by Geoff Chester, USNO Public Affairs | 25 January 2023 Finder Chart for Comet C/2022 E3 (ZTF), 2023 January 24 - February 5 View looking north at 10:00 pm local time The Moon spends the rest of January brightening the evening sky as she climbs northward toward the bright constellations of winter. First Quarter occurs on the 28th at 10:19 am Eastern Standard Time. Luna pays a close visit to bright Jupiter on the evening of the 25th. On the 30th you will find her waxing gibbous just a degree west of ruddy Mars. Residents of the southwestern U.S. will see the Moon cover Mars later that evening. By now you have probably heard that we will be visited by an emissary from the outer reaches of the solar system. Headlines in the popular press are touting it as the “Green Comet”, which is certainly easier to remember than its true name of C/2022 E3 (ZTF). Comets are generally designated by their order of discovery in a given year along with the name(s) of the people or places of their discovery. In this case the interloper’s name comes from the observatory where it was first identified. That institution is known as the Zwicky Transient Facility, or ZTF. This facility combines a state-of-the-art wide-field camera made up of a mosaic of 16 large CCDs attached to the venerable 1.22-meter (48-inch) Oschin Schmidt telescope at Palomar Observatory. This telescope, dedicated in 1948, was designed for wide-field photographic surveys of the sky. It was fitted with its current camera in 2017, giving it the ability to image a large swath of the sky looking for transient phenomena such as novae, supernovae, near-Earth asteroids, and comets. Comet C/2022 E3 (ZTF) was discovered on March 2nd, 2022 and passed perihelion on January 12th. It will pass closest to Earth on February 1st at a distance of about 33 million kilometers (20.5 million miles). It will be visible in our skies during the late evening hours for the next couple of weeks. It should be visible to the naked eye from very dark locations, and it should be a fairly easy target for binoculars as it courses through the northern sky. As the week opens you can spot it between the “handle” of the Big Dipper and the “bowl” of the Little Dipper in the northeast at around midnight. It will then move toward the North Star, Polaris, lying between the “pointer” stars of the Big Dipper and Polaris on the 29th. After it passes by Polaris it will head toward the bright star Capella, passing very close to the star on February 5th. As with most comets, its glow comes from the excitation of carbon compounds by solar radiation, which give off a characteristic greenish light. It should remain visible in binoculars for a few more weeks, but it will fade as it recedes to the depths of the solar system, most likely never to be seen again. While the waxing Moon will gradually brighten the sky during the evening hours, you can still enjoy the bright stars of the Great Winter Circle, which dominate the sky throughout the evening. Nine of the 25 brightest stars in the heavens may be found within the confines of the circle. One of these, Sirius, is the brightest star in the entire sky, beaming down from a position southeast of Orion’s famous belt. Sirius is bright because it is one of the closest stars to the solar system and a distance of just over 8 light years. I like to compare it to Rigel, the bright blue star that marks one of Orion’s knees. Both stars have a dazzling blue tint, but Rigel is 100 times more remote than Sirius. It’s brightness is due to its intrinsic luminosity, which is over 100,000 times that of the Sun. If we could somehow drag Rigel to the distance of Sirius, its light would rival that of the full Moon! Venus should be easy to find in the western sky shortly after sunset. You may even be able to spot her before the Sun goes down if you know where to look. She begins the week drawing away from Saturn, which won’t appear until deeper twilight falls. The two planets are about two degrees apart on the evening of the 24th. By week’s end Venus will leave the ringed planet some 10 degrees in her wake. Jupiter also appears in the southwest shortly after sunset, his luster only outdone by Venus. Old Jove still offers a pleasing sight for the small to moderate aperture telescope, but his viewing “window” is rapidly getting shorter. The giant planet now sets at around 10:00 pm local time. Point the telescope at him during twilight and you’ll have a bit over two hours to see him clearly. Mars passes high overhead at around 8:00 pm. The red planet is still brighter than the nearby star Aldebaran, but he is fading quickly as Earth pulls away from him. Mars’ apparent disc is becoming smaller with each passing night, making his surface features difficult to see in smaller telescopes. One thing that you probably will notice, though, is the distinct gibbous phase that the planet presents. Just over 90 percent of his face is illuminated.