Re: Online calculators for position angle

In article <dfanm1huah0pl7q77hd7jseqk05rhmdgnc@xxxxxxx>,
Chris L Peterson <clp@xxxxxxxxxxxxxxxxxx> wrote:

> On 4 Nov 2005 10:14:47 -0800, "canopus56" <canopus56@xxxxxxxxx> wrote:
>
>> What is the story behind this. I notice that Meeus insists it is
>> measured from the south meridian, not the north. What is the correct
>> way and how did it get that way? Is there some IAU position on it. -
>
> Meeus is a contrarian here. Nearly everyone uses north as the origin,
> measuring clockwise to 360 deg. This is the system recommended by the IAU
> (but not enforced as a standard). Occasionally you still see azimuth
> measured from zero to +/-180 deg, but this is fortunately becoming very
> rare. It is best to stick with 0 deg as north and 360 deg CW range if you
> want to be interpreted correctly by the largest number of people. Of course,
> there is no harm in explicitly defining your usage.

Jean Meeus is a contrarian in other matters too.

Earlier, astronomers counted longitudes on Earth as positive if west
and negative if east, i.e. "west positive". Geographers have always
counted west longitudes as negative and east longitudes as negative,
i.e. "east positive". Some time in the 1980's astronomers switched
from west positive to east positive longitudes on Earth, so today
astronomers and geographers agree on whether a longitude on Earth is
positive or negative. But this change was only for the Earth - for
the other planets astronomers contunied using west positive longitudes.

Jean Meeus refused to accept this change in terrestial longitudes,
and has continued using west positive longitudes for the Earth. He
does so in his "Astronomical Algorithms" and other books. And in
doing so, he goes against the astronomical community.

So what are the arguments for east positive and west positive longitudes?

The argument for east positive longitudes is that if you set up a
local cartesian coordinate system, with x pointing towards increasing
longitude, y towards increasing latitude and z towards increasing
altitude, east positive longitudes will give you a normally ("right
handed") oriented system, while west positive longitudes will give
you a mirrored ("left handed") system. I.e. the argument is
mathematical.

The argument for west positive longitudes is that when the planet is
viewed from a stationary point outside the planet, the longitude of
the central meridian will increase with time if the longitude is west
positive. East positive longitudes will make the longitude of the
central meridian decrease with time. I.e. the argument is
observational (assuming a traditional Earth-based observer).

Jean Meeus' argument for keeping with the older west positive
terrestial longitudes is that he wants to use the same standard for
the Earth as for the other planets. However, there seems to be a
change also for some of the other planets: people sending spacecrafts
to Mars are switching to east positive longitudes also for Mars,
because they want to use the same standard as is being used here on
Earth.

The situation remind a bit about east and west on the Moon when
astronauts started to land there: traditionally, lunar east and west
were based on the apparent directions in our sky when we viewed the
Moon. But once you actually land on the Moon, east and west will
switch, if the astronaut wants to use Earthly standards also while on
the Moon. For a number of years, both "astronomical" and
"astronautical" east and west were used on the Moon -- today
astronomers have changed their definition of lunar east and west,
making it agree with the astronautical definition. I don't know if
this includes Jean Meeus though.... :-)

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