Crater Copernicus on October 22, 2021. | SkyNews
Crater Copernicus on October 22, 2021 (Chris Vaughan, Starry Night Education)

This Week’s Sky: October 18 to 24, 2021

Look for the full Hunter’s Moon and the Orionids meteor shower this week.

Monday, October 18 – Spotty Jupiter completes a retrograde loop (11:10 to 11:25 p.m. PDT)

Spotty Jupiter completes a retrograde loop on October 18, 2021. | SkyNews
Spotty Jupiter completes a retrograde loop on October 18, 2021 (Chris Vaughan, Starry Night Education)

On Monday, October 18, Jupiter will appear to stop moving with respect to the distant stars of eastern Capricornus – marking the end of a westward retrograde loop that began in June. Meanwhile, starting at 11:10 p.m. PDT (or 06:10 GMT on Tuesday, October 19), observers with telescopes in western North America and the Pacific Ocean region can see the small, round, black shadows of the Galilean moons Ganymede and Io cast upon Jupiter’s disk at the same time for approximately 15 minutes. Ganymede’s shadow will begin to cross alone at 7:55 p.m. PDT. At the end of the double shadow transit event, Ganymede’s shadow will lift off Jupiter, leaving Io’s shadow to complete its solo transit at 1:25 a.m. PDT. By then, Jupiter will have set for observers in Canada and the continental United States.

Wednesday, October 20 – Full Hunter’s Moon (at 14:56 GMT)

Full Hunter’s Moon on October 20, 2021. | SkyNews
Full Hunter’s Moon on October 20, 2021 (Chris Vaughan, Starry Night Education)

The full Moon of October, which occurs at 10:56 a.m. EDT, or 14:56 GMT, on Wednesday, October 20, is traditionally called the Hunter’s Moon, Blood Moon or Sanguine Moon. The Anishinaabe people of the Great Lakes region call this Moon Binaakwe-giizis, the Falling Leaves Moon, or Mshkawji-giizis, the Freezing Moon. The Cree Nation of central Canada calls it Opimuhumowipesim, the Migrating Moon — the month when birds are migrating. The Haudenosaunee (Iroquois / Mohawk) of eastern North America use Kentenha, the Time of Poverty Moon. Full moons in October always shine in or near the stars of Cetus and Pisces. Since it’s opposite the Sun on this day of the lunar month, the full Moon will rise at sunset and set at sunrise.

Thursday, October 21 – Orionids meteor shower peak (pre-dawn)

Orionids Meteor shower peak on October 21, 2021. | SkyNews
Orionids meteor shower peak on October 21, 2021 (Chris Vaughan, Starry Night Education)

The annual Orionids meteor shower is produced when the Earth crosses through a cloud of small particles dropped by repeated passages of Comet Halley in its orbit. The shower runs from September 23 to November 27 and will peak between midnight and dawn on Thursday, October 21. At that time the sky overhead will be moving directly into the densest region of the particle field, producing 10-20 fast meteors per hour. Orionids meteors can appear anywhere in the sky, but will seem to be streaking away from the constellation of Orion. Unfortunately, a very bright, waning gibbous Moon will overwhelm many of the meteors on the peak night.

Thursday, October 21 – Bright Moon below Uranus (all night)

Bright Moon below Uranus on October 21, 2021. | SkyNews
Bright Moon below Uranus on October 21, 2021 (Chris Vaughan, Starry Night Education)

In the eastern sky after dusk on Thursday night, October 21, the very bright, waning gibbous Moon will shine two finger widths to the lower left (or 2.5 degrees to the celestial southeast) of magnitude 5.7 Uranus. By dawn on Friday morning, the Moon’s orbital motion will shift it farther from Uranus in the western sky, and the diurnal rotation of the sky will move it above the planet. While Uranus’ blue-green dot can be seen in binoculars (red circle), I recommend noting its location between the stars of Aries and Cetus and seeking it out a few nights later, when the bright Moon will have moved away.

Friday, October 22 – Crater Copernicus (all night)

Crater Copernicus on October 22, 2021. | SkyNews
Crater Copernicus on October 22, 2021 (Chris Vaughan, Starry Night Education)

The prominent crater Copernicus is located in eastern Oceanus Procellarum — due south of Mare Imbrium and slightly northwest of the Moon’s centre. This 800 million year old impact scar is visible with unaided eyes and binoculars — but telescope views will reveal many more interesting aspects of lunar geology. Several nights before the Moon reaches its full phase, Copernicus exhibits heavily terraced edges (due to slumping), an extensive ejecta blanket outside the crater rim, a complex central peak, and both smooth and rough terrain on the crater’s floor. Around full Moon, Copernicus’ ray system, extending 500 miles (800 kilometres) in all directions, becomes prominent. Use high magnification to look around Copernicus for small craters with bright floors and black haloes — impacts through Copernicus’ white ejecta that excavated dark Oceanus Procellarum basalt and even deeper highlands anorthosite.

Monday, October 25 – Mercury at greatest western elongation (pre-dawn)

Mercury at greatest western elongation on October 25, 2021, pre-dawn. | SkyNews
Mercury at greatest western elongation on October 25, 2021, pre-dawn (Chris Vaughan, Starry Night Education)

On Monday, October 25, the planet Mercury will reach a maximum angle of 18 degrees from the Sun, and peak visibility for the current morning apparition. Look for the innermost planet shining brightly, very low in the east-southeastern sky between about 6:15 a.m. and 7 a.m. in your local time zone. In a telescope (inset) Mercury will exhibit a 57-per-cent illuminated, waxing gibbous phase. Mercury’s position above the nearly upright morning ecliptic (green line) will make this an excellent apparition for Northern Hemisphere observers, but a poor one for those located near the Equator and farther south.

Chris Vaughan is a science writer, geophysicist, astronomer, planetary scientist and an “outreach RASCal.” He writes Astronomy Skylights, and you can follow him on Twitter at @astrogeoguy. He can also bring his Digital Starlab portable inflatable planetarium to your school or other daytime or evening event. Contact him through AstroGeo.ca to tour the universe together.

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