When 252P/LINEAR passed just 14 lunar distances from Earth on the afternoon of Monday, 21 March, the comet was close to the south celestial pole and unobservable from the British Isles. At closest approach, 252P/LINEAR galloped across the Southern Hemisphere sky at a rate of almost ten degrees per day. As the comet recedes from both the Earth and Sun that rate has slowed somewhat (it’s presently 5.3 degrees/day, dropping to 1.8 degrees/day in a week’s time), but fortunately for us its motion is now almost entirely northward in declination.
By the small hours of Sunday, 27 March comet 252P/LINEAR will be in southern Ophiuchus — the unofficial 13th zodiacal constellation — and finally visible from the UK on successive mornings around 5am BST when astronomical twilight begins to assert itself for the centre of the British Isles. Sadly, the comet will have to contend with the glare of the waning gibbous Moon until 4 April (the 20-day-old lunar orb lies just two degrees from 252P/LINEAR at 5am BST on Wednesday, 30 March). However, the good news is that this periodic comet with a 5⅓-year orbit is 100-fold brighter than pre-perihelion predictions owing to increased activity as a result of passing closest to the Sun on 15 March.
Were it not for the Moon’s glare, 252P/LINEAR would be on the verge of naked-eye visibility over the next week or so, but it should still make a good binocular object if you can find an observing location that is devoid of streetlights and other sources of artificial illumination. Reports indicate that the comet currently sports a diffuse round coma some two-thirds of a degree across (somewhat larger than the Moon in angular size), with a total magnitude exceeding +6. In a large telescope, 252P/LINEAR might also have a greenish hue caused by diatomic carbon (C2) molecules fluorescing in the sunlight, but don’t expect a long tail.
Inside the magazine
Find out all you need to know about observing comet 252P/LINEAR and the other solar system bodies in the night sky in the April 2016 edition of Astronomy Now.
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