Planetary Motion Basics?

I'd like to understand how astronomers "know where things are", what
coordinate systems are used and how time is "used" in whatever
reference methods exist in astronomy. Since my curiosity stems from
recently trying to understand the Keplerian Elements in satellite
orbits, my curiosity now extends to planetary motion and the stars.
I'm no scientist or mathematician, though, but want to build a
foundation of the topics.

I'd also like to develop a matching vocabulary to go with building a
mind's-eye understanding of how things move and how they're defined.

So, without stomping my curiosity with the elephant's foot of
complexity, can anyone make some textbook recommendations to suit my
needs?

"Wayne R." wrote in message
...
I'd like to understand how astronomers "know where things are", what
coordinate systems are used and how time is "used" in whatever
reference methods exist in astronomy. Since my curiosity stems from
recently trying to understand the Keplerian Elements in satellite
orbits, my curiosity now extends to planetary motion and the stars.
I'm no scientist or mathematician, though, but want to build a
foundation of the topics.

I'd also like to develop a matching vocabulary to go with building a
mind's-eye understanding of how things move and how they're defined.

So, without stomping my curiosity with the elephant's foot of
complexity, can anyone make some textbook recommendations to suit my
needs?

Fundamentals of Astrodynamics, Bate, Mueller, White.

Good introduction to all the above with plenty of
practical examples and exercises. Amazon has it.

Perfect, thanks!

(Did you know Amazon has over 6000 titles by "et.al."?)


On Mon, 8 Sep 2003 09:51:49 -0400, "Greg Neill"
wrote:

Fundamentals of Astrodynamics, Bate, Mueller, White.

Good introduction to all the above with plenty of
practical examples and exercises. Amazon has it.

The following is from the TheSky Softwa

The horizon coordinate system (altitude and azimuth) is not convenient for
specifying the location of celestial objects because the horizon coordinates
of stars and other objects are continuously changing with time (due to the
rotation of the Earth). For example, at Sunrise, the Sun is near zero
degrees altitude in the East, but a short 6 hours later it is high in the
sky, with a completely different altitude and azimuth.

The exception occurs with objects near the North and South celestial poles.
These are unique since they are close to the axis of rotation of the Earth
and therefore move only in small circular paths. For example, Polaris, the
North Star remains at a nearly constant altitude and azimuth. All celestial
objects that are not near the poles change position from hour-to-hour.

In the equatorial coordinate system, the coordinates of all celestial
objects remain fixed* from hour-to-hour, day-to-day and so on. An object's
equatorial coordinates remain the same regardless of from where on Earth the
object is viewed. This allows us to create star maps that apply to any
place on Earth, or publish the anticipated position of an upcoming comet so
that astronomers everywhere know where it is located among the stars.

* Equatorial coordinates change over long periods of time due to precession
(wobbling of the Earth). TheSky computes this change in stars' position for
the input date and time. Precession, however, does not change the relative
positions of objects with respect to one another.

The equatorial coordinate system used to specify the positions of celestial
objects is directly analogous to the latitude-longitude coordinate system
used on Earth. In fact, if you were to expand the latitude and longitude
grid of the Earth so that it is out beyond all stars, you would have a
sphere with identical geometry to the celestial sphere.

We suppose that all stars and deep-sky objects are located on a very large
sphere (out beyond all stars). We call this the celestial sphere. For
purposes of describing the positions of celestial objects, we consider all
stars and deep sky objects to be on the celestial sphere, when actually they
are all positioned at varying distances from the Earth.


The Declination lines on the celestial sphere are similar to the latitude
lines on Earth, ranging in value from -90 degrees to +90 degrees. The
"declination" of an object is the angle measured from the celestial equator
(0 degrees declination) along a meridian line through the object. Polaris,
the North Star has a declination of 89.26 degrees so it is very close to the
North Celestial Pole. Mintaka, the western-most star in Orion's belt has a
declination of about 0 degrees 17 minutes south so it is very near the
celestial equator.


Objects with a declination below the latitude of the observer less 90
degrees will never rise at that latitude. For example, if you live at 40
degrees North latitude, objects with a declination below minus 50 degrees
(40 minus 90) will never rise. If you are located at 90 degrees latitude
(the North Pole), you will never see any objects with a negative
declination.

We have defined the declination of stars to be the angle measured from the
equator, but we need a second coordinate to completely state the positions
of celestial objects.

The geometry of the right ascension (RA) lines on the celestial sphere is
identical to the longitude lines on Earth. Longitude lines on Earth divide
one rotation into 360 degrees, but RA lines on the celestial sphere divide
one rotation into 24 hours. Therefore one hour equals 15 degrees (360
divided by 24). See the definition of Local Sidereal Time for additional
information on why 24 hours are used for right ascension instead of 360
degrees.

Zero degrees longitude passes through Greenwich, England and is the
designated reference line for longitude. What then is the reference line for
zero hour's right ascension? Astronomers use the vernal equinox, the
location where the Sun crosses the celestial equator during its apparent
annual motion against the background stars, as a "starting point" for right
ascension.

The term "Right Ascension" comes from the fact that when viewed from the
equator, all stars rise (or ascend) at right angles to the horizon, so their
times of rising are called their times of right ascension.
"Greg Neill" wrote in message
...
"Wayne R." wrote in message
...
I'd like to understand how astronomers "know where things are", what
coordinate systems are used and how time is "used" in whatever
reference methods exist in astronomy. Since my curiosity stems from
recently trying to understand the Keplerian Elements in satellite
orbits, my curiosity now extends to planetary motion and the stars.
I'm no scientist or mathematician, though, but want to build a
foundation of the topics.

I'd also like to develop a matching vocabulary to go with building a
mind's-eye understanding of how things move and how they're defined.

So, without stomping my curiosity with the elephant's foot of
complexity, can anyone make some textbook recommendations to suit my
needs?

Fundamentals of Astrodynamics, Bate, Mueller, White.

Good introduction to all the above with plenty of
practical examples and exercises. Amazon has it.