diff --git a/src/gooey.cpp b/src/gooey.cpp
index 9aeea30..8382758 100644
--- a/src/gooey.cpp
+++ b/src/gooey.cpp
@@ -41,18 +41,20 @@ gooProcessEvent(SDL_Event& e)
 		|| ImGui::GetIO().WantCaptureKeyboard);
 }
 
+void drawSimulationWindow(bool& running, u64 sim_time_ms, float& sim_speed);
 void drawEllipseParameters(ellipse_parameters& ep);
 void drawGravitaationalBody(grav_body& body);
 void drawOrbitalElements(orbital_elements& el);
 void drawSatelliteWindow(satellite& sat);
-void drawSystemWindow(system_2body& sys, bool& running);
+void drawSystemWindow(system_2body& sys);
 
 const static int G_WIDTH = 300;
 const static ImVec2 V_SPACER(0, 10);
 const int H_FLAGS = ImGuiTreeNodeFlags_DefaultOpen;
 
 void
-gooDraw(SDL_Window* window, system_2body& sys, bool& running)
+gooDraw(SDL_Window* window, system_2body& sys,
+	bool& running, u64 sim_time_ms, float& sim_speed)
 {
 	ImGui_ImplOpenGL3_NewFrame();
 	ImGui_ImplSDL2_NewFrame(window);
@@ -64,7 +66,8 @@ gooDraw(SDL_Window* window, system_2body& sys, bool& running)
 	ImGui::SetNextWindowSize(ImVec2(G_WIDTH, y));
 	ImGui::Begin("Gooey");
 
-	drawSystemWindow(sys, running);
+	drawSimulationWindow(running, sim_time_ms, sim_speed);
+	drawSystemWindow(sys);
 	drawSatelliteWindow(sys.sat);
 	drawGravitaationalBody(sys.body);
 	drawOrbitalElements(sys.elements);
@@ -75,6 +78,21 @@ gooDraw(SDL_Window* window, system_2body& sys, bool& running)
 	ImGui_ImplOpenGL3_RenderDrawData(ImGui::GetDrawData());
 }
 
+void drawSimulationWindow(bool& running, u64 sim_time_ms, float& sim_speed)
+{
+	if (ImGui::CollapsingHeader("Simulation:", H_FLAGS)) {
+		if (ImGui::Button("||")) running = false;
+		ImGui::SameLine();
+		if (ImGui::Button("|>")) running = true;
+		ImGui::SameLine();
+		ImGui::Text("running: %s", (running > 0) ? "true" : "false");
+
+		ImGui::Text("sim time:			%.3f s",
+					double(double(sim_time_ms) / 1000));
+		ImGui::Text("sim speed:			%.1f x", sim_speed);
+	}
+}
+
 void drawEllipseParameters(ellipse_parameters& ep)
 {
 	if (ImGui::CollapsingHeader("Ellipse Parameters:", H_FLAGS)) {
@@ -126,19 +144,9 @@ drawSatelliteWindow(satellite& sat)
 	}
 }
 
-void drawSystemWindow(system_2body& sys, bool& running)
+void drawSystemWindow(system_2body& sys)
 {
 	if (ImGui::CollapsingHeader("2 Body System:", H_FLAGS)) {
-		if (ImGui::Button("||")) running = false;
-		ImGui::SameLine();
-		if (ImGui::Button("|>")) running = true;
-		ImGui::SameLine();
-		ImGui::Text("running: %s", (running > 0) ? "true" : "false");
-
-		ImGui::Text("time_step:");
-		ImGui::SameLine();
-		ImGui::InputDouble("##", &sys.time_step, 5);
-
 		ImGui::Text("orbital period:             %f", sys.orbital_period);
 		ImGui::Text("epsilon, spec. orb. energy: %f", sys.epsilon);
 		ImGui::Text("h, angular momentum:        %f", sys.h);
diff --git a/src/gooey.h b/src/gooey.h
index 153f45d..26ba73a 100644
--- a/src/gooey.h
+++ b/src/gooey.h
@@ -16,5 +16,5 @@ bool
 gooProcessEvent(SDL_Event &e);
 
 void
-gooDraw(SDL_Window* window, system_2body& sys, bool& running);
-
+gooDraw(SDL_Window* window, system_2body& sys,
+	bool& running, u64 sim_time_ms, float& sim_speed);
diff --git a/src/main.cpp b/src/main.cpp
index bcb70fe..59e8320 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -38,6 +38,10 @@ struct GameState
 	bool running;
 	MemoryArena* arena;
 
+	u64 game_time_ms;
+	u64 sim_time_ms;
+	float sim_speed;
+
 	system_2body system;	// FIXME: we're going to need more than 1 system...
 	Entity* satellite_entity;
 };
@@ -181,9 +185,11 @@ loadScene(GameState* gs, RenderState* rs)
 }
 
 void
-updateSatelliteModel(const system_2body& sys, satellite& sat)
+updateSatelliteModel(const system_2body& sys,
+	satellite& sat,
+	double time_step)
 {
-	sat.theta = getPropagatedTrueAnomaly(sys, sat.theta);
+	sat.theta = getPropagatedTrueAnomaly(sys, sat.theta, time_step);
 	sat.gamma = orbitGetFlightPathAngle(sys.ep.e, sat.theta);
 	sat.r = orbitGetRadialDistance(sys.ep.e, sys.ep.p, sat.theta);
 	sat.v = orbitGetVelocity(sys.epsilon, sys.body.mu, sat.r);
@@ -193,7 +199,6 @@ updateSatelliteModel(const system_2body& sys, satellite& sat)
 void
 updateSatelliteEntity(Entity* e, const satellite& sat)
 {
-	// TODO: decouple framerate from time_step
 	const static mat4 xform =
 		glm::rotate(mat4(1.0), (float) M_PI_2, vec3(1, 0, 0));
 	const vec3& v = sat.position;
@@ -222,39 +227,50 @@ preFrameCallback(RenderState* rs, void* user_data = nullptr)
 	bool gooey_wants = false;
 
 	while (SDL_PollEvent(&e)) {
+		// FIXME: we should check for escape key here first
 		gooey_wants = gooProcessEvent(e);
 		if (!gooey_wants) inputProcessEvent(&is, e);
 	}
 
 	if (is.window_closed || is.escape)
 		rs->running = false;
-}
 
-void
-postFrameCallback(RenderState* rs, void* user_data = nullptr)
-{
+	// TODO: sim time stuff should have dedicated functions, and maybe a test
+	// 	since it's pretty important that the simulation is accurate-ish
+	// TODO: verify time to apoapsis/periapsis are correct
 	assert(user_data != nullptr);
 	GameState* gs = (GameState*) user_data;
-	gooDraw(rs->handles.window, gs->system, gs->running);
+	u64 last_sdl_tick = gs->game_time_ms;
+	gs->game_time_ms = SDL_GetTicks64();
+	u64 current_tick = gs->game_time_ms - last_sdl_tick;
 
+	// NOTE: update sim time base on time since last frame * sim_speed
 	if (gs->running) {
-		updateSatelliteModel(gs->system, gs->system.sat);
+		u64 last_game_tick = gs->sim_time_ms;
+		gs->sim_time_ms = gs->sim_time_ms + current_tick * gs->sim_speed;
+		u32 diff_ms = gs->sim_time_ms - last_game_tick;
+		double time_step = double(diff_ms) / 1000;
+
+		updateSatelliteModel(gs->system, gs->system.sat, time_step);
 		gs->system.ep = ellipseInitAE(gs->system.ep.a, gs->system.ep.e);
 		updateSatelliteEntity(gs->satellite_entity, gs->system.sat);
 		//updateOrbit(gs->system, ellipse_entity);
 	}
+}
 
-	// FIXME: testing sim time
-	u64 ticks = SDL_GetTicks64();
-	static u32 last_T;
-	u32 T = ticks / 1000;
-
-	if (T != last_T) {
-		LOGF(Debug, "seconds since sim start: %d\n", T);
-		last_T = T;
-	}
+void
+postFrameCallback(RenderState* rs, void* user_data = nullptr)
+{
+	assert(user_data != nullptr);
+	GameState* gs = (GameState*) user_data;
+	gooDraw(rs->handles.window,
+			gs->system,
+			gs->running,
+			gs->sim_time_ms,
+			gs->sim_speed);
 }
 
+#define DEFAULT_SIM_SPEED 100
 int
 main()
 {
@@ -274,6 +290,7 @@ main()
 
 	GameState gs = {0};
 	gs.arena =  arenaInit(16);
+	gs.sim_speed = DEFAULT_SIM_SPEED;
 	loadScene(&gs, rs);
 	doRenderLoop(rs, 60, preFrameCallback, postFrameCallback, &gs);
 	gooFree();
diff --git a/src/orbits.cpp b/src/orbits.cpp
index f09a9d6..c465fab 100644
--- a/src/orbits.cpp
+++ b/src/orbits.cpp
@@ -6,7 +6,7 @@ const static uint ELLIPSE_VERT_COUNT = 256;
 
 
 system_2body
-systemInit(grav_body gb, orbital_elements el, float time_step)
+systemInit(grav_body gb, orbital_elements el)
 {
 	system_2body s = {0};
 	s.body = gb;
@@ -18,7 +18,6 @@ systemInit(grav_body gb, orbital_elements el, float time_step)
 	s.orbital_period = orbitGetPeriod(s.ep.a, gb.mu);
 	s.r_apoapsis = s.ep.a - s.ep.c;
 	s.r_periapsis = 2 * s.ep.a - s.r_apoapsis;
-	s.time_step = time_step;
 
 	return s;
 }
@@ -214,13 +213,14 @@ getNextTrialValue(double err, double ecc, double test_anom, double mean_anom)
 
 double
 getPropagatedEccAnomaly(system_2body sys,
-	double initial_anom)
+	double initial_anom,
+	double time_step)
 {
 	double e = sys.ep.e;
 	double E1 = getEccAnomFromTrueAnom(e, initial_anom);
 	double M1 = getMeanAnomFromEccAnom(E1, e);
 	double n = getMeanMotion(sys.body.mu, sys.elements.a);
-	double M2 = getPropagatedMeanAnom(M1, n, sys.time_step);
+	double M2 = getPropagatedMeanAnom(M1, n, time_step);
 	double E2_1 = getInitialTrialValue(M2, e);
 
 	// test if guess is a solution to kepler's equation
@@ -241,13 +241,15 @@ getPropagatedEccAnomaly(system_2body sys,
 
 double
 getPropagatedTrueAnomaly(system_2body sys,
-	double initial_anom)
+	double initial_anom,
+	double time_step)
 {
+	// FIXME: I don't think we need this now that we have gs->running?
 	// NOTE: 'pause' simulation when time_step is set close to 0
-	if (sys.time_step < 1e-8 && sys.time_step > -1e-8)
+	if (time_step < 1e-8 && time_step > -1e-8)
 		return initial_anom;
 
-	double ecc_anom = getPropagatedEccAnomaly(sys, initial_anom);
+	double ecc_anom = getPropagatedEccAnomaly(sys, initial_anom, time_step);
 	return getTrueAnomFromEccAnom(sys.ep.e, ecc_anom);
 }
 
diff --git a/src/orbits.h b/src/orbits.h
index 2328c2a..0ca1e86 100644
--- a/src/orbits.h
+++ b/src/orbits.h
@@ -67,16 +67,11 @@ struct system_2body
 	double r_apoapsis;		// NOTE: apoapse distance from body center
 	double r_periapsis;		// NOTE: periapsis distance from body center
 	double orbital_period;	// NOTE: in seconds
-
-	double time_step;
 };
 
 
-#define DEFAULT_TIME_STEP 1
 system_2body
-systemInit(grav_body gb,
-			orbital_elements el,
-			float time_step = DEFAULT_TIME_STEP);
+systemInit(grav_body gb, orbital_elements el);
 
 grav_body
 gravBodyInit(double mu, double r);
@@ -152,7 +147,8 @@ orbitUpdate(orbital_elements& o, double a, double e);
  */
 double
 getPropagatedTrueAnomaly(system_2body sys,
-	double initial_anom);
+	double initial_anom,
+	double time_step);	// NOTE: in seconds
 
 // NOTE: aka) the trajectory equation (eq. 2.45)
 double