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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: An assumption about the moon
From: George Huxtable
Date: 2010 Feb 18, 10:17 -0000

```John Huth wrote-
"The orbital inclination of the moon is 28.5 degrees, so it's going to
wander all over the place during the course of a lunar month."

It's not really true; or perhaps I should say only occasionally true, that
the Moon's orbital inclination is 28.5�.

VERY roughly speaking, the Moon's orbit, around the Earth, is in the same
plane as the Earth's orbit around the Sun. If that were exactly the case,
the "orbital inclination" of the Moon, with respect to the plane of the
Earth's equator, would be the same as that of the Sun, at  23.5�.

But, to be a bit more precise, the Moon's orbital plane is tilted by about
5� with respect to that of the Earth. And the direction of that tilt isn't
constant, though the amount is. The tilt direction wanders, just like that
of a wobbling top (and for similar reasons), making a complete circle in
18.6 years. The direction of tilt of the Earth's equatorial plane, in space,
with respect to that orbit, remains pretty constant. So some years, the
moon's orbital plane is tilted in the away from that of the Earth's equator,
in which case there's an angle between them of the sum, 28.5�, which is the
amount John has quoted. In those years, the Moon's declination shows large
swings, over each month, between N28.5� and S28.5�. That was the case around
2006. Then, 9 years later, the Moon's orbit has tilted the other way, so
that it subtracts from the tilt of the equator, the angle between them being
only 18.5�, so the Moon's declination swings between N18.5� and S18.5�. That
will happen around 2015.

The sequence, of big-swings and small-swings, repeats over that 18.6-year
cycle. On long-term average, though, the orbital inclination of the Moon is
23.5�, not John's quoted 28.5�. At present, in 2010, the Moon's orbital
inclination is just a bit more than that of the Sun, and reducing year by
year.

====================

However all that was a bit of a diversion, so back to the original question,
about wheter "the path of the moon indicated the season.".

Consider the Sun first. The direction of Sunset changes over the year, with
the Sun's declination, and that swing, between Summer and Winter, is greater
in higher latitudes. Where I live, in N51-and-a-bit, it swings between NW in
Summer and SW in Winter, roughly speaking.

And the Moon goes through that cycle once in a month, so haematomus is
correct when he recalls-

"By my recollection though I believe that the moons path ( well at least the
rising and settings points) on the horizon move back and forth fairly
quickly - probably over a lunar months time."

Indeed, those swings in moonset direction are so great, day by day, that
from your front door, say, it's easy to see the change, against a horizon
marker, from one day to the next.

However, it isn't such a simple business, to follow those movements of
moonset over the cycle of a month, as it is for the Sun over a year.
For one thing, near New Moon, it can't be seen at all. At other moonsets,
sunlight will be bright enough to render a setting Moon invisible. That
blots out many moonsets in a row; and those lost moonsets would have been at
different directions on the skyline, from one month to the next. Other
Moonsets will be at an awkward time, such as 3am, when most observers would
rather be in bed.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

----- Original Message -----
From: "Apache Runner"
To:
Sent: Thursday, February 18, 2010 12:50 AM
Subject: [NavList] Re: An assumption about the moon

| Wow, that's a tough one.   The orbital inclination of the moon is 28.5
| degrees, so it's going to wander all over the place during the course of a
| lunar month.
|
| The only "primitive" use of the moon I can do (no tables) is to look at
the
| illuminated side knowing roughly the time of night, and "follow the horns"
| of the crescent moon to find south.   I guess I could be more
sophisticated,
| but don't really bother.
|
| On Wed, Feb 17, 2010 at 7:34 PM,  wrote:
|
| > Greetings,
| >
| > recently on a camping trip in baja we were looking at the moon and
during
| > the conversation one person commented that the path of the moon
indicated
| > the season.
| >
| > By my recollection though I believe that the moons path ( well at least
the
| > rising and settings points) on the horizon move back and forth fairly
| > quickly - probably over a lunar months time.
| >
| > Is there any pattern that I could describe to someone so that they could
| > over a months time see the changes in the path of the moon without
resorting
| > to tables or even compass.
| >
| > Thanks
| >
| > ----------------------------------------------------------------
| > NavList message boards and member settings: www.fer3.com/NavList
| > Members may optionally receive posts by email.
| > To cancel email delivery, send a message to NoMail[at]fer3.com
| > ----------------------------------------------------------------
| >
|

```
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