Reiner Ruthenbeck; Julio Le Parc review – elegant, charming, fetchingly simple

Serpentine Gallery; Serpentine Sackler Gallery, London W2
The first big UK shows for German artist Reiner Ruthenbeck, 77, and Argentina’s Julio Le Parc, 86, are long overdueAt dusk in Hyde Park, the sun hangs so low among the winter trees it is little more than a fingernail of light against the darkness. Night is gradually setting in. Soon it will cancel the day completely, but the exact moment when this happens is beautifully mysterious – and the subject of a work inside the Serpentine Gall