of
the year - spring, summer, autumn (fall) and
winter. Since the axis is tilted, different
parts of the globe are oriented towards the Sun
at different times of the year."
"Summer is warmer than winter (in each
hemisphere) because the Sun's rays hit the
Earth at a more direct angle during summer than
during winter and also because the days are
much longer than the nights during the summer.
During the winter, the Sun's rays hit the Earth
at an extreme angle, and the days are very
short. These effects are due to the tilt of the
The Moon also is also tilted in relation to
the Earth's orbital plane:
[ http://nfo.edu/Moonview.htm ]
"The Moon's path on the celestial sphere is close to the ecliptic
but tilted some 5 degrees. For this reason, sometimes when the
Moon crosses the meridian for an observer in the U.S. it is
higher in the sky than at other times."
The relationship between Sunrise and Sunset,
Moonrise and Moonset is the complex
paths of each the Moon and Earth as they
wobble around the Sun. All risings and
settings are timed by the Earth's rotation.
Positional changes throughout the seasons
come about because of Earth's tilt. To
visualize this process you can experiment
with these JAVA applets collected by
Juergen Giesen as well as some other Moon
and timing related sites:
You can get a sense of Moon phases and how they
[ http://www.jgiesen.de/Moonyear/index.htm ]
"Sun, Moon and Earth Applet"
[ http://www.jgiesen.de/SME/index.htm ]
"Moon Phases" animated
[ http://www.astro.wisc.edu/~dolan/java/MoonPhase.html ]
"Phases of the Moon" - USNO
[ http://aa.usno.navy.mil/data/docs/MoonPhase.html ]
It's military, so it's gotta be right... :)
View from Earth: 388671 km above 5°17'N 7°19'E
[ http://www.fourmilab.ch/cgi-bin/uncgi/Earth/action?opt=-m&img=Moon.evif ]
CLEMENTINE - USGS IMAGES
[ http://astrogeology.usgs.gov/Projects/Clementine/index.html ]
Glossary of Night Sky Terms
[ http://www.souledout.org/nightsky/nsglossary.html ]
-= search techniques =-
Google searches based on:
[ ://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=Moon+conjunction ]
Sun Moon Earth
Moon OR lunar phases timing -tarot -Magick
Earth Moon relationship
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This is abridged to highlight the points you were
interested in. If you need clarification on any
part of my answer please ask before accepting it
as the final answer.
hope this sorts it out for you,