A diagram of the full harvest Moon on September 20, 2021. | SkyNews
Full harvest Moon on September 20, 2021. (Chris Vaughan, Starry Night Education)

This Week’s Sky: September 20 to 26, 2021

This week, the September equinox occurs, marking the beginning of autumn in the north. Also, the Moon reaches its full phase and hops past Uranus.

Monday, September 20 – Full harvest Moon (at 23:54 GMT)

A diagram of the full harvest Moon on September 20, 2021. | SkyNews
Full harvest Moon on September 20, 2021. (Chris Vaughan, Starry Night Education)

The September full Moon, traditionally known as the “Corn Moon” and “Barley Moon,” always shines in or near the stars of Aquarius and Pisces. The Anishinaabe people of the Great Lakes region call this Moon Waatebagaa-giizis or Waabaagbagaa-giizis, the Leaves Turning or Leaves Falling Moon. Full moons always rise around sunset and set around sunrise. Because this is the closest full Moon to the autumnal equinox in 2021, it is also the harvest Moon. On the evenings around its full phase, the Moon normally rises about 50 minutes later than the previous night. But the shallow slope of the evening ecliptic (and the Moon’s orbit) around the equinox causes harvest moons to rise at almost the same time each night – only delayed by as little as 10 minutes, depending on your latitude. This phenomenon traditionally allowed farmers to work easily into the evening under bright moonlight, hence the name.

Tuesday, September 21 – Mercury speeds past Spica (after sunset)

A diagram of Mercury speeding past Spica on September 21, 2021. | SkyNews
Mercury speeds past Spica on September 21, 2021. (Chris Vaughan, Starry Night Education)

In early September, bright Venus passed closely above Virgo’s brightest star, Spica. Now, it’s Mercury’s turn. On Tuesday, September 21, the speedy planet’s path (red line with labels) will carry it only a thumb’s width below Spica (or 1.4 degrees to the celestial south). That is just close enough for them to share a low power eyepiece view, but telescopic views will be blurry because they are shining through so much of Earth’s distorting atmosphere. The pair will be nearly as close on Monday, too. Magnitude 0.25 Mercury will appear twice as bright as Spica – helpful to know since most telescopes will flip or mirror the binoculars’ view (red circle). Start searching the sky above the west-southwestern horizon starting about 20 minutes after the Sun has completely set. Skywatchers at southerly latitudes will see the pair more easily – higher, and in a darker sky.

Wednesday, September 22 – September equinox (at 19:21 GMT)

A diagram of the autumnal equinox on September 22, 2021. | SkyNews
Autumnal equinox on September 22, 2021. (Chris Vaughan, Starry Night Education)

On Wednesday, September 22 at 3:21 p.m. EDT, or 19:21 GMT, the Sun’s apparent motion along the ecliptic (green line) will carry it across the celestial equator travelling southward, marking the autumnal equinox in the Northern Hemisphere and the beginning of autumn here. On the equinoxes in March and September, day and night are near equal length, and the Sun rises due east and sets due west (yellow arc).

Thursday, September 23 – Bright Moon hops past Uranus (all night)

A diagram of the bright Moon hopping past Uranus on September 23, 2021. | SkyNews
Bright Moon hops past Uranus on September 23, 2021. (Chris Vaughan, Starry Night Education)

On Thursday night, September 23, the bright, waning gibbous Moon will shine a generous palm’s width to the right (or 7.5 degrees to the celestial southwest) of magnitude 5.7 Uranus. By dawn on Friday morning, the Moon’s orbital motion will carry it closer to Uranus in the west-southwestern sky, and the diurnal rotation of the sky will move it below the planet. When they rise again on Friday night, the Moon will sit 5 degrees to Uranus’ lower left (celestial east) – close enough for them to share the view in binoculars. While the blue-green dot of Uranus can be seen in binoculars, I recommend noting its location between the stars of Aries and Cetus and seek it out on the weekend, when the bright Moon will have moved away from it.

Saturday, September 25 – Bright Moon between the Pleiades and Hyades (all night)

A diagram of Bright Moon between the Pleiades and Hyades on September 25, 2021. | SkyNews
Bright Moon between the Pleiades and Hyades on September 25, 2021. (Chris Vaughan, Starry Night Education)

When the waning gibbous Moon rises in mid-evening on Saturday, September 25, it will shine several finger widths below (or 5 degrees to the celestial south of) the bright Pleiades star cluster, also known as Messier 45, the Seven Sisters, Subaru, and Matariki. The broad Hyades star cluster that forms the triangular face of Taurus the bull will be located below the Moon. The Moon and the Pleiades will share the field of view of binoculars (red circle). By dawn, the rotation of the sky will lift the Hyades to the left of the Pleiades, with the Moon midway between 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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