Morning zodiacal light for mid-northern observers on October 4, 2021. | SkyNews
Morning zodiacal light for mid-northern observers on October 4, 2021 (Chris Vaughan, Starry Night Education)

This Week’s Sky: September 27 to October 3, 2021

Look for the old crescent Moon in Leo, and the zodiacal light in the pre-dawn sky.

Tuesday, September 28 – Third quarter Moon again (at 9:57 p.m. EDT)

Third quarter Moon on September 28, 2021. | SkyNews
Third quarter Moon on September 28, 2021 (Chris Vaughan, Starry Night Education)

When a lunar phase occurs in the first few days of a calendar month, it can re-occur at month’s end. For the second time in September, the Moon will reach its third quarter phase at 9:57 p.m. EDT on Tuesday, September 28 (or 01:57 GMT on Wednesday, September 29). The ensuing week of moonless evening skies will be ideal for observing deep-sky targets.

Wednesday, September 29 – The Andromeda Galaxy (all night)

The Andromeda Galaxy on September 29, 2021. | SkyNews
The Andromeda Galaxy on September 29, 2021 (Chris Vaughan, Starry Night Education)

September evenings feature the Andromeda Galaxy, which is already climbing the northeastern sky after dusk. This large spiral galaxy, also designated Messier 31 (or M31) and NGC 224, lies only 2.5 million light years from us, and covers an area of sky measuring 3 by 1 degrees (or six by two full Moon diameters). Under dark skies, the galaxy can be seen with unaided eyes as a faint smudge located 1.4 fist diameters to the left (or 14 degrees to the celestial northeast) of Alpheratz, the star that forms the left-hand (northwestern) corner of the square of Pegasus. The three westernmost stars of Cassiopeia, Caph, Shedar, and Navi (Gamma Cas), also conveniently form a triangle that points towards M31. Binoculars (red circle) will reveal the galaxy better. In a telescope, use low magnification and look for M31’s two smaller companion galaxies, the foreground Messier 32 and more distant Messier 110 (inset).

Friday, October 1 – Crescent Moon passes the Beehive (pre-dawn)

Crescent Moon passes the Beehive on October 1, 2021. | SkyNews
Crescent Moon passes the Beehive on October 1, 2021 (Chris Vaughan, Starry Night Education)

When the waning crescent Moon rises over the east-northeastern horizon during the wee hours of Friday, October 1, it will be positioned several finger widths above (or 4.5 degrees to the celestial northwest of) the large open star cluster in Cancer known as the Beehive Cluster and Messier 44. By the time the sky begins to brighten before dawn, the pair will be higher and the Moon’s orbital motion will have carried it slightly closer to the cluster. To better see the “bees,” hide the Moon beyond the edge of your binoculars’ field of view (red circle).

Sunday, October 3 – Old Moon is the Lion’s Heart (pre-dawn)

Old Moon is the Lion’s heart on October 3, 2021. | SkyNews
Old Moon is the Lion’s heart on October 3, 2021 (Chris Vaughan, Starry Night Education)

In the eastern pre-dawn sky on Sunday, October 2, the slim crescent of the old Moon will resemble the smile of the Cheshire Cat. Fittingly perhaps, the Moon will also temporarily become the heart of Leo, the Lion since it will be positioned between the bright stars Regulus (to the Moon’s right) and Algieba (to its upper left). The stars that form the lion’s neck and head are arranged in a fist-sized curve that extends upward from Algieba. The rest of the beast will extend downwards to the lower left (celestial east), ending at his tail star Denebola.

Monday, October 4 – Morning zodiacal light for mid-northern observers (pre-dawn)

Morning zodiacal light for mid-northern observers on October 4, 2021. | SkyNews
Morning zodiacal light for mid-northern observers on October 4, 2021 (Chris Vaughan, Starry Night Education)

During autumn at mid-northern latitudes every year, the ecliptic extends nearly vertically upward from the eastern horizon before dawn. That geometry favours the appearance of the faint zodiacal light in the eastern sky for about half an hour before dawn on moonless mornings. Zodiacal light is sunlight scattered by interplanetary particles that are concentrated in the plane of the Solar System — the same material that produces meteor showers. It is more readily seen in areas free of urban light pollution. Between now until the full Moon on October 20, look for a broad wedge of faint light extending upwards from the eastern horizon and centred on the ecliptic (the green line). It will be strongest in the lower third of the sky, around the bright star Regulus. Try taking a long exposure photograph to capture it. Don’t confuse the zodiacal light with the Milky Way, which is positioned off to the south-southeast.

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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