diff --git a/modernc/03/.gitignore b/modernc/03/.gitignore
index 946b089..211e8e5 100644
--- a/modernc/03/.gitignore
+++ b/modernc/03/.gitignore
@@ -1 +1,2 @@
 challenge-1
+heron
diff --git a/modernc/03/heron.c b/modernc/03/heron.c
new file mode 100644
index 0000000..5af6e0c
--- /dev/null
+++ b/modernc/03/heron.c
@@ -0,0 +1,40 @@
+#include <stdlib.h>
+#include <stdio.h>
+
+/* lower and upper iteration limits centered around 1.0 */
+static double const eps1m01 = 1.0 - 0x1P-01;
+static double const eps1p01 = 1.0 + 0x1P-01;
+static double const eps1m24 = 1.0 - 0x1P-24;
+static double const eps1p24 = 1.0 + 0x1P-24;
+
+int main(int argc, char* argv[argc+1]) {
+	for (int i = 1; i < argc; ++i) {
+		double const a = strtod(argv[i], 0);
+		double x = 1.0;
+
+		for (;;) {
+			double prod = a*x;
+			if (prod < eps1m01) {
+				x *= 2.0;
+			} else if (eps1p01 < prod) {
+				x *= 0.5;
+			} else {
+				break;
+			}
+		}
+
+		for (;;) {
+			double prod = a*x;
+			if ((prod < eps1m24) || (eps1p24 < prod)) {
+				x *= (2.0 - prod);
+			} else {
+				break;
+			}
+		}
+
+		printf("heron: a=%.5e,\tx=%.5e,\ta*x=%.12f\n",
+				a, x, a*x);
+	}
+
+	return EXIT_SUCCESS;
+}