Waning Moon close to Messier 35. | SkyNews
Waning Moon close to Messier 35 on September 1, 2021. (Chris Vaughan, Starry Night Education)

This Week’s Sky: August 30 to September 5, 2021

This week, as the Moon wanes after reaching its third quarter, check out some clusters and deep-sky objects.

Monday, August 30 – Third quarter Moon (at 07:13 GMT)

A diagram of the third quarter Moon on August 30, 2021. | SkyNews
Third quarter Moon on August 30, 2021. (Chris Vaughan, Starry Night Education)

The Moon will officially reach its third quarter phase at 3:13 a.m. EDT or 07:13 GMT on Monday, August 30. At third quarter, our natural satellite always appears half-illuminated, on its western side towards the pre-dawn Sun. It rises in the middle of the night and remains visible in the southern sky all morning. The name for this phase reflects the fact that the Moon has completed three quarters of its orbit around Earth, measuring from the previous new Moon. The ensuing week of moonless evening skies will be ideal for observing deep sky targets.

Wednesday, September 1 – Waning Moon close to Messier 35 (pre-dawn)

Waning Moon close to Messier 35. | SkyNews
Waning Moon close to Messier 35 on September 1, 2021. (Chris Vaughan, Starry Night Education)

When the waning crescent Moon rises among the stars of Gemini at about 1 a.m. local time on Wednesday, September 1, it will be positioned a finger’s width to the upper left (or 1 degree to the celestial north) of the large open star cluster named Messier 35. The two objects will share the view in binoculars and telescopes (red circle) until the dawn twilight overwhelms the stars. To better see the cluster, which is nearly as wide as the Moon, try hiding the Moon just outside the upper left edge of your binoculars’ field.

Thursday, September 2 – Io’s shadow on Jupiter (from 10:45 p.m. to 12:55 a.m. PDT)

A diagram of Io's shadow transit on Jupiter on September 2, 2021. | SkyNews
Io’s shadow transit on Jupiter on September 2, 2021. (Chris Vaughan, Starry Night Education)

From time to time, the small, round black shadows cast by Jupiter’s four Galilean moons become visible in amateur telescopes as they cross (or transit) the planet’s disk. On Thursday night, September 2, observers in the Americas can watch Io’s small shadow on Jupiter from approximately 10:40 p.m. PDT (or 1:45 a.m. EDT and 05:45 GMT) until 12:55 a.m. PDT (or 3:55 a.m. EDT and 7:55 GMT). Io itself will be visible just before it moves onto Jupiter at 10:17 p.m. PDT and again after it moves off Jupiter at 12:38 a.m. PDT.

Saturday, September 4 – Crescent Moon buzzes the Beehive (pre-dawn)

A diagram of the Crescent Moon buzzing the Beehive on September 4, 2021. | SkyNews
Crescent Moon buzzes the Beehive on September 4, 2021. (Chris Vaughan, Starry Night Education)

For about an hour before dawn on Saturday morning, September 4, look in the eastern sky for the slim crescent of the waning Moon shining several finger widths to the left (or 3 degrees to the celestial northeast) of the huge open star cluster known as the Beehive, Praesepe and Messier 44. The Moon and cluster will be close enough to share the field of binoculars (red circle), but you’ll see more of the “bees” if you tuck the Moon just out of sight on the left. Hours earlier, observers in Europe will see the pair somewhat closer together.

Sunday, September 5 – Venus gleams above Spica (after sunset)

A diagram of how Venus gleams above Spica on September 5, 2021. | SkyNews
Venus gleams above Spica on September 5, 2021 (Chris Vaughan, Starry Night Education)

Above the west-southwestern horizon on the evenings surrounding Sunday, September 5, the orbital motion of the very bright planet Venus will carry it closely past Virgo’s brightest star, Spica. At closest approach on Sunday evening Venus will shine only a thumb’s width above (or 1.5 degrees to the celestial north of) Spica, allowing them to appear together in binoculars and low power telescopes (red circle). Venus will pop into view first after sunset, but you’ll need to let the sky darken more to see 100 times fainter Spica with your unaided eyes. Start looking at about 8 p.m. local time. The pair will be binoculars-close from September 3-8, but ensure that the Sun has fully set before using optical aids to view them.

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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