Newly discovered Comet Nishimura is quickly brightening in the morning sky and may be faintly visible with the naked eye next month. On August 24th observers in North America will be treated to an occultation of Antares.
It's no small thing for an amateur to wrest a comet from the all-seeing eyes of robotic surveys. But Japanese observer Hideo Nishimura beat those odds on August 12.8 UT. That morning he took three 30-second exposures with a Canon 6D digital camera and 200-mm lens and captured images of a new 11th-magnitude comet in the constellation Gemini. It was his third comet discovery after Comet Nakamura-Nishimura-Machholz (C/1994 N1) and an earlier Comet Nishimura (C/2021 O1). I have nothing against automated hunting, but seeing an amateur's name on a comet is not only a breath of fresh air but a great achievement.
Comets are like presents under the Christmas tree waiting to be opened. On August 15th the sky cleared and I found a location with an unobstructed view to the east to get my first look. At 4 a.m. local time Comet Nishimura stood just 8° high in the northeastern sky in Gemini. Through my 15-inch (38-cm) Dob I saw it right away at 64× at magnitude 9.8 with a 2.3′ moderately condensed coma. A Swan band filter enhanced its visibility and intensified the apparent brightness of the inner coma, a sign that Nishimura's comet was rich in volatile carbon gas.
Since then the comet's magnitude has brightened to about 8.5 with binocular sightings becoming increasingly more common. Nishimura should top out around 4th magnitude before the solar glare claims it. The bad news is that its altitude will be rather low throughout its apparition and decrease as the comet approaches perihelion on September 18th. Crucial to spotting the new object will be an unobstructed view to the east-northeast. Across the continental U.S. the comet will stand about 15° high just before the start of astronomical twilight for the next two weeks. Currently, twilight begins in the northern part of the country about 1 hour 50 minutes before sunrise and 1 hour 20 minutes before sunrise in the southern U.S.
After about September 5th, the comet's rapidly decreasing solar elongation chips away at its elevation. By the time it's within naked-eye range it will stand just a few degrees high at the start of dawn. For example, we'll see Nishimura about 10° above the horizon on September 5th; 6° on September 8th; and just 2° high on September 10th. Given that the comet will brighten from about magnitude 5 to 4 during this time it should still remain visible with binoculars. Northern Hemisphere observers will likely get their last looks on or close to September 13th.
Through the end of August a 4-inch telescope should be adequate to track down our temporary guest. In September 50-mm binoculars may do the trick but I'd still keep a scope handy to explore details of the inner coma, pseudo-nucleus, and better appreciate the character and extent of the tail. Astrophotographers will have opportunities to capture nice views of the comet paired with the Beehive Cluster (M44) in Cancer on and around August 31st and the 9th-magnitude galaxy NGC 2903 in Leo on September 5th.
Its perihelion will be a scorcher with the comet passing just 0.2 a.u. (30 million kilometers) from the Sun, well inside Mercury's orbit. There's a fair chance this first-time visitor from the Oort Cloud may not survive the brush and fall to pieces. But if it can keep itself together the fragile mess will swing into the evening sky in Virgo after perihelion. Mid-northern latitude observers will lose the comet to glare and low altitude, but prospects are more sanguine for the Southern Hemisphere where it will appear very low in the western sky at dusk through month's end. Catch it soon before it fades.
Given the ubiquity of wildfire smoke I encourage everyone with good skies to share their observations in the comments section so other readers can keep track of developments. You'll also find current information about the comet on Seiichi Yoshida's Weekly Bright Comets and the Comet Observation database (COBS).
Heads up! Antares occultation
On August 24th, for the first time since February 2010, the Moon will occult the 1st-magnitude star Antares in Scorpius. Observers from the East Coast (except most of Florida) to the mountain states will see the waxing gibbous Moon slowly approach and then suddenly cover the star during early evening viewing hours. The visibility zone also includes much of Canada and a portion of Mexico.
From locations in the central U.S. both the star's disappearance at the Moon's dark limb and reappearance at the bright limb will be visible. To find out your viewing prospects, go to the International Occultation Timing Association's (IOTA) page devoted to the event and look up your city. Times shown are Universal Time. To convert to EDT subtract 4 hours; 5 hours for CDT; 6 hours for MDT; and 7 hours for PDT.
Antares is a close double star with a 5.4-magnitude companion 2.7″ away in P.A. (position angle) 277°. Notoriously difficult to split, the Moon won't offer any help this time around because it first occults the secondary followed by the bright primary. Although the secondary star reappears at the opposite limb moments before the glary primary, light from the bright limb may mask it from view. Give it a try anyway — I'd love to hear if you spot it both coming and going. For those keeping track, the August 24th Antares occultation is the first in a series that will end on August 27, 2028.