David Hockney created a glorious depiction of a sunrise on his iPad in April and emailed it from his lockdown in Normandy to the Guardian’s art critic Jonathan Jones. He has made pictures from nature every day through this bitter spring as his artistic stand against despair – and what is more hopeful than the sun coming up? Jones describes how the picture reminded him of all the sunrises shut away inside the National Gallery, in London. From Bellini to Monet, Titian to Turner, a private view of some of the greatest masters’ sunrises