Coordinates on mars

Hi.

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?

On 14 Jul 2006 01:26:57 -0700, 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.

William Hamblen (or somebody else of the same name) wrote thusly in message
:

On 14 Jul 2006 01:26:57 -0700, 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?

[snip]
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.

So what defines 0° longitude on Mars?
--
Warning: keel away from child for hot bulb

Interchange the alphabetic letter groups to reply

Prai Jei wrote:
So what defines 0° longitude on Mars?

In theory, the center of crater Airy-0. In practice, the coordinate
system is defined implicitly by the expression for the angle W:

W = 176.630 + 350.89198226 d

where d is the number of days since the standard epoch 2000 Jan 1 12:00.
W is measured in the plane of Mars' equator, from the ascending node of
Mars' equator on the fundamental x-y plane to the location of the
Martian prime meridian. To put it another way, the transformation
from ICRS coordinate (Earth equator and equinox of J2000.0) to Mars
body-fixed coordinates is the matrix product R3(W) R1(90-Dec) R3(RA+90)
where RA and Dec are the direction of the Martian north pole.

-- Bill Owen

On Fri, 14 Jul 2006 23:01:01 +0100, Prai Jei
wrote:

William Hamblen (or somebody else of the same name) wrote thusly in message
:

On 14 Jul 2006 01:26:57 -0700, 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?

[snip]
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.

So what defines 0° longitude on Mars?

There's a small crater named Airy that marks the zero point.

BTW, please read since as sense.

Bud

William Hamblen wrote:
On 14 Jul 2006 01:26:57 -0700, 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? And what "historic
reasons" are there to keep the current system for Earth?

On 15 Jul 2006 13:45:38 -0700, wrote:

How does it simplify astronomical observations? And what "historic
reasons" are there to keep the current system for Earth?

You figure the longitude of the central meridian from the time of the
observation. You have some commonly agreed upon zero time and just
count rotations from there. This is about the only way you can do it
when there is no sharply defined zero point visible on the surface.
It is less trouble to count up continuously rather than count up a
half rotation then count down for the other half.

The historic reason on the Earth is that there are a gazillion charts
and land records you would have to change.

On Sun, 16 Jul 2006 09:13:34 GMT, (Paul Schlyter)
wrote:

"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.

Of course the foot and pound have been defined as ratios of the
standard meter and kilogram since the 1890s, so they are "derived
units".

Paul Schlyter wrote:
In article ,
Bill Owen wrote:

Prai Jei wrote:
So what defines 0 deg longitude on Mars?


I don't think anyone has pointed this out yet - That crater was
selected for consistency with a much older definition (going
back a good century) that put zero longitude through the center
of the two forklike projections of the dark albedo feature
Sinus Meridiani (hence the name), which is the "pipestem"
extending from Syrtis Major.

Bill Keel

Paul Schlyter wrote:
In article .com,
wrote:

William Hamblen wrote:
On 14 Jul 2006 01:26:57 -0700, 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.


Why though is the +/- 180 degree system used for Earth while a
full-round
360-degree system is used everywhere else? Ie. why do we just talk
about
"X degrees" on Mars, etc. vs. "X degrees West/East" on Earth?


----------------------------------------------------------------------
(*) 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.


Laziness

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