1. Orbit is a game for two players, Black and White, who alternately put stones of their color on the board. Once placed, stones do not move. They are, however, subject to capture during the course of the game, as well as to automatic removal at game's end, prior to counting territory.
2. The board is a 16x16 grid (counting the points rather than the squares), as shown in Diagram 1. At the beginning the board is empty, with play taking place on the points.
Diagram 1 --- the board for playing Orbit
3. A corner point is considered to belong to both edges that meet there.
4. It is permissible to pass. If both players pass consecutively, the game concludes.
5. It is not permissible to "mirror" the opponent's moves ten or more turns in succession.
6. The beginning of the game is governed by a "refined" pie rule. Player 1 plays two moves for Black and one move for White. Player 2 then decides which side to take, with White to move. Note that the standard pie rule is encapsulated within the refined rule, because it is permissible to pass a turn. (I.e., Player 1 could play one move for Black, then pass for White, then pass for Black.)
7. For the purpose of defining the formations used in the game, two stones are considered to be connected if they're orthogonally adjacent or diagonally adjacent.
8. A half-orbit is a connected set of stones which, together with one side of the board, completely encircles one or more points. When a half-orbit is formed, no opposing stones are captured. However, it is thereafter prohibited for the opposing player to play within that formation. An orbit is a connected set of stones completely encircling one or more points of the board. When an orbit is formed, any opposing stones contained within it are captured. (Captured stones are simply returned to their owner and do not count in scoring the game.) It is thereafter prohibited for the opposing player to play within that formation.
In Diagram 2 there are some examples of half-orbits. At the lower left Black has completed a half-orbit. The four white stones are not captured, but White cannot play inside the formation. Note that the white stones are doomed, since Black cannot be prevented from playing on the points marked in orange, forming an orbit. At the top White has completed a half-orbit. Observe that the six-stone black group can never be captured, but the two-stone black group is doomed since White cannot be prevented from playing on the point marked in orange, forming an orbit. On the right White has completed another half-orbit. The three black stones are not captured, but Black cannot play inside the half-orbit. He may, however, play on the point marked in orange, completing an orbit and capturing the three middle white stones. White's half-orbit would then be destroyed.
Diagram 2 --- half-orbits
In Diagram 3 there are some examples of orbits. At the upper left Black has completed an orbit. At the lower left, if White plays on the point marked in orange he will complete an orbit, capturing five black stones. Consider the situation on the right. If it is Black's turn, he can play on M8 and capture a white stone. However it is futile, because White can respond at J7 and thereby capture the black orbit.
Diagram 3 --- orbits
9. There is often the arising of "shared territory" --- empty points where neither player is allowed to play. Since neither player controls these points exclusively, they do not count as actual territory and thus add nothing to a player's score. (Note that shared territory is not the same thing as dame, the Go term for neutral points. It is permissible --- though useless --- to play on dame points, but playing on shared territory is prohibited.)
In Diagram 4 there are some examples of shared territory. At the upper left, White has a half-orbit that controls five points of territory. Black has a half-orbit controlling zero points of territory. The two points marked in orange are shared territory --- they are within both players' half-orbits and thus do not add to either player's score. Note that since White cannot play on them, he cannot capture the two middle black stones and destroy Black's half-orbit. On the left Black and White both have formed half-orbits. The six points marked in orange are shared territory, off limits to both players and counting for neither player. White has only four points of actual territory (i.e., places where he can play and Black cannot); Black has only three points of actual territory (at A10, B10 and D9). At the bottom a black orbit exists within a white half-orbit. The point marked in orange is at the moment shared territory. But White can claim it for his own (and capture four black stones in the process), since he cannot be prevented from connecting underneath, forming an orbit. Observe that if Black had a stone at I1 he would be safe, and the point marked in orange would remain shared. In the upper right, White and Black both have a half-orbit. White has two points of territory (at N14 and O16); Black has none. M16 and M15 are shared territory. If it is Black's turn, he will play on the point marked in orange, forming an orbit and capturing three white stones; now he will have six points of actual territory to White's zero. At the right a white orbit exists within a black half-orbit; N7 and O7 are shared territory. If it is White's turn he will play on the point marked in orange, capturing a black stone and claiming the shared territory for his own.
Diagram 4 --- shared territory
10. The game concludes when both players pass consecutively. (At this stage there are no more profitable moves to be made.) Any stones that cannot avoid capture are automatically removed from the board. Each player then figures his score, which is simply his territory (vacant surrounded points). Captured stones do not count, and shared territory does not count. Note that there may be a number of empty points which are neutral --- that is, they aren't part of anyone's territory and their occupation, though permissible, would be useless. These points, known as dame, can be quickly filled in by either player (or just left empty, it makes no difference).
Diagram 5 shows a position just after both players have passed. The following stones cannot avoid capture and are automatically removed from the board: the lone black stone at C13, the eleven-stone white orbit on the left, and the three-stone black group near the upper right (at J14, J13 and K13). Note that the white stones at N11 and N10 are alive and will stay on the board. Also note that there's a total of 9 points of shared territory in this game: at F2 and G2; at L2, M2, L1 and M1; and at O11, O10 and P10. The points marked in orange are all neutral points (dame) which will have no bearing on the score. Black has 58 points of territory to White's 38. (In practice, it is generally not necessary to actually count the score of an Orbit game --- quite often one player will succeed in forming and protecting a giant half-orbit, effectively winning the game. This is therefore the best overall strategy to adopt, provided it is implemented patiently. When victory does come it will come suddenly, making Orbit feel more like a connection game than a territorial game, though it is technically the latter. On the odd occasion, a Go-like endgame will be required.)
Diagram 5 --- both players pass
Orbit can be neatly played with pen-and-paper. (Although stones are captured during the course of the game, their positions are not subsequently re-occupied --- it is illegal for the captured player to do so, and it is a wasted move for the capturing player to do so.) Mark an open circle for White, a closed circle for Black, and an "X" through dead stones. Diagram 6 shows a completed pen-and-paper game, which Black has won by a score of 72 to 27. Some "X"ed stones were killed during the course of the game, and some were automatically killed at the end of the game (because they could not escape capture); as an exercise, you may wish to investigate for yourself which are of necessity which. Also note that there's a total of 25 points of shared territory on the board.
Diagram 6 --- a completed pen-and-paper game