Observers in the British Isles with an unobscured view of the southwest horizon should direct their telescopes at the setting 4-day-old crescent Moon by 7:40pm, for if skies are clear you may get a chance to see magnitude +3.9 star gamma (γ) Librae (aka Zuben Elakrab in Arabic) slip behind the darkened lunar hemisphere. The time at which this occultation occurs is just before 7:55pm in London, or 7:45pm in Edinburgh (all times quoted in this article are in British Summer Time). AN graphic by Ade Ashford/Stellarium.A waxing crescent Moon sliding serenely toward the horizon in evening twilight is always a pleasant sight to behold, but observers in the UK watching the 4-day-old Moon through a telescope around 40 minutes after sunset on Sunday, 24 September have an additional treat in store. The lunar crescent lies in the zodiacal constellation of Libra (the Scales) and passes in front of naked-eye star gamma (γ) Librae, also known by its Arabic name of Zuben Elakrab, soon after 7:45pm BST — though the precise time depends on where you live in the British Isles.
For observers in the London area, the magnitude +3.9 star winks out behind the darkened lunar hemisphere shortly before 7:55pm, while skywatchers closer to Edinburgh will see the occultation occur at 7:45pm BST. For observers nearer the centre of the British Isles, the event happens close to 7:49pm when the 20 percent illuminated Moon lies just 9 degrees — barely the span of a fist held at arm’s length — above the southwest horizon.
Gamma Librae lies about 160 light-years from Earth. Interestingly, it is also a double star, though its magnitude +11 companion some 42½ arcseconds distant will not be visible owing to the glare from the adjacent Moon and atmospheric extinction so close to the horizon. Furthermore, the star’s reappearance at the bright lunar limb close to 8:55pm BST is unlikely to be seen so close to moonset.
While looking for Geminid meteors, observers in Western Europe should also look to the southeast about 90 minutes before sunrise to see a beautiful conjunction of a 25-day-old Moon, Jupiter and Mars on Thursday, 14 December.
When a nearby astronomical body passes between the observer and a more distant object, see say that an occultation is taking place. Since the Moon is our nearest celestial neighbour, it regularly passes in front of planets and stars. Observers in the British Isles can see naked-eye star Zeta (ζ) Tauri glide behind the Moon on Saturday, 19 October 2019.
Set your alarm for 6am GMT if you wish to see three naked-eye planets in the UK dawn sky this week. Find a location that offers an unobstructed view of the horizon from southeast to south and let the waning Moon be your guide to locating Jupiter, Mars and Saturn on successive mornings from 7 to 11 February.
