NavList:

   

Reply

At 07:18 PM 11/30/04 +0000, George wrote...

>Michael Dorl offered his own calculations of when the Right-Ascensions of
>Sun and Mars differed by 180 degrees, about 400 years ago.
>
>In another message, he asked me about the table of Mars oppositions, of the
>same era, that I had quoted extracts from, in Meeus' "Astronomical Tables"-
>
>"Do you now how these times were calculated or how opposition is defined
>here?"
>
>Meeus defines his values as "the instant when the true heliocentric
>longitudes of the Earth and Mars, referred to the mean equinox of the date,
>are equal."
>
>I suggest that Michael, on one hand, and Meeus and Tycho, on the other
>hand, are referring to rather different quantities.

I was aware that opposition is defined in terms of ecliptic coordinates but
a first glance, I though I didn't have the requisite data so I used RA. I
also had no idea what Brahe might have been thinking. On closer
examination, I see that Mosier's routines do produce ecliptic coordinates
so I modified my almanac to show difference in ecliptic longitude between a
reference object and a target object.  So, I get a time of 11:44:00 TDT dT
= 95.4 for the 11/18/1580 opposition TDT JDate 2298474.98889.  George's
earlier message gave a time of 11:41.

I still don't know exactly what I have since the routines produce something
called 'ecliptic longitude' for all bodies and something called 'apparent
ecliptic longitiude' for only the Sun.  The Astronomical Almanac says
opposition is defined in terms of apparent ecliptic longitude. At the
instant in question, the ecliptic longitude is about 9 seconds of arc
larger than the apparent ecliptic longitude; I suspect the difference is
annual aberration.  I'll examine the code and correspond with Mosier to see
if I can figure out what is and isn't included.  At this particular time
the difference in ecliptic longitude is changing by about 1.25 arc seconds
for each minute of time.

   

Reply