Greek sporting events were closely linked to religion, and therefore games were held at religiously important places or sacred sites. The competitions, held every two or four years and lasting for centuries, included Zeus at Olympia (from 776 B.C.), Apollo at Delphi (from 586 B.C.), Poseidon at Isthmia (from B.C.). It was to honor a particular god or demi-god, such as (from 580 BC) and Opheltes of Nemea (from 573 BC). These constituted the four great Pan-Hellenic Games; However, other locations in the Greek homeland and islands also hosted regular sports festivals. The oldest competition at these Games was a foot race over a certain distance - the stade or stadium, which was 600 meters long. Since the foot (or pous) was not strictly standardized across Greece, a stadion could vary in different places, from 177 m long at Delphi to 192 m at Olympia. At Nemea, small poles survive along the side of the track marking every 100 meters across the stadium