This May offers observers in the northern hemisphere their best chance of the year to see the planet Mercury in the evening sky. Being the closest planet to the Sun, Mercury is an elusive target because, despite being bright, it never appears in a fully dark sky. When it appears after sunset, it dips below the horizon before nightfall, and when it appears in the morning, the twilight sky is already preparing for dawn. This week, however, Mercury is climbing ever further from the Sun, and will reach its greatest elongation on 17 May. But don’t wait until then: start tonight if it’s clear. Find an unobstructed western horizon and look in the direction of the sunset. Wait until the Sun has set to begin your search. Remember never to look at the Sun directly as this can cause permanent eye damage. The chart shows the view looking west from London this evening at 21:30BST. Although Venus shows on the chart, it is unlikely to be visible in practice. However, Mars and a beautiful new moon are easy extra rewards. Viewers in the southern hemisphere can also see Mercury but the viewing angles are not as favourable, and September will offer a better chance of success.