Comet C/2015 V2 Johnson has been sliding south down the side of Boötes the Herdsman all month and slowly brightening. The “2015” in its name was the year of its discovery. On Nov. 3, 2015, astronomer J.A. Johnson, working with the Catalina Sky Survey, found the then-faint new comet in the constellation Lynx the Lynx not far from the bowl of the Big Dipper.
The comet stayed faint until earlier this year when amateur astronomers picked it in more modest-sized telescopes. Johnson’s now visible in little more than a pair of binoculars. I had to bend my neck way back to see it, since it’s high up in the southern sky at nightfall not far from the brilliant, orange star, Arcturus. In 10x50s, I easily spotted a faint, fuzzy patch of light with a somewhat brighter center about ⅔ the size of the full moon from my home.
Tonight (May 28), Johnson’s located 10° north-northeast of Arcturus and glows at magnitude +7.5 or 8. As it continues moving south, the comet will brighten a bit more, peaking around magnitude +7-7.5 around mid-June. As you’ve already noticed, the moon has returned to the evening sky. While still just a crescent and not a threat to faint fuzzy things yet, see if you can spot the comet in the next few nights.
Through a 6-inch or larger telescope, Johnson will show a distinctly brighter head and fainter tail pointing toward the northwest. There’s a second tail also, but it’s a faint thread of a thing composed of carbon monoxide gas that fluoresces in UV light from the sun.
It wasn’t long ago that Johnson’s brighter tail, composed of dust, was broad and fan-shaped and generally pointed opposite the sun, the way comet tails typically do. That’s now changing quickly. On May 31, Earth will pass through the plane of Comet Johnson’s orbit. For a short time, we stare straight across and through dust boiled off the comet’s core earlier this year which lies in the comet’s orbital plane. Since the dust is confined to the comet’s flat orbit, the fanned-out tail plus the earlier debris will morph into a narrower, ray-like tail. Meanwhile, our changing perspective causes the dust tail to appear to point toward the sun.
A sunward-pointing dust or gas tail is called an anti-tail. Johnson’s other tail, the skinny gas one, continues to point opposite the sun. But the closer we get to the comet’s orbital plane, the more the two tails will appear to diverge until they’ll be directly opposite each other on the 31st.
Earth passed through the orbit of Comet C/2011 L4 PANSTARRS (above) in May 2013 — for a brief time, the comet looked just like a straight line drawn with chalk. To better picture an orbital crossing, recall that Saturn’s rings look like a straight line when we see them edge-on, ie. when Earth passes through their orbital plane.
Good luck in finding the comet the next few nights. If you’re using binoculars, look for a faint, fuzzy spot. Telescope users should see the anti-tail.