Re: Coordinates on mars

In article <1152996338.554226.176780@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
<mike4ty4@xxxxxxxxx> wrote:

William Hamblen wrote:
On 14 Jul 2006 01:26:57 -0700, mike4ty4@xxxxxxxxx wrote:

Why is longitude on Mars measured 360 degrees all the way around, going
west, instead of like on Earth where we have two zones of 180 degrees
each, going both east and west? Why don't we uniformize these systems?

Longitudes on all the planets other than the Earth is measured that
way because it is simpler for astronomical observations. One thing
about Mars is that there are two coordinate systems in use and
measurements in one are slightly different from measurements in the
other. Selenographic and lunar coordinates on the Moon work like
latitude and longitude on maps of the Earth. This makes since because
the Moon always presents the same face to the Earth with the zero
point more or less in the middle of the disk. Lunar coordinates have
east and west defined as they look in the sky. The sun rises in the
west on the Moon in the lunar coordinate system. Selenographic
coordinates have east and west defined as they look on the Earth, The
sun rises in the east on the Moon in the selenographic coordinate
system. Longitude is measured on the Earth the way it is for historic
reasons. I suppose it might be easier for navigation, too.

How does it simplify astronomical observations?

Perhaps by removing one computational step in common navigational
computations? Of course this mattered only during the age of hand
computations.....

Actually, there were two different ways of representing longitude on Earth
until about one decade ago:

1. East longitude is positive, west longitude negative. This is the
convention used by geographers.

2. East longitude is negative, west longitude positive. This was the
convention used by astronomers until about a decade ago, when the IAU
decided to switch the astronomical convention to make it agree with
the geographical convention.

(This difference matters also if you measure longitudes from 0 to 360
rather than -180 to +180 degrees: it determines the direction to go
when starting from 0 deg longitude.)

Some astronomical traditionalists refused to switch the astronomical
convention - the most noteable example is Jean Meeus, who still in all
his books insist on using west positive and east negative longitudes.
His argument is that if he switched to east positive longitudes, then
the direction of longitude on Earth would be opposite to the direction
of longitude on all the other planets. Meeus is right about that of
course - at least until astronauts may start landing on these other
planets and the direction of longitude is switched there too, as what
seems to happen at Mars.


So why should east longitudes be positive? Well, here on Earth, if
we then travel towards numerically bigger longitudes, the local
sidereal time as well as the local solar time increases too. The
formula for the local sidereal time then becomes:

LST = GMST0 + UT(*) + long

rather than:

LST = GMST0 + UT(*) - long

and to some the "+ long" could be preferable to "- long".



But why should west longitudes be positive? Well, if we observe the
planet from space or from some other planet, if we use the convention
"west longitude positive", then the longitude of the central meridian
will increase with time instead of decrease with time. And to some,
a central meridian longitude increasing with time could be preferable
to its longitude decreasing with time.



So it's really a matter of perspective: planets we observe from the
outside could have west longitudes positive, while the planets we're
actually visiting could have east longitides positive. And it can
be acceptable to use a different direction of the longitudes for the
Earth than for the other planets, since the Earth is a special place
after all: that's were we live.


----------------------------------------------------------------------
(*) To those who wonder why I write:

LST = GMST0 + UT + long

instead of:

LST = GMST0 + UT*(366.2422/365.2422) + long

no, I'm not ignoring the different rates between sidereal and solar
time. The reason I do it is this: GMST0 (Greenwich Mean Sidereal Time
at 0h UT) is commonly computed by adding (or subtractiong) 180 degrees
to the longitude of the mean Sun, corrected for aberration. Since
it's the "sidereal time at 0h UT", it appears natural to compute it
only for 0h UT - if one does that, the (366.2422/365.2422) factor is
indeed needed. But the GMST0 quantity increases slowly, at about one
degree (= 4 minutes of time) per day, so one can compute it for any
moment, including when UT is different from zero. If one computes
GMST0 for the "now moment" rather than "the previous 0h UT moment",
then the factor (366.2422/365.2422) shouldn't be used, and the formula
can be simlified to:

LST = GMST0 + UT + long

----------------------------------------------------------------------





And what "historic reasons" are there to keep the current system for Earth?

"Historic reasons" is just another way of saying "people don't want to
change their habits". And that's why e.g. the US still uses pounds,
inches, etc instead of metric units.

--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/
.

Relevant Pages

  • Re: Pioneer 10 test of light speed delay
    ... Imagine the earth rotating and orbiting around the sun inside a big ... At some instant the longitude of the earth at Madrid and the ... so contributes zero to the Doppler shift of radiation sent fromor ... received at this earthsite longitude. ...
    (sci.astro)
  • Re: Coordinates on mars
    ... instead of like on Earth where we have two zones of 180 degrees ... Longitude is measured on the Earth the way it is for historic ... convention used by astronomers until about a decade ago, ... of longitude on all the other planets. ...
    (sci.astro)
  • Re: How do I convert Longitude/Latitude/Altitude to XYZ Coordinates? (starter info included)
    ... For a fun project in Flash, I'm plotting satellites in 3D space. ... *Latitude measures vertically on the Earth, ... *Longitude measures horizontally on the Earth, ...
    (sci.math)
  • Re: How do I convert Longitude/Latitude/Altitude to XYZ Coordinates? (starter info included)
    ... For a fun project in Flash, I'm plotting satellites in 3D space. ... *Latitude measures vertically on the Earth, ... *Longitude measures horizontally on the Earth, ...
    (sci.math)
  • Re: Empirical cult
    ... of the planets against the stellar background,the Ptolemaics drew ... Copernican astronomers correctly identified a forward moving Earth ... "For to the earth planetary motions appear sometimes direct, ...
    (uk.religion.christian)