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We present an analysis of the relatively low mass ($\sim2400$~M$_{\odot}$),$\sim800$~Myr, Galactic open cluster, NGC~2818, using Gaia DR2 results combinedwith VLT/FLAMES spectroscopy. Using Gaia DR2 proper motions and parallaxmeasurements we are able to select a clean sample of cluster members. Thiscluster displays a clear extended main sequence turn-off (eMSTO), a phenomenonpreviously studied mainly in young and intermediate age massive clusters in theMagellanic clouds. The main sequence of NGC~2818 is extremely narrow, with awidth of $\sim0.01$ magnitudes (G$_{\rm BP} - $ G$_{\rm RP}$), suggesting verylow levels of differential extinction. Using VLT/FLAMES spectroscopy of 60cluster members to measure the rotational velocity of the stars (Vsini) we findthat stars on the red side of the eMSTO have high Vsini ($>160$~km/s) whilestars on the blue side have low Vsini ($<160$~km/s), in agreement with modelpredictions. The cluster also follows the previously discovered trend betweenthe age of the cluster and the extent of the eMSTO. We conclude that stellarrotation is the likely cause of the eMSTO phenomenon.

Extended Main Sequence Turnoffs in Open Clusters as Seen by Gaia: I. NGC 2818 and the Role of Stellar Rotation