diff --git a/src/main.zig b/src/main.zig
index 806c219..e856191 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -18,6 +18,62 @@ pub const std_options: std.Options = .{
 // these are all in degrees
 pub const AzEl = struct { az: f64, el: f64 };
 
+pub const Log = struct {
+    file: std.fs.File,
+
+    pub fn init() !Log {
+        var buf: [64]u8 = undefined;
+        const now = std.time.milliTimestamp();
+        const filename = try std.fmt.bufPrint(buf[0..], "rotint-{d}.eventlog", .{now});
+        return .{
+            .file = try std.fs.cwd().createFile(filename, .{ .exclusive = true }),
+        };
+    }
+
+    pub fn writeEvent(self: Log, event: Event) void {
+        var buf: [128]u8 = undefined;
+        var fbs = std.io.fixedBufferStream(buf[0..]);
+        const writer = fbs.writer().any();
+
+        const now = std.time.milliTimestamp();
+        writer.print("{d},", .{now}) catch return;
+        event.write(writer) catch return;
+        writer.writeByte('\n') catch return;
+        self.file.writeAll(fbs.getWritten()) catch return;
+    }
+
+    pub const Event = union(enum) {
+        offset_updated: AzEl,
+        request_updated: AzEl,
+        command_sent: AzEl,
+        position_updated: AzEl,
+
+        pub fn designator(self: Event) []const u8 {
+            return switch (self) {
+                .offset_updated => "O",
+                .request_updated => "R",
+                .command_sent => "C",
+                .position_updated => "P",
+            };
+        }
+
+        pub fn write(self: Event, writer: std.io.AnyWriter) !void {
+            switch (self) {
+                .offset_updated,
+                .request_updated,
+                .command_sent,
+                .position_updated,
+                => |pos| {
+                    try writer.print(
+                        "{s},{d:.1},{d:.1}",
+                        .{ self.designator(), pos.az, pos.el },
+                    );
+                },
+            }
+        }
+    };
+};
+
 pub const RotInt = struct {
     allocator: std.mem.Allocator,
     // TODO: associate timestamps with this somehow
@@ -40,6 +96,8 @@ pub const RotInt = struct {
     poller: xev.Timer,
     poll_completion: xev.Completion = undefined,
 
+    log: Log = undefined,
+
     pub const State = enum {
         initial,
         rotator_connected,
@@ -50,6 +108,7 @@ pub const RotInt = struct {
     pub fn initInPlace(self: *RotInt) !void {
         self.server.rotint = self;
         self.rotator.rotint = self;
+        self.log = try Log.init();
 
         const connect_addr = try std.net.Address.parseIp("127.0.0.1", 4533);
         try self.rotator.connect(self.loop, connect_addr);
@@ -130,9 +189,11 @@ pub const RotInt = struct {
         };
 
         switch (reply) {
-            .okay => {},
+            .okay => if (self.last_command == .set_position)
+                self.log.writeEvent(.{ .command_sent = self.last_command.set_position }),
             .get_position => |pos| {
                 self.current_posture = pos;
+                self.log.writeEvent(.{ .position_updated = pos });
                 if (self.state == .rotator_connected) self.stateEvent(.rotator_ready);
                 self.draw() catch {};
             },
@@ -165,6 +226,7 @@ pub const RotInt = struct {
             .set_position => |pos| {
                 self.requested_posture = pos;
                 self.server.respond(self.loop, .okay);
+                self.log.writeEvent(.{ .request_updated = pos });
                 self.draw() catch {};
             },
 
@@ -261,6 +323,7 @@ pub const RotInt = struct {
 
                         self.offsets.az += delta.az;
                         self.offsets.el += delta.el;
+                        self.log.writeEvent(.{ .offset_updated = self.offsets });
                         self.draw() catch {
                             self.warn("draw failure", .{});
                         };