diff --git a/last_checkpoint.npz b/last_checkpoint.npz
new file mode 100644
index 0000000..1edcfaa
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diff --git a/src/__init__.py b/orbiter/__init__.py
similarity index 100%
rename from src/__init__.py
rename to orbiter/__init__.py
diff --git a/orbiter/__pycache__/__init__.cpython-312.pyc b/orbiter/__pycache__/__init__.cpython-312.pyc
new file mode 100644
index 0000000..7d52799
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diff --git a/orbiter/__pycache__/units.cpython-312.pyc b/orbiter/__pycache__/units.cpython-312.pyc
new file mode 100644
index 0000000..bfa835f
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diff --git a/src/orbits/__init__.py b/orbiter/orbits/__init__.py
similarity index 100%
rename from src/orbits/__init__.py
rename to orbiter/orbits/__init__.py
diff --git a/orbiter/orbits/__pycache__/__init__.cpython-312.pyc b/orbiter/orbits/__pycache__/__init__.cpython-312.pyc
new file mode 100644
index 0000000..7c8f0c8
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diff --git a/orbiter/orbits/__pycache__/body.cpython-312.pyc b/orbiter/orbits/__pycache__/body.cpython-312.pyc
new file mode 100644
index 0000000..0ab8563
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diff --git a/orbiter/orbits/__pycache__/simulator.cpython-312.pyc b/orbiter/orbits/__pycache__/simulator.cpython-312.pyc
new file mode 100644
index 0000000..5749f93
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diff --git a/src/orbits/body.py b/orbiter/orbits/body.py
similarity index 89%
rename from src/orbits/body.py
rename to orbiter/orbits/body.py
index c2b5236..57028a6 100644
--- a/src/orbits/body.py
+++ b/orbiter/orbits/body.py
@@ -1,7 +1,7 @@
 from enum import Enum
 import numpy as np
 
-from ..units import Position, Velocity, Mass, Acceleration, Force
+from ..units import Position, Velocity, Mass, Acceleration
 
 class Body:
     """
diff --git a/src/orbits/simulator.py b/orbiter/orbits/simulator.py
similarity index 57%
rename from src/orbits/simulator.py
rename to orbiter/orbits/simulator.py
index a75249f..317e2d6 100644
--- a/src/orbits/simulator.py
+++ b/orbiter/orbits/simulator.py
@@ -1,7 +1,7 @@
 from pathlib import Path
+import numpy as np
 
-from body import Body
-from ..units import Position, Velocity, Mass, Acceleration, Force
+from .body import Body
 
 class Simulator:
     """
@@ -31,7 +31,8 @@ class Simulator:
                 bodies: list[Body],
                 step_size: float,
                 steps_per_save: int,
-                output_file: Path
+                output_file: Path,
+                current_step: int = 0,
                 overwrite_output: bool = False
     ):
         if output_file.exists() and not overwrite_output:
@@ -40,43 +41,63 @@ class Simulator:
         self.output_file = output_file
         self.bodies = bodies
         self.step_size = step_size
-        self.steps_to_take = steps_to_take
         self.steps_per_save = steps_per_save
+        self.current_step = current_step
 
         if output_file.exists() and overwrite_output:
             print(f"Warning! Overwriting file: {output_file}")
 
-        _save_body_masses_to_file()
-        _checkpoint()
+        #self._save_body_masses_to_file()
+        self._checkpoint()
 
     
-        self.current_step = 0
-
     @classmethod
-    def from_checkpoint(cls, input_file: Path):
+    def from_checkpoint(cls, output_file: Path):
+        data = np.load("last_checkpoint.npz")
+        positions = data["positions"]
+        velocities = data["velocities"]
+        masses = data["masses"]
+        step_size = data["steps"][0]
+        current_step = data["steps"][1]
+        steps_per_save = data["steps"][2]
 
+        bodies = [
+            Body(val[0], val[1], val[2]) for val in zip(
+                positions, velocities, masses
+            )
+        ]
+        return cls(
+            bodies, 
+            step_size, 
+            steps_per_save,
+            output_file,
+            current_step,
+        )
 
-    def _save_body_masses_to_file(self):
-        masses_str = ' '.join([
-            str(body.m) for body in bodies
-        ]) + '\n'
-        #if saving masses, we are always starting a new file,
-        #this will overwrite file.
-        self.output_file.write_text(masses_str)
     
     def _checkpoint(self):
         """
         Two things - save high precision last checkpoint for resuming
         then save lower precision text for trajectories
         """
-        body_X_V_np = np.array([
-            [body.X, body.Y] for body in self.bodies
+        body_X_np = np.array([
+            body.X for body in self.bodies
         ])
-        body_ints_np = np.array([
+        body_V_np = np.array([
+            body.V for body in self.bodies
+        ])
+        body_m_np = np.array([
             body.m for body in self.bodies
         ])
-        body_m_np.append(self.step_size, self.current_step)
-        np.savez("last_checkpoint.npz", 
-                array=body_X_V_np,
-                ints = body_m_np)
+        stepsz_n_np = np.array([
+            self.step_size,
+            self.current_step,
+            self.steps_per_save
+        ])
+
+        np.savez("last_checkpoint.npz", 
+                positions=body_X_np,
+                velocities=body_V_np,
+                masses=body_m_np,
+                steps=stepsz_n_np)
 
diff --git a/orbiter/units.py b/orbiter/units.py
new file mode 100644
index 0000000..9ab7ee7
--- /dev/null
+++ b/orbiter/units.py
@@ -0,0 +1,9 @@
+import numpy as np
+
+Position = np.array
+
+Velocity = np.array
+
+Acceleration = np.array
+
+Mass = int
diff --git a/src/units.py b/src/units.py
deleted file mode 100644
index 02bddb9..0000000
--- a/src/units.py
+++ /dev/null
@@ -1,16 +0,0 @@
-import numpy as np
-
-class Position(np.array):
-    pass
-
-class Velocity(np.array):
-    pass
-
-class Acceleration(np.array):
-    pass
-
-class Force(np.array):
-    pass
-
-class Mass(int):
-    pass