Directional Coordinates

I live in Western Washington State where over half of the year is
cloudy and rainy. Clear skies are rare most of the year. In which case,
I've never found it practical to own a good telescope and don't have
much experience with one.


However, I have read about the complex system that the astronomy
community uses, which is necessary to figure directional reference in
the universe. Like others, I have a very general idea of how it works,
but still lack much in the way of being able to think of it in a
practical manner.


It would seem that in some way a method could be devised so that any of
these coordinates might be better visualized. For us laymen, one of
many confusing things about the universe is there is no up or down --
north, east, south or west.


Knowing where the HST is located in relation to the earth at the time a
particular image is taken along with the direction the telescope is
aimed, would be very interesting for me. Earth based telescopes would
be equally as important. Still there are others who would say, "The
pictures are coming from such and such a place out there and that's all
I care about."


Computer software may have already been created to show what direction
each of the various telescope images are coming from ..... ?


I've often thought that 3 dimensional software models might be designed
with the ability to show the actual aim trajectory and in some cases,
even the target of most telescopes. I'd think that this software could
be designed, so that by entering the HST and any earth based telescope
coordinates into the software program, you'd have the desired results
for that particular day.


Needless to say, a 3D model of the Solar System in combination with the
Milky Way Galaxy, the Local Group and so on would certainly help to
better envision the BIG picture of at least a very small part of the
universe.

Chuck wrote:
I live in Western Washington State where over half of the year is
cloudy and rainy. Clear skies are rare most of the year. In which case,
I've never found it practical to own a good telescope and don't have
much experience with one.


However, I have read about the complex system that the astronomy
community uses, which is necessary to figure directional reference in
the universe. Like others, I have a very general idea of how it works,
but still lack much in the way of being able to think of it in a
practical manner.

I suspect you are thinking of equitorial coodinates--Those appearing on
star charts, atlases and planispheres.

Coordinate Systems

There are at least five coordinate systems used in astronomy. We will
concern our selves the first three of them.

Geodetic - First consider geodetic coordinates. "Geo" refers
to the Earth. The most common geodetic coordinates used all over the
world are Latitude and Longitude. The MCC campus has geodetic
coordinates very close to 42° North Latitude and 93° West Longitude.
This means that MCC is located 42° north of the Earth's equator and
that it is located 93° west of the "Prime Meridian", that great
circle that passes through Greenwich England and both the north and
south poles. Remember it is 360° all the way around the Earth. See
Figure 1.7 on page 5 of your textbook--substituting "MCC" for
"Riverside Observatory"

Horizon (Horizonal) - Horizonal coordinates consist of an
azimuthal angle (like a magnetic compass) and an altitude, an angle
from the horizon up to an object in the sky. See Figure 1.14 on page
8 of your textbook. When you face north, you are looking in the 0°
direction, east 90°, and so on. What is the azimuth when you are
facing south-west? Every object from your perspective has a unique
pair of horizonal coordinates. An hour later the same object will
have a different set of horizonal coordinates because the Earth (and
you) are turning eastward underneath the sky.

Equatorial - Like the state map of Iowa and country maps with
Latitude and Longitude coordinates, sky charts have a similar grid
system called equatorial coordinates based on the Earth's equator and
poles. The coordinate that corresponds to the north and south of
latitude is call Declination (Dec). Declination is zero at the
celestial equator and is positive up to +90° toward the north and down
to -90° toward the south. In stead of longitude starting at the prime
meridian going through Greenwich England, the left-right scale is
called Right Ascension going from 0-24 hours (this is the same as going
from 0-360°) starting at a point in the sky called the Vernal Equinox
which is where the Sun appears to be in the sky on the first day of
spring. The equatorial coordinate system is essentially fixed on the
stars and the whole coordinate system appears to rotate with the sky
as the Earth (and you) are turning eastward underneath it.



Ecliptic - Similar to equatorial coordinates but centered on the
ecliptic instead of the celestial equator.

Galactic - Similar to equatorial coordinates but centered on the
galactic equator of the Milky Way.

On 10 Dec 2004 07:47:45 -0800, "Chuck" wrote:

I have read about the complex system that the astronomy
community uses, which is necessary to figure directional reference in
the universe. Like others, I have a very general idea of how it works,
but still lack much in the way of being able to think of it in a
practical manner.

It would seem that in some way a method could be devised so that any of
these coordinates might be better visualized. For us laymen, one of
many confusing things about the universe is there is no up or down --
north, east, south or west.

The system is not at all complex. It is precisely the system we use on Earth,
latitude and longitude (for astronomical coordinates, latitude is called
declination, and longitude is called right ascension). And just like the
terrestrial coordinate system, there are fixed directions north, south, east,
and west.

There is a slight complication that the Earth rotates, which means that the
position of a given astronomical coordinate changes with time, but it is simple
enough to run a planetarium program to see what the sky looks like at any
particular time. The object coordinates don't change with time, so a star chart
is fixed.


Knowing where the HST is located in relation to the earth at the time a
particular image is taken along with the direction the telescope is
aimed, would be very interesting for me.

There is no need to know the exact position of the HST when a picture is made,
only the direction it is pointed. That's because everything it is aimed at is so
far away that the range of positions it can be in make no difference to the
image. And the direction of every image is available as a coordinate pair, right
ascension and declination.


I've often thought that 3 dimensional software models might be designed
with the ability to show the actual aim trajectory and in some cases,
even the target of most telescopes. I'd think that this software could
be designed, so that by entering the HST and any earth based telescope
coordinates into the software program, you'd have the desired results
for that particular day.

From a practical standpoint, astronomers (and the HST) essentially live in a
two-dimensional environment. With the exception of our own solar system,
virtually every other object can usually be treated as existing on the inside of
a sphere whose surface is infinitely distant.


Needless to say, a 3D model of the Solar System in combination with the
Milky Way Galaxy, the Local Group and so on would certainly help to
better envision the BIG picture of at least a very small part of the
universe.

Such programs exist. They allow you to simulate space travel, and watch how the
relative positions of things change as you travel cosmic distances.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Thank you Sam and Chris for your professional knowledge -------------
what a privilege! Your explanations make a lot of sense over what I
had originally read and thought to be down right confusing.

After reading your replies, I formed a pair of 180 degree arcs (about
18 inches across) from a soft metal bar and I'll use a protractor to
cut in gradations. One arc will be placed horizontally (flat) for
declination and one placed vertically (standing) for ascension.
Although this combination isn't extensive enough for all possible
directions, for starters it gives me something to help visualize the
coordinates.

I have another question ---- It would be easier for me to draw a
picture ha, ha, rather than trying to write the question, but I'll give
it a try:

In relation to the plane on which the Solar System rotates, how does
the tilt of the Milky Way Galaxy plane compare, in degrees? I'd also
like to be able to figure the Canis Major plane, the Andromeda plane
and so on using the methods you've explained.

I'm a real hands on guy and I like to built things. When the planes
have been figured, I would simply make different sizes of plastic disks
for each galaxy plane and then position them on a stand with the Solar
system. Of course, nothing would even be close to scale, but the
directions from the Solar System disk and the tilt of each galaxy plane
should be close. I'd like to position all known galaxies in The Local
Group in an area measuring about ten feet in length, width and height.
On each galaxy disk I'd also like to show the direction of rotation. I
tend to learn more when I use numbers to create in three dimesions.

Is there a book available for the greenest of amateurs to help learn
more about this particular subject

Chuck wrote:
Thank you Sam and Chris for your professional knowledge -------------
what a privilege! Your explanations make a lot of sense over what I
had originally read and thought to be down right confusing.

After reading your replies, I formed a pair of 180 degree arcs (about
18 inches across) from a soft metal bar and I'll use a protractor to
cut in gradations. One arc will be placed horizontally (flat) for
declination and one placed vertically (standing) for ascension.
Although this combination isn't extensive enough for all possible
directions, for starters it gives me something to help visualize the
coordinates.

I have another question ---- It would be easier for me to draw a
picture ha, ha, rather than trying to write the question, but I'll give
it a try:

In relation to the plane on which the Solar System rotates, how does
the tilt of the Milky Way Galaxy plane compare, in degrees? I'd also
like to be able to figure the Canis Major plane, the Andromeda plane
and so on using the methods you've explained.

Guy Ottewell's "The Astronomical Companion" is exactly what you
are looking for... excellent graphic representations depicting
the horizonal plane of an observer, the ecliptic plane of the
solar system, and the galactic plane of the Milky Way. He even
includes the orientation of Andromeda's galactic plane.

See: http://www.astromax.com/go-companion.htm


I'm a real hands on guy and I like to built things. When the planes
have been figured, I would simply make different sizes of plastic disks
for each galaxy plane and then position them on a stand with the Solar
system. Of course, nothing would even be close to scale, but the
directions from the Solar System disk and the tilt of each galaxy plane
should be close. I'd like to position all known galaxies in The Local
Group in an area measuring about ten feet in length, width and height.
On each galaxy disk I'd also like to show the direction of rotation. I
tend to learn more when I use numbers to create in three dimesions.

Is there a book available for the greenest of amateurs to help learn
more about this particular subject