See comet 252P emerge in the UK predawn sky
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.
This graphic shows the track of comet 252P/LINEAR at daily intervals around 5am BST when it is best placed for observation from the British Isles over the coming week. The comet’s diffuse round coma is about 40 arcminutes in diameter and currently has an integrated magnitude of around +5, so it would be a naked-eye object from dark sky sites if it did not have to contend with the glare of a nearby waning gibbous Moon. For scale, the view is about 40 degrees wide, or twice the span of an outstretched hand at arm’s length. The Moon is enlarged for clarity and the positions of Saturn and Mars are shown for 30 March. Click the graphic for a greyscale version suitable for printing.
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.
This daily topocentric ephemeris of 252P/LINEAR is computed for 5am BST for the centre of the British Isles. Observers in the UK with computerised GoTo mounts or digital settings circles can therefore use the J2000.0 epoch coordinates directly with minimal parallax error. The stated integrated magnitudes are best estimates, but the glare from an adjacent waning gibbous Moon will make observations harder. The Delta column is the distance to the comet in astronomical units, where one AU is 92,955,807.3 miles (149,597,870.7 kilometres).
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.