Flat Disc with Triangular lighting on top

Driving home in car 4 miles north of Longmont;
granddaughter and I sighted large immobile object;
stopped car and used binoculars to verify.

Ten minutes later, used larger binoculars and compass
to re identify

Granddaughter most skeptical, then astounded at
what was irrefutably present.

Large flat glowing disc with triangular lighted structure above.
Unfortunatly no camera capable of capturing image.
Remained in southwest position for approximately 40 minutes,
suddenly vanished.