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Q: What happens when we cross the "Galactic Plane" ( Answered 5 out of 5 stars,   4 Comments )
Subject: What happens when we cross the "Galactic Plane"
Category: Science > Astronomy
Asked by: citizena-ga
List Price: $70.00
Posted: 05 Oct 2005 01:41 PDT
Expires: 04 Nov 2005 00:41 PST
Question ID: 576560
I have come across several sources that state our solar system crosses
the Galactic Plane periodically. Quoting an unknown blogger: "...Every
65 million years or so, the solar system passes through the galactic
plane, our next trespassing will be in 2012. The galactic plane is the
most dense part of the galactic disc. That will be the best time for
something to run into us, or us to run into something..." I assume the
writer means there is an increased chance of Earth nearing NEO objects
or being impacted by them. So, my questions in general: I would like
someone to explain briefly the Galactic Plane travel, what does 2012
have to do with it, and is the Earth more or less in danger as a
result of this event when it happens? Also, have major extiction evens
coincided with crossing the Galactic Plane? Thanks.
Subject: Re: What happens when we cross the "Galactic Plane"
Answered By: answerfinder-ga on 05 Oct 2005 07:37 PDT
Rated:5 out of 5 stars
Dear citizena-ga,

There are a number of aspect to your question so I hope that this
research should be able to separate them and inform you as to how the
earth is possibly affected by its journey through the galaxy.

Our sun is part of a large mass of stars, interstellar gas and dust,
and possibly dark matter. This is known as a galaxy. Ours we call the
Milky Way. The Sun orbits the centre of the Milky Way at a distance of
about 25,000 to 28,000 light-years from the centre, and it takes about
226 million years to complete a revolution. This graphic shows in a
simplistic way the shape of the galaxy and the sun?s position in the
This photograph is a side on view of a similar galaxy.

You can read more about different galaxy shapes here.
?Our own galaxy, the Milky Way, sometimes simply called the Galaxy
(with uppercase), is a large disk-shaped barred spiral galaxy about 30
kiloparsecs or 100,000 light years in diameter and 3,000 light years
in thickness. It contains about 3x1011 stars and has a total mass of
about 6x1011 times the mass of the Sun.?

Viewing the galaxy from the side, astronomers draw an imaginary line
along length of it called the ?plane of the galaxy? or ?galactic
plane?. The line is used as a co-ordination system for astronomers.
See this for additional information.

During its journey around the galaxy, the sun wobbles above and below
the galactic plane. This is how one theorist describes it, ?so the
Solar System bobs up and down like a horse on a carrousel through the
dense, central portion of the galactic disk? (Michael Rampino - see

As the quote mentions, the central part of the disc is the densest
part of the galaxy and it is the existence of the dense area and the
sun?s journey through it which has given rise to theories that this
may explain life extinction events on earth.

Before moving to these theories you may wish to view the information
and pictures on the below links to give you some additional background
information. These links are to Canadian Galactic Plane Survey which
is part of a project to map to the interstellar matter of the Milky

As this web site notes, scientists are still undecided on the exact
size and shape of the galaxy and the suns? position in the galaxy, and
exact measurements are problematic.

?The structure of our galaxy is hard to determine from the inside,
especially because distances are often difficult to measure in
astronomy. The situation is somewhat analogous to maps of the Earth in
the early days of cartography.?

This article from American Scientist I would also recommend reading. 
?The Galactic Environment of the Sun?.

The article tells us that this up and down movement of the earth means
that the earth crosses the plane every 33 years. Other sources
referred to below, give a figure of 27 - 36 million years, so we see
here an example of the difficulties involved in astronomical

Page 2
?The sun's path is inclined about 25 degrees to the plane of the
galaxy and is headed toward a region in the constellation of Hercules
near its border with Lyra. The sun oscillates through the plane of the
galaxy with an amplitude of about 230 light-years, crossing the plane
every 33 million years.?

The various extinction events which have occurred during the world?s
history have attracted a number explanations. One such hypothesis is
that these events coincided with the earth crossing the galactic plane
and encountering the dense matter in the form of comets which struck
the earth. This is known as the Shiva Hypotheisis. Its author is
Michael Rampino, Associate Professor of Earth and Environmental

The citation is:
Rampino, M. R. and Haggerty, B. M. (1996) The "Shiva Hypothesis":
impacts, mass extinctions and the galaxy. Earth, Moon & Planets

A summary of the Shiva Hypotheisis written by Rampino appears here.

This is another summary.
Rampino, M.R., and B.M. Haggerty 1996. The "Shiva Hypothesis":
Impacts, mass extinctions, and the galaxy. Earth, Moon, and Planets
72, 441-460.
?The mass extinction record shows evidence for a periodic component of
about 26 to 30 Myr, and an ~30 Myr periodic component has been
detected in impact craters by some workers, with recent pulses of
impacts in the last 2-3 million years, and at ~35, 65, and 95 million
years ago. A cyclical astronomical pacemaker for such pulses of
impacts may involve the motions of the Earth through the Milky Way
Galaxy. As the Solar System revolves around the galactic center, it
also oscillates up and down through the plane of the disk-shaped
galaxy with a half-cycle ~30±3 Myr. This cycle should lead to
quasi-periodic encounters with interstellar clouds, and periodic
rotations in the galactic tidal force with maxima at times of plane
crossing. This "galactic carousel" effect may provide a viable
perturber of the Oort Cloud comets, producing showers of comets in the
inner Solar System. These impact pulses, along with stochastic
impactors, may represent the major punctuations in earth history. ?

This page carries a diagram and some other information on periodic
mass extinctions.
?One hypothesis is that this corresponds the the solar system
oscillating through the galactic plane as it orbits the Milky Way. 
Rampino notes that the last crossing of the galactic plane occurred a
few million years ago and it has been suggested that this led to a
disturbance of comets in the Oort Cloud, some of which could now be
approaching the inner solar system.?

Other scientists have followed up on this work and have made the
following comments.

?The Sun's motion perpendicular to the galactic plane
John N. Bahcall* & Safi Bahcall?
For all the models we consider, the most recent passage of the Sun
through the galactic plane occurred in the past 3 Myr provided only
that the present position of the Sun is between 0 and 20 pc above the
plane. We extend the argument of Thaddeus and Chanan8 to show that the
apparent periodicity in the mass extinction and cratering records
cannot be caused by an population of objects (observed or unobserved)
that contributes a major fraction of the total mass density at the
solar vicinity.?;jsessionid=C466C8A80B8306FC1CEC42B97A84C268

?We find this terrestrial cycle to be strongly correlated with the
time needed for the Solar System to oscillate vertically about the
plane of the Galaxy, which is 33 plusminus 3 Myr according to the best
current astronomical evidence.?

Large Meteorite Impacts and Planetary Evolution II 
p242 ?the sun?s last crossing of the galactic plane occurred in the
last million years or so?

In these and other sources it is suggested that earth crossed the
galactic plane within the past 3 millions years (depending on which
model or measurements are used). So how does the year 2012 come into
the equation? As far as I can establish this date comes from a theory
that the Mayan?s calendar was due for renewal in 2012. One particular
writer, John Major Jenkins, has written that this coincides with the
crossing of the galactic plane and the aligning of earth?s solar
plane. I have not read them in any detail and may have simplified it.
You can read of his theories on these links.

THE HOW AND WHY OF THE MAYAN END DATE IN 2012 A.D. by John Major Jenkins

Galactic Alignment: The Transformation of Consciousness According to
Mayan, Egyptian, and Vedic Traditions (Paperback) by John Major

Other references to 2012 may be found in the search term at the
bottom. Some of theories are confusing and most of them appear
apocalyptic. For instance, in Answers for Everything by Joe Paladin,
p85, of the Mayan Prophecy he writes that 2012 will bring the end of
the world as crossing the galactic plane will change the
electromagnetic fields.

In conclusion, I should obviously state that the Shiva Hypothesis is
only a theory and is open to debate and challenge.
I hope this answers your question. If it does not, or the answer is
unclear, then please ask for clarification of this research before
rating the answer. I shall respond to the clarification request as
soon as I receive it.
Thank you

Search strategy
"Galactic Plane"
"Galactic Plane " crossing
"shiva hypothesis"
mayan 2012 "galactic plane"

Request for Answer Clarification by citizena-ga on 05 Oct 2005 12:07 PDT
Thank you for the quick response, answerfinder-ga. I will need about a
day to look through, if you don't mind, as there is quite a bit of
info to go through. If I have any further questions, I will ask then,
or close and rate  the answer. Thank you for your patience.-Citizen-ga

Request for Answer Clarification by citizena-ga on 06 Oct 2005 02:32 PDT
You have provided fascinating links and clearly there are many
theories to consider; Very good synopsis of extinction events. I have
a final question. I am aware that the whole issue of a supposed
"galactic crossing in 2012" is a "byproduct" of the Mayan Calendar
literature frenzy that is proliferating these days. I was already
somewhat aware of Jenkins'ideas, and though they are complicated, they
seem to be intertwined with various mythologies, which, as you have
probably surmised in your own research, leaves out science.  So,
getting back: is it possible to determine, from what the science of
astronomy has to say, if there is any "galactic significance" (for
lack of a better term) in 2012? In other words, stripping away the
Mayan Calendar speculations, is it really an "eventful" year for the
galaxy, or is this just part of the 2012 hoopla?  I have just seen a
comment posted stating the crossing "cannot be pinned down to a year".
Would appreciate a check on what astronomy has to say. Thanks.

Clarification of Answer by answerfinder-ga on 06 Oct 2005 05:55 PDT
Thank you for your clarification request. I have searched and the only
notable astronomical event for 2012 that I can find is the Transit of
Venus on 2012 June 06. I could find no notable galactical event.

While researching your clarification I found the following which will
be of interest. Dr. Louis Strous of the Astronomical Institute,
University of Utrecht, ( has examined the interest in the
date 21 December 2012. .

Jenkins? replied to his web page and they entered into email correspondence.

citizena-ga rated this answer:5 out of 5 stars
Very timely in answering questions and correspondence, and has done a
wonderful job providing links that I didn't even know existed.
Professionally presented and documented.

Subject: Re: What happens when we cross the "Galactic Plane"
From: elids-ga on 05 Oct 2005 08:07 PDT
Good explanation,

answerfinder did you mean to say  'In conclusion, I should obviously
state that the Shiva Hypothesis is not yet a theory....'


   1. A tentative explanation for an observation, phenomenon, or
scientific problem that can be tested by further investigation


   1. A set of statements or principles devised to explain a group of
facts or phenomena, especially one that has been repeatedly tested or
is widely accepted and can be used to make predictions about natural
Subject: Re: What happens when we cross the "Galactic Plane"
From: answerfinder-ga on 05 Oct 2005 08:46 PDT
Oops, my error. Non-scientist using the wrong words I'm afraid.
Subject: Re: What happens when we cross the "Galactic Plane"
From: rracecarr-ga on 05 Oct 2005 14:45 PDT
Given that the Milky Way is kind of a fuzzy blob, and that the solar
system orbits with a time scale of tens of millions of years, it is
silly to suppose that the time when the earth crosses the galactic
plane can be pinned down to a year.  It is like claiming to know where
the middle of the Pacific Ocean is with an accuracy of 1 cm.
Subject: Re: What happens when we cross the "Galactic Plane"
From: iang-ga on 05 Oct 2005 15:11 PDT
>One such hypothesis is that these events coincided with the earth
crossing the galactic plane and encountering the dense matter in the
form of comets which struck the earth.

That's not what the Shiva Hypothesis says - the comets are already
orbiting the sun in the Oort cloud.  It's the perturbations of the
Oort cloud which might cause an increase in the number of impacts, not
the Earth running into an area with more comets in it.

Ian G.

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