For the moonset at your location nearest the equinox, the moon will
have azimuth 273:32:30. This is Three and a half degrees north of
west. This is probably not accurate enough for your purposes.
Center Equ: Topo 2000.0 3/20/2000 6:57:02 AST:
RA Dec Az Alt Elong Phase RiseTm SetTm
Sun 0:00:32.53 0:03:22.9 95:00:34 5:04:59 6:26 18:33
Moon 12:16:50.50 2:03:28.5 273:32:30 0:00:01 -174.5 100 19:13 6:57
However, twenty minutes earlier the moon will be due west, with an
altitude of three degrees. You could cast a shadow at that time and
check for perpendicularity. Does this help?
Center Equ: Topo 2000.0 3/20/2000 6:36:20 AST:
RA Dec Az Alt Elong Phase RiseTm SetTm
Sun 0:00:29.39 0:03:02.5 91:17:18 1:38:59 6:26 18:33
Moon 12:16:06.89 2:07:45.7 270:00:00 3:11:50 -174.6 100 19:13 6:57
Perhaps it would be useful to use the time when the moon is due
south. Then you can make a direct comparison--no need for a
perpendicularity check.
Center Equ: Topo 2000.0 3/20/2000 0:35:36 AST:
RA Dec Az Alt Elong Phase RiseTm SetTm
Sun 23:59:34.22 -0:02:53.8 2:17:25 -44:12:30 6:26 18:33
Moon 12:06:07.02 3:22:27.2 180:00:00 47:33:11 175.7 100 19:13 6:57
Finally, perhaps the sun would be easier to use than the moon. The
sun will be due south
Center Equ: Topo 2000.0 3/20/2000 12:28:53 AST:
RA Dec Az Alt Elong Phase RiseTm SetTm
Sun 0:01:22.51 0:08:50.6 179:59:59 44:19:29 6:26 18:33
Moon 12:30:45.30 0:56:11.9 349:57:48 -42:47:42 -172.4 100 19:13 6:57
These calculations are relative to your location, AST (adjusted four
hours from UTC). Of course, nothing in these calculations requires
that this comparison be done near equinox. Should the day prove to be
cloudy, let me know and I can calculate some more for another day.
Now I will try to more directly address your questions.
"Steve Lelievre" <[EMAIL PROTECTED]> wrote:
> I take this to mean that the moon's plane of orbit is tilted too far from
> Earth's equatorial plane for a moon shadow to give a good East-West line.
> Presumably it only works when both solar and lunar declination are zero,
> which I believe to be the equinox and lunar eclipse combination I asked
> about already.
>
> How often does that happen, provided I don't mind a few hours gap between
> the two events? My own answer is:
>
> The moon is on our equatorial plane twice a month, but I'm only facing it at
> night once a month. These would be the strange "27 1/3" day months,
> wouldn't they? Equinoxes are twice a year, so the two must coincide every
> 27.33 / 2 = 13 2/3 years. If I want bright moonlight from a near full moon,
> then I need it to be 3 or 4 days either side or about one week in 4. So that
> means suitable conditions arise once every 55 years or so.
>
> Is my thinking right?
As an item of vocabulary, the two points of the lunar orbit where it
intersects the ecliptic (the plane containing the earth's orbit) are
called the nodes. As you infer, there are (approximately) two nodes
per month. I don't have my reference books here, so I cannot tell you
how many days separate the nodes, though of course it is not a
constant amount (Kepler and all that...).
Of course, there are times when the moon has zero declination
throughout the year. One of these near full moon could be used in the
way you were wanting with no need to wait for an equinox.
My point is that you really don't need the equinox to do what you're
attempting. Just use a convenient time when the sun or moon is in a
cardinal direction with a fairly low altitude (so the shadow will be
long).
> [...]
> Steve
Jim
---------- ---------------------- --------------------
| Jim Cobb | 540 Arapeen Dr. #100 | [EMAIL PROTECTED] |
| PTC | Salt Lake City, UT | (801)-588-4632 |
| | 84108-1202 | Fax (801)-588-4650 |
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Whenever I watch TV and see those poor starving kids all over the
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but not with all those flies and death and stuff. -- Mariah Carey
