@vv Based on your mention of the role of the intersection, here is another way to determine phi1

phi1 := solve(add(`~`[rhs](el)[i]^2, i = 1 .. 3) = add(`~`[rhs](sp)[i]^2, i = 1 .. 3), phi)[1];
          
 

@vv Please explain the derivation of angle phi1 such that it limits the display of the ellipsoid to the portion of its surface which lies outside of the sphere.

Can you also show the derivation of a comparable angle which will limit the display of the sphere to the portion of its surface which lies inside the ellipsoid? 

@MapleMathMatt Thanks to you I now understand the meaning of map's index. BTW this also works:

`~`[P]([A1, A2, A3, A4], [B1, B2, B3, B4])

@MapleMathMatt I have a lot to learn about map. How does its action change when map is followed by a tilde?

@Joe Riel There is a lot to learn about how map performs. The advice below by MapleMathMatt 55 is informative

@Joe Riel Interesting that the A's must be a list of lists but the B's are a simple list.

Thank you, Acer. These York University tutorials with examples will help me greatly to understand the basics of Special Relativity.

@Pascal4QM The examples you have given will help.

@Pascal4QM I am a beginner working only with special relativity.

I have loaded the Physics package and I think I understand the use of the Coordinates command to define the components of a spacetime vector.

Please show me the Physics commands or a sample worksheet, which define the space time interval and the 4-Velocity.

Later, I will probable seek help with the definition and manipulation of tensors.

@acer You have generously increased my understanding of Maple's solve and eliminate commands. Much appreciation.

@John Fredsted Your solution was the one I expected from my command. I'll use your method in the future.

@vv I didn't realize that parametric had to be stated when the equations for solve, as in your example, include an implied condition.

It is interesting that to obtain a correct solution one must solve for all "variables" even though one of them, y in your example, already has a stated value. 

@Carl Love 

@Kitonum Please explain how your commands below cause Maple to determine the period. I particularly do not understand how f-1 represents an equation.

f:=s->eval(y(t),sol(s)):
T:=fsolve(f-1, 2..2.5);  # The period

@Kitonum I believe I now fully understand your excellent graphical application of my original pair of equations.