[TowerTalk] North? Use the Moon!

[Bill Coleman AA4LR]

On 3/14/00 11:26 PM, Jim Reid at kh7m@hsa-kauai.net wrote:

>
>> >Look up at the moon.  The line dividing the light and
>> >the dark of the moon is running exactly N-S!  That
>> >dividing line is called the "Terminator",  same as
>> >hear on our world.
>>
>> While the terminator on the moon is exactly parallel to the terminator on
>> the Earth,

Actually, on reflection, this isn't precisely true, either, since the 
plane of the moon's orbit doesn't coincide with the plane of the earth's 
orbit. However, the error is just going to be a few minutes of arc, since 
the sun is so distant from either the Earth or the Moon.

>> the terminator only runs north/south twice a year -- the first
>> day of spring and the first day of fall.
>
>Hmmmm?  Does the moon also have seasons?  Or,  to say
>it another way,  is the moon also tipped 23.5 degrees on
>it's rotational axis?  Seems that would impact which way
>the terminator line would project.

Immaterial. The EARTH's terminator doesn't line up north/south but twice 
a year. 

>What is the inclination of the rotational axis of the moon?

The moon rotates in lock step with its revolution. (Which is why one side 
of the moon is always away from the earth) I don't believe it has much, 
if any, inclination. (Which is why almost half of the moon's surface 
isn't visible from the earth) 

But the moon's inclination is immaterial in measuring True North.

>However,  right now,  and for the last several and the next
>many days,  the moon is a great compass in the sky.  But,
>as more of the moon's disk gets illuminated by the Sun,  it
>will become much less useful as a North direction compass.

I'm just saying that in the Winter and Summer, observing the Moon's 
Terminator will point as many as 23 degrees AWAY from True North.

>Anyway,  it works here in the tropics,  where it does not
>work to site the Sun at sidereal noon. The Sun gets too high
>overhead,  and the shadows very,  very short!  In fact,  for
>several weeks of the year,  the silly Sun goes to the North
>of our zenith.

Measuring north from the terminator on the moon only works right at the 
turn of spring and fall. Measuring from the shadow at sidereal noon works 
during local winter everywhere, and outside the tropics, it works all the 
time. During the summertime in the tropics could be a problem....



Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@radio.org
Quote: "Boot, you transistorized tormentor! Boot!"
            -- Archibald Asparagus, VeggieTales


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