Watch the crescent Moon graze a double star at dawn on 27 August
Depending on where you live in the British Isles, you may be fortunate to view a lunar occultation of naked-eye double star delta (δ) Geminorum at dawn on Tuesday, 27 August 2019. Observers fortunate enough to lie on the so-called graze line will
Will you see it or won’t you? This looping animation shows that the occultation of magnitude +3.5 double star delta (δ) Geminorum (aka ‘Wasat’) at dawn on Tuesday, 27 August 2019 is only visible from parts of the UK (for the extent of visibility, see the map below). Those observers fortunate enough to lie on the so-called graze line can watch Wasat appear to flicker on and off as it is alternately visible in lunar polar valleys and hidden by adjacent peaks as the Moon drifts by. AN animation by Ade Ashford. Mere days after the Moon’s close encounter with three stars in the constellation of Taurus, the waning lunar crescent has another dawn rendezvous with a naked-eye star – this time it’s delta (δ) Geminorum , otherwise known as Wasat in the constellation of Gemini, on Tuesday, 27 August . The exciting thing about this particular occultation – when a nearby celestial body passes between the observer and a more distant object, from the Latin occulo, ‘to hide’ – is that not all observers in the British Isles will see it: those south of the line drawn between Swansea Bay on the southern coast of Wales and The Wash on the Norfolk/Lincolnshire border will see Wasat hidden by the Moon, but everyone else will see a near miss. Observers located in the greyed out part of the UK will see delta (δ) Geminorum [aka ‘Wasat’] hidden by the Moon around 5:15am BST on Tuesday, 27 August, but everyone to the north and west will not. The boundary of the grey and white areas is the so-called graze line, where lucky observers will see the star flicker on and off as the mountains and valleys of the lunar polar regions drift by. AN graphic by Ade Ashford based on data provided by Guide 9.1. Occultation timings For an observer in Plymouth, the disappearance and reappearance times of δ Geminorum are 5:04am and 5:20am, respectively. In Southampton, expect Wasat’s occultation shortly before 5:04am with the star hidden for about 20 minutes. As seen from London, the star disappears at the Moon’s illuminated edge at 5:05am and reappears at the darkened limb at 5:25am (all times are British Summer Time). Wasat – a close binary A bonus for those observers equipped with modest telescopes is that δ Geminorum is an attractive double star. The primary is a magnitude +3.5 subgiant 59 light-years from the Sun separated by 5.5 arcseconds from a magnitude +8.2 companion. The stars of δ Geminorum orbit each other every 1,200 years. At magnifications of around 100×, see if you can detect a yellowish tint to the primary star and a pale bluish hue to the fainter companion.