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Brad,

> If the orbital period of Mercury is 88 days, does it not move (360/88)

No. Because it does not make the full circle (360d) around the sky,
like the Moon does. It moves on a small elongated ellipse around the Sun,
and the size of this ellipse is between 17 to 27 degrees.

All other motions (of Sun, of Mars) can be neglected here, they are slower.
The conclusion is that the speed is not sufficient for determination
of time.

Alex.



> about 4 degrees per day?  So that's about 10 arc minutes per hour of time
> or 10 arc seconds per minute of time.  Only 3x slower than the Moon.
>
> Then we must consider the Earth is moving 1 degree per day, but the
> objects
> in question (Mars,Mercury) will indeed show parallax effects.  They really
> aren't close, they just look that way.  So doesn't the parallax help us?
>
> Best Regards
> Brad
>
>
>
> On Feb 3, 2013 12:50 AM, "Alexandre Eremenko" 
> wrote:
>
>> ------------------------------
>>
>>
>> It is not possible to determine longitude from the distance
>> between Mars and Mercury with an ordinary sextant with
>> reasonable accuracy.
>>
>> Here is a very crude estimate.
>> Mercury has period about 88 days.
>> And the maximum elongation from the Sun is about 20 degrees in the
>> average. Which means that Mercury moves less than 1/4 degree per day.
>> While the Moon moves roughly 12 degrees per day.
>> Thus determination of the longitude from the Mercury distance
>> to anything will be
>> about 50 times less accurate than from the Moon distance.
>>
>> Definitely out of the limit of the Longitude prize:-)
>>
>> I am not even mentioning the difficulty if seeing Mercury
>> through the sextant telescope.
>>
>> Alex.
>>
>> On Sat, 2 Feb 2013, Brad Morris wrote:
>>
>> >
>> > Coming up this week, Mars and Mercury will be in close angular
>> proximity to each other.  Mercury, of course, is one of the swiftest
>> moving planets in the solar system.   Both Mercury and Mars are in the
>> ecliptic plane.
>> >
>> > The question is basic.  Can we tell our longitude by the angular
>> separation of the two bodies, similar to a lunar?  Can we do this with
>> a sextant?  We might be able to tell our longitude using this method,
>> but its below the measurement sensitivity of a sextant.
>> >
>> > Just asking!
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>> > : http://fer3.com/arc/m2.aspx?i=122239
>> >
>> >
>> >
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>> : http://fer3.com/arc/m2.aspx?i=122241
>>
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> : http://fer3.com/arc/m2.aspx?i=122243
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