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.
Skywatchers in Western Europe looking at the rising 13-day-old gibbous Moon in the south-southeast at dusk on Sunday, 27 May can also see prime-time Jupiter within the same binocular field of view. But look closer in the vicinity of the solar system’s largest planet and you’ll see an easily resolved double star – alpha Librae.
Have you ever seen the closest planet to the Sun? If you wish to tick Mercury off your To-See list, particularly if you live in the Southern Hemisphere, now until the middle of May is the time to be scrutinising the eastern sky about an hour before sunrise. Mercury also has a close encounter with an old crescent Moon on 14 May.
An Israeli-built spacecraft seeking to become the first privately-developed probe to land on the Moon crashed on descent Thursday, but the mission was widely lauded as a breakthrough for the commercial space industry, and Israeli prime minster Benjamin Netanyahu said the country would try again.
