(Above: An image of comet C/2016 M1 PanSTARRS taken on May 18, 2018 by Raffaele Esposito)

A Binocular Comet

Comets are among the most captivating of sights for sky watchers. Their glowing green heads and glorious tails, powered by the sun’s warmth and buffeted by its solar wind, sweep across the sky. The brightest comets, easily visible with naked eyes, are legendary. But those are extremely rare, often only visible from limited regions of the globe, and for only a brief time. The arrival of new bright comets is completely unpredictable, adding to their mystique. But one or two dimmer comets are usually observable in binoculars or small telescopes every month, if you know where to find them.

Comets can be one-time visitors. Those ones drop into the inner solar system from the distant Oort cloud and then are flung out of the solar system by the sun’s gravity or destroyed by its heat when they pass too close to our star. Other comets traverse the solar system like interplanetary shuttle-craft, returning to view every few years or decades. Halley’s comet is the most famous of these periodic comets. The Rosetta spacecraft and its little Philae lander studied another — comet 67P/Churyumov-Gerasimenko.

The best times to hunt comets are during dark moonless nights. Expect a comet to appear as a faint greenish blob (quite different from a star, but resembling a galaxy). If it has a tail, it will be much fainter, and pointing away from the sun. If you use a telescope, be sure to search at low power (50x) initially, and then magnify once you have it in view, to 150X or more. Don’t be afraid to try long exposure photographs, either through the eyepiece, or using a tripod-mounted camera.

Astronomers use a term called visual magnitude to indicate the brightness of a celestial object. The visual magnitude number increases as the brightness decreases. A small pair of binoculars should show comets to visual magnitude of 9.5 under dark skies. Large binoculars and small telescopes will work to about magnitude 11. And an 8” (203 mm) reflector or SCT telescope can reach magnitude 14.

(Above: The path of Comet C/2016 M1 PanSTARRS through the sky is represented here using a yellow dot every consecutive day. The time shown is for Monday, June 4 at 2 am Eastern Daylight time.)

Right now, there is a faint comet that is observable using binoculars and low power telescopes in the hours after midnight. And it will become easier to see as the moon wanes this week. Comet C/2016 M1 (PanSTARRS) is moving southwestward (towards the lower right) through Sagittarius (the Archer). In the name, C/2016 means that it is a non-periodic comet discovered in 2016 by the robotic sky camera system called PanSTARRS. The M1 indicates that it was the first comet discovered in the second half of June that year.

Comet C/2016 M1 has an orbit that is at right angles to the plane of our solar system. (Comets can drop towards the sun from any direction — not just the plane that the planets orbit in.) Because it is about as close to Earth as it will get (roughly as far as the main asteroid belt beyond Mars), it’s probably near its peak brightness now, at a visual magnitude of about 9.0. It will swing around the Sun during August. There is a very nice 3D model of the comet’s orbit here.

(Above: A 3D rendered model of the orbit of Comet C/2016 M1 PanSTARRS. The comet’s orbit is at right angles to the plane of our solar system. At present, the comet is crossing the asteroid belt beyond Mars.)

The comet will rise about 11:30 pm local time, reach its highest point about two fist diameters above the southern horizon at about 3:30 am local time. The coming sunrise will brighten the sky soon after that time. Overnight tonight (early Monday morning), the comet will be positioned 1.25° (or a generous finger’s width) to the upper left of the medium-brightness star Namalsadirah (aka T Sagittarii), which marks the lower corner of the Teapot asterism’s handle. From Tuesday to Wednesday morning, it will hop over that star as it drops lower.

Future, potentially even better comets this year include: Comet 46P/Wirtanen will brighten in Sept-October, and peak at visual magnitude 3 at year’s end, when it should become an evening target visible to unaided eyes; comet 21P Giacobini-Zinner will peak at magnitude 6 in September; and comet C/2017 S3 (PanSTARRS) will peak in the August pre-dawn sky.

Dwarf Planet Ceres Kisses the Lion’s Nose

This evening (Sunday, June 3), in the western sky, the eastward orbital motion of the dwarf planet Ceres (visual magnitude 8.7) will carry it very close to Leo’s (the Lion) brightest star Ras Elased Australis (also known as Algenubi). Viewing the encounter through a backyard telescope at medium-high power will readily show the motion of Ceres. Just after dusk in eastern North America, at around 10 pm EDT, Ceres will be positioned 4 arc-minutes southwest of the star. In the Eastern Time zone, the pair of objects will set by the time of closest separation at about 2:30 am EDT, when Ceres will move to a position only 30 arc-seconds south of the star. Observers located farther west will be able to see the entire encounter.

(Above: The sky near the star Ras Elased Australis, which marks Leo, the Lion’s nose. On June 3, the orbit (red line, with the times) of the dwarf planet Ceres will skim past the star. Parts of the world will see the closest approach, while others will merely see the two objects very close together. The green circle represents the field of view of a telescope.)

The Moon and Planets

This is the week in the moon’s monthly orbit around Earth when it rises in the late evening and post-midnight time-frame, and then lingers in the morning daytime sky as a waning, partially illuminated orb. When the waning gibbous moon rises after midnight tonight (Sunday), it will sit a fist’s diameter to the left of bright, reddish Mars, which sits among the stars of Capricornus (the Sea-Goat).

On Wednesday afternoon, the moon will officially reach its Last Quarter Phase, but it will have set by then for most of North America. Hours earlier, on Wednesday before dawn, the moon will sit a palm’s width to the right of Neptune. On Saturday morning, June 10, the old crescent moon will hover over the pre-dawn eastern sky. If you are up around 4 am local time, you might find Uranus, which will be sitting just less than a palm’s width above the moon. The next morning (Sunday), look very low in the east for the very slim, old crescent moon.

(Above: Saturn and Mars will appear in the post-midnight sky this week, as shown here at 1:30 am local time. The waning moon will join them on Monday, June 4.)

Venus continues to gleam in the western evening sky this week as it climbs away from the sun. The planet sets at about 11:30 pm local time all week because it is travelling east, while the entire sky is shifting west, holding it in place.

Jupiter will be the brightest object you see shining brightly in the southeast after dusk this week. It will reach its highest elevation (about three fist diameters) above the southern horizon around 11:15 pm local time, and then descend into the southwestern horizon before the sun rises. Once it’s dark enough, look for the bright star sitting just to the lower right of Jupiter. That’s Zubenelgenubi, the brightest star in Libra (the Scales). In binoculars or a small telescope, it splits into a closely separated pair of stars.

(Above: The bright planets Venus, in the northwest, and Jupiter, in the south, will dominate the evening sky this week, as shown for 10 pm local time.)

On Thursday, June 7 between 1:01 am and 3:11 am, the black shadow of Jupiter’s moon Io and its little round black shadow will cross (or transit) Jupiter’s disk. A reasonable backyard telescope will show the black shadows, but a very good telescope is needed to see the moons themselves.

The Great Red Spot takes about three hours to cross Jupiter’s disk. But the planet’s 10-hour rotation period (i.e., its day) means that the spot is only observable from Earth every 2–3 nights. If you’d like to see the GRS, use a medium-sized telescope (or larger). You’ll have your best luck on evenings with steady air — when the stars are not twinkling too much. The best times to try this week are: Sunday, June 3 at 10:57 pm, Wednesday, June 6 at 12:36 am, Friday, June 8 at 10:06 pm, and Sunday, June 10 at 11:44 pm. All times are given in Eastern Daylight Time (EDT), so adjust for your local time zone. Try to look within an hour before or after the times I’ve given.

Yellow-tinted Saturn will be rising in the southeastern sky shortly after 10 pm local time this week. It will spend all summer near the Milky Way and above the Teapot-shaped stars that form Sagittarius (the Archer). You should be able to see Saturn itself until about 5 am local time, when it will sit about 1.5 fist widths above the southwestern horizon.

The summer of Mars is about to begin in earnest! After this week, the Red Planet will begin to rise in the east before midnight. Mars, noticeably brighter than Saturn, will continue to steadily brighten and increase in apparent size as the Earth’s faster orbit brings us closer to the red planet. (We will pass it on the “inside track” in late July.) In the meantime, look for Mars shining over the southern horizon just before dawn.

(Above: Mars will dramatically increase in size and brightness during June, as shown in this simulated view using Starry Night software.)

Distant blue Neptune, still among the modest stars of Aquarius (the Water-bearer), has become observable in telescopes in the pre-dawn eastern sky after it rises just before 2 am local time. Uranus is in Pisces and Mercury is lost from view beside the sun.

Astronomy Skylights for this week (from June 3rd, 2018) by Chris Vaughan. Keep looking up to enjoy the sky!