Bryn Celli Ddu is the only tomb on Anglesey which is accurately aligned to coincide with the rising sun on the longest day of the year.
Bryn Celli Ddu ydy'r unig feddrod ar Ynys Môn sydd wedi cael ei alinio'n gywir i gyd-daro â'r haul yn codi ar ddiwrnod hiraf y flwyddyn.