Two comets spark excitement for the coming week — NEOWISE might reach naked-eye visibility at dawn, while Lemmon will be visible in binoculars at dusk.
Dare we hope? Expectations were high for comets ATLAS (C/2019 Y4) and SWAN (C/2020 F8) last season but both ultimately fizzled. Will Comet NEOWISE (C/2020 F3) finally take us to the naked-eye finish line?
Comet NEOWISE (C/2020 F3) has a close brush with the Sun on July 3rd when it reaches perihelion at a distance of 44 million kilometers, some 14 million km closer on average than the planet Mercury. If it survives the solar onslaught skywatchers in the Northern Hemisphere could see it peep over the northeastern horizon at dawn glowing at first magnitude. Its unusual name comes from NASA's Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE), which discovered the comet back in March.
By now we all know that comet magnitude predictions should be taken with a proverbial grain of cometary ice. But there's cause for optimism: When last photographed in the field of SOHO's C3 coronagraph NEOWISE appeared intact and was still climbing in brightness, both great signs. The most recent ephemeris from the Central Bureau for Astronomical Telegrams, dated June 27th, includes a cautious peak magnitude estimate of +0.6 on July 5th, followed by +0.8 on July 7th, the expected date of the comet's first appearance at dawn.
During its debut week NEOWISE will hug the northeastern horizon. From the southern states it will stand only about 2°–3° high at the start of morning twilight. The central and northern U.S. fair better with altitudes between 3° and 7°. The situation improves considerably once the comet emerges in the evening sky at mid-month.
If NEOWISE stood high in a dark sky at 1st magnitude it would be incredibly obvious even from the suburbs. But while shining at its brightest it hovers near the horizon through mid-July, so it will appear about two magnitudes fainter than the published estimates. Denser, dustier air near the horizon both scatters and absorbs light from celestial objects compared to overhead views, where we look through much less atmosphere.
Assuming NEOWISE will look about 3rd magnitude I encourage you to bring along a pair of binoculars or a small, wide-field telescope for a satisfying view. Comet heads can be bright while tails are often faint and wispy, making them difficult to see with the naked eye. I can't tell you how many times my 10×50 binoculars have revealed a beautiful feathery appendage that my unaided eye struggled to see. Based on the SOHO images, NEOWISE should be wagging a tail pointing upward from the northern horizon.
A splendid evening appearance
Bright moonlight will compromise the dawn view until about July 11th, which happens to be the same time that NEOWISE emerges into the evening sky. There, it quickly gains altitude while zipping across Ursa Major under moonless skies. Although the comet fades to magnitude 2 by mid-month and magnitude 3 by month's end, its increasing elevation will help offset its diminishing light, making the latter half of July the prime time to enjoy the comet.
For the best views of NEOWISE at both dawn and dusk find a location with a wide-open northern horizon such as a lake or field. More happy news: The comet's dawn and dusk appearances coincide with the best time for noctilucent cloud watching. Keep a lookout.
(Special note! Carl Hergenrother, ALPO Comet Section coordinator, managed to spot the comet in 30×125 binoculars shortly before sunrise in a rapidly brightening sky on July 1 around 11:45 UT from Arizona. He described it as "easy" in the large instrument and "difficult but visible" in a pair of 10×50s. NEOWISE was just 3.6° high at the time with a magnitude of ~1.0.)
Lemmon joins the scene
If you're like me and burn the candle at both ends, be sure to set aside time in the evening for the arrival of Comet Lemmon (C/2019 U6). After a splendid run in the Southern Hemisphere, where it maxed out at 6th magnitude, it's now on its way north. Although the comet has dimmed to magnitude 7, it still sports a gorgeous tail and should be a lovely sight in binoculars and telescopes from a dark sky.
Lemmon makes its initial appearance low in the western sky around July 4th in Sextans and gradually ascends, crossing the rich Virgo Cluster of galaxies at mid-month before reaching Coma Berenices at month's end. The comet passed perihelion on June 18 and swung closest to Earth at 124 million km on June 29th. Like NEOWISE, it will slowly fade, dimming by about 1.5 magnitudes during July.
Although Southern Hemisphere observers will miss the best days of NEOWISE, their consolation prize is Comet 2P/Encke, currently making a bright but low-altitude appearance as it jogs across Cancer at dusk. On June 30th, amateur Chris Wyatt estimated its magnitude at 7.3 and described the comet as a "bright, starlike pseudo-nucleus surrounded by a thin, hazy coma." This apparition will be a poor one for the Northern Hemisphere, with Encke only showing up in late August after fading to around magnitude 12. Click here for a finder map.