diff --git a/python/PiFinder/main.py b/python/PiFinder/main.py
index fb2491d2..fc26e16c 100644
--- a/python/PiFinder/main.py
+++ b/python/PiFinder/main.py
@@ -26,6 +26,7 @@
 from timezonefinder import TimezoneFinder
 
 # Keep numpy from using multiple threads
+# https://rhodesmill.org/skyfield/accuracy-efficiency.html
 os.environ["OPENBLAS_NUM_THREADS"] = "1"
 os.environ["MKL_NUM_THREADS"] = "1"