## Re: [guide-user] Earth- Sun conjunction May 11 1984 as seen from Mars

Bill Gray Jun 9, 2014

Hello all,

A bit further on this oddity :

For magnitudes of the planets as seen from Earth, Guide uses "standard" formulae
as described in Jean Meeus' _Astronomical Algorithms_ and in the _Explanatory Supplement
to the Astronomical Almanac_. The formulae have been used in the _Astronomical
Almanac_ since 1984.

The Moon is a somewhat tricky case. I've been using a table of observed values
from Allen's _Astrophysical Quantities_, posted by Paul Schlyter on this list
(along with most of the formulae for planetary magnitudes) :

https://groups.yahoo.com/neo/groups/guide-user/conversations/messages/3410

I simply interpolate within those observed values. (Allen's table stopped
within 20 degrees of the sun; I added magnitude drops for 170 and 180 degrees
to make sure the moon would be around mag -3 at new moon.) Along with this, there
are some bits of code to add a special term for Saturn's moon Iapetus (which is
quite bright on one hemisphere and very dark on the other), and so forth.

All this is to say that there are agreed-upon formulae for the magnitudes of
planets as seen from the earth. However, there are no such formulae for magnitudes
of planets as seen from anywhere else. In cases where I have no "standard"
formula, I've used one I derived myself, based on the assumption of a diffuse
sphere. This gives good results at high elongations. At low elongations, it
gives values that become much too faint.

It occurs to me that I can probably get around this by saying that the brightness
of a planet with an atmosphere consists of the diffusely-reflected light, plus
a faint "baseline" magnitude below which it should not go. Its total brightness
is the sum of those two components. That should enable me to at least ensure that
planets don't drop to magnitude -infinity when close to, or in transit over, the sun.

-- Bill

On 06/09/2014 08:32 AM, 'Luc Desamore' luc_desamore@... [guide-user] wrote:
> Hi Gehrhard and Larry,
>
> Indeed it seems related to magnitudes.
> In display data shown with planets "on", there is no problem but with
> planets "fixed" or "auto" the Earth disappears:
> With "fixed" you need a magnitude of at least 17 to have the Earth displayed
> during the whole transit:
> As my setting was "on", I did not notice the disappearance.
>
> Luc
>
>
> ----- Original Message -----
> From: "Gerhard Dangl gerhard@... [guide-user]"
> <guide-user@yahoogroups.com>
> To: <guide-user@yahoogroups.com>
> Sent: Monday, 09 June, 2014 11:20
> Subject: Re: [guide-user] Earth- Sun conjunction May 11 1984 as seen from
> Mars
>
>
>> Luc,
>>
>> set the mag limit for planets to a fainter value. Right click on earth
>> disk and left click on display. It seems that the earth brightness is
>> going below a limit during the transit as seen from Mars.
>>
>> I can show the same effect as described by you if the limit value is set
>> too bright.
>>
>> I am using guide9.exe from April 2014.
>>
>> Regards
>> Gerhard
>> www.dangl.at
>>
>>
>> Am 09.06.2014 10:14, schrieb Woodchuk woodl@... [guide-user]:
>>> I am at level 6.0 and the Earth disappears when still 2' off from the
>>> limb of the
>>> Sun. The Moon travels across the face but no Earth any where to be found
>>> but it
>>> still tracks the Earth when animation is set to moving Earth. If I am out
>>> much
>>> further the Lunar + crosses the Sun but there is no label or + for earth.
>>> Anyone
>>> else with this oddity?
>>>
>>> Larry Wood
>>>
>>> On 08/06/2014 4:21 PM, 'Luc Desamore' luc_desamore@... [guide-user]
>>> wrote:
>>>> Hi Larry,
>>>>
>>>> Here it is fine, but maybe difficult to see on a black sky background.
>>>>
>>>> >From a "geocentric" ( center of Mars ) point of view the transit begins
>>>>> at
>>>> about UT 04:41
>>>>
>>>> Luc
>>
>>
>>
>> ------------------------------------
>> Posted by: Gerhard Dangl <gerhard@...>
>> ------------------------------------
>>
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>>
>>
>>
>
>
>
> ------------------------------------
> Posted by: "Luc Desamore" <luc_desamore@...>
> ------------------------------------
>
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>
>
>
>