Sep22b-2020-at-1331-GMT-Equinox
Equinox (Chris Vaughan, Starry Night Education)
This Week’s Sky: September 21 to 27, 2020

As the autumnal equinox is marked in the Northern Hemisphere, the Moon passes near Saturn and Jupiter.

Monday, September 21 after sunset – Mercury moves past Spica

Mercury moves past Spica (Chris Vaughan, Starry Night Education)

Low in the west-southwestern sky after sunset on Monday, September 21, Mercury’s rapid orbital motion (red curve) will bring it very close Spica, Virgo’s brightest star. Look for brighter Mercury sitting just 40 arc-minutes (about 1.3 times the diameter of a full Moon) to Spica’s right (celestial west), allowing both object to fit into the field of view of a backyard telescope at medium magnification. On the following evening, Mercury will climb to sit a similar distance above Spica. Observers viewing from southerly latitudes will be able to see the duo more easily.

Tuesday, September 22 pre-dawn – Venus passes Vesta

Venus passes Vesta (Chris Vaughan, Starry Night Education)

In the eastern pre-dawn sky on the mornings surrounding Tuesday, September 22, very bright Venus will overtake and pass the slower-moving main belt asteroid Vesta. Venus will be dropping sunward while Vesta climbs in the opposite direction. At closest approach on Tuesday morning, Venus will be positioned about two finger widths to the right (or 2 degrees to the celestial south) of Vesta. Magnitude -4.14 Venus will outshine magnitude 8.16 Vesta by more than 8300 times! To see how both objects move compared to the stars around them, try to view the event on several mornings – ideally before 6 a.m. local time, when the sky will be starting to brighten.

Tuesday, September 22 at 13:31 GMT – Equinox

Equinox (Chris Vaughan, Starry Night Education)

On Tuesday, September 22 at 13:31 GMT, the Sun will cross the celestial equator moving southward, marking the autumnal equinox in the Northern Hemisphere and the beginning of autumn there. On the equinoxes in March and September, day and night are of equal length and the Sun rises due east and sets due west.

Wednesday, September 23 at 9:55 p.m. EDT – First quarter Moon

First quarter Moon (Chris Vaughan, Starry Night Education)

When the Moon completes the first quarter of its orbit around Earth at 9:55 p.m. EDT on Wednesday, September 23 (or 1:55 GMT on Thursday, September 24), the relative positions of the Earth, Sun and Moon will cause us to see it half-illuminated on its eastern side. At first quarter, the Moon always rises around noon and sets around midnight, so it is also visible in the afternoon daytime sky. The evenings surrounding first quarter are the best ones to see the lunar terrain when it is dramatically lit by low-angled sunlight.

Thursday, September 24 evening – Gibbous Moon near Jupiter

Gibbous Moon near Jupiter (Chris Vaughan, Starry Night Education)

The Moon’s monthly visit with the gas giant planets Jupiter and Saturn will commence on the evening of Thursday, September 24. As the evening sky darkens, the bright planet Jupiter will become visible several finger widths to the upper left (or 4.5 degrees to the celestial northeast of) the gibbous Moon in the southern sky. The Moon and Jupiter will fit into the field of view of binoculars (red circle). Look for somewhat dimmer Saturn sitting off to their upper left (east). By the time they set soon after midnight local time, the Moon will slide east, closer to Jupiter — and the diurnal rotation of the sky will raise Jupiter above the Moon. This conjunction will make a beautiful wide field image when composed with some interesting foreground scenery.

Friday, September 25 evening – Bright Moon below Saturn

Bright Moon below Saturn (Chris Vaughan, Starry Night Education)

The Moon’s monthly visit with the gas giant planets Jupiter and Saturn will continue on Friday, September 25. After 24 hours of eastward motion, the bright waxing Moon will sit several finger widths to the lower left (or 3.6 degrees to the celestial south of) yellowish Saturn in the southern sky after dusk. The Moon and Saturn will fit into the field of view of binoculars (red circle). Look for much brighter Jupiter sitting to their right (west). By the time the Moon and Saturn set shortly before 1 a.m. local time, the Moon will be farther from Saturn – and the diurnal rotation of the sky will lower Saturn to the Moon’s right. This conjunction will make a beautiful wide field image when composed with some interesting foreground scenery.

Sunday, September 27 all night – Sinus Iridum’s Golden Handle

Sinus Iridum’s Golden Handle (Chris Vaughan, Starry Night Education)

On Sunday night, September 27, the terminator on the waxing gibbous Moon will fall just west of Sinus Iridum, the Bay of Rainbows. The circular 155 mile (249 km) diameter feature is a large impact crater that was flooded by the same basalts that filled the much larger Mare Imbrium to its east – forming a rounded handle-shape on the western edge of that mare. The “Golden Handle” effect is produced by way the slanted sunlight brightly illuminates the eastern side of the prominent Montes Jura mountain range surrounding the bay on the north and west, and by a pair of protruding promontories named Heraclides and Laplace to the south and north, respectively. Sinus Iridum is almost craterless, but hosts a set of northeast-oriented dorsae or “wrinkle ridges” that are revealed at this phase.

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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