# Lab 2, due Monday, 9 Feb 04

General Considerations: Your program code should be placed at \$HOME/CZ3102/lab2/ (and set the permission to world readable). Programming can be done in C, MATLAB, Mathematica, or Java.

Q1: In class we have discussed the modeling of planetary motion. In this lab, we like to follow the foot-steps of ancient astronomers and try our best to model the motions of the Sun, Moon, and Mars, based only on observations.

(1) First, we need a good coordinate system of the sky. In astronomy, people use the Right Ascension (RA) and Declination to specify the position of the stars and planets. Search the web to understand how they are defined. We have used 'Ephemeris Generator' of Jet Propulsion Lab to give you the positions of planets. You don't need to hand-in anything for this part.

(2) In the second part of this lab, you are asked to model the motions of the three celestial objects: the Sun, the Moon, and Mars. You can choose from the following four possible models (with increased level of difficulties)

I. Ancient geocentric perfect circular motion

II. Ptolemy's geocentric circular motion with one epicenter

III. Copernicus helicentric model (still perfect circular motion)

IV. Helicentric model with Kepler's elliptic orbitals

You can make a choice from I to IV, and do only one of them. However, only the last choice IV will have the potential to get full marks.

We assume that we are in the Middle Age (with the exception that we have computer at our disposal) and we don't know the theory of Newton and we don't know a lot about astronomy except the empirical facts of the locations the planets. They are the ephemeris of the Sun, Moon, and Mars of the past, which is given here by the links. You should NOT use any other astronomical facts such as the masses of the planets, the periods, and semi-axes etc. Discuss in detail how you derived your model and how you determine the constants or parameters in your model. Your model should fit the observation reasonably well. Discuss discrepancies and accuracies of your model. Justify the method that you use to measure accuracy. Submit your code and make it available on softcopy in your lab directory. Draw a figure to show your model results and the actual observations (i.e. compare your model prediction with the ephemerides).

(3) A good model must predict well. Predict the locations (in apparent Right Ascension and Declination) of the Sun, Moon, and Mars on Monday, 9 Feb 2004, at 21:00 SST (Singapore local time), from the model you have constructed in part 2. Check your prediction with reality (e.g. go out and look at the sky).

I hope this lab is fun.

Hint: The circular or elliptical orbit in a two-dimensional plane is easy to write down. To get the same orbit that is tilted with respect to x-y plane, you need to apply a three-dimensional coordinate system rotation of the form

(x,y,z)^T = R (x',y',0)^T

where ^T is transpose. R is a 3x3 orthogonal matrix, R*R^T = 1. The matrix R can be parametrized by two Euler angles (a rotation about x and rotation about z axis).

Without actually carrying out the transformation, for the case of circle (a great circle of the sphere), the equations x' = R cos(t), y' = R sin(t), z' = 0, imply x, y, or z is a linear combination of sin and cos. So we can write,

x = x0 * cos(t+d1), y = y0 * cos(t+d2), z = z0 * cos(t+d3)

where x0, y0, z0, and d1, d2, d3 are constants. However, these constants are not independent.

On the other hand, a great circle must satisfy simultaneously

x^2+y^2+z^2 = R^2, and a*x + b*y + c*z = 0.

Since three functions of the form A * cos(t + d) are not linearly independent, the linear equation is satisfied automatically. A great circle in parametric form is obtained if we take x = x0 * cos(t+d1), y = y0 * cos(t+d2), with x0, y0, d1, and d2 as fitting parameters, and compute z from z = + or - sqrt(R^2-x^2-y^2). We don't fit for z = z0 cos(t+d3), because z0 and d3 are not independent from the parameters for x and y. [If you do the fit for z, then the orbit will be elliptical, I think]. Of course, you need the relation between Cartesian coordinates (x,y,z) and spherical polar coordinates (r, theta, phi).

Unless you are very brave, you should try model I, at least first.