**Proof: Kepler's 1st Law ( Planetary Law of Ellipses: Sun - Centered Model )**

**"**Geometry is one and eternal shining in the mind of God. That share in it accorded to men is one of the reasons that Man is the image of God.**"** - Johannes Kepler ( 1571 - 1630 )

Since we are trying to determine a planet's orbital path as an __elliptical conic section__ with the sun at one foci with __eccentricity__, it's incumbent to express a planet's orbital radius as a function of angle and eccentricity as follows:

Therefore

However from

we also know that

Hence, a new relationship emerges

Let's now simplify the above 2nd - order differential equation into an even more simple 1st order differential equation as follows:

Now, as before in the derivation of **"**Mercury's Orbital Precession Advance of its Perihelion**"**, we set

Therefore,

**Some Further Eccentricity Relationships**

**ยง References:**

**"**Mathematical Physics**"**, 2nd edition, by Donald H. Menzel ( 1901 - 1976 ), Professor of Astrophysics, Harvard University, Dover Publications, Inc., first published in 1947 under the title**"**Theoretical Physics**"**, Prentice-Hall, Inc.**"**Calculus and Analytic Geometry**"**, 8th, 9th, 10th or 11th editions, by George Brinton Thomas, Jr. ( 1914 - 2006 ), M.I.T., Ross L. Finney, U.S. Naval Postgraduate School, and Maurice D. Weir, Professor Emeritus, U.S. Naval Postgraduate School, Addision-Wesley Publishing Company.