Old Moon meets Venus near M44 (Chris Vaughan, Starry Night Education)

This Week’s Sky: September 7 to 13, 2020

In This Week’s Sky, Mars starts a retrograde loop while Jupiter completes one. Neptune reaches opposition near Phi Aquarii, and a waning crescent Moon meets Venus.

Wednesday, September 9 all night – Mars stands still

Mars stands still (Chris Vaughan, Starry Night Education)

On Wednesday, September 9, Mars will cease its eastward motion through the distant background stars of Pisces, and commence a retrograde loop (red path with labeled dates) that will last through opposition in October, and end in mid-November. To see Mars’ motion, note its position compared to the stars around it and check back on subsequent evenings to see the difference.

Thursday, September 10 at 9:26 GMT – Last quarter Moon

Last Quarter Moon (Chris Vaughan, Starry Night Education)

At its last quarter phase at 9:26 GMT on Thursday, September 10, the Moon will rise around midnight and remain visible in the southern sky all morning. At this phase, the Moon is illuminated on its western side, towards the pre-dawn Sun. Last quarter moons are positioned ahead of the Earth in our trip around the sun. About 3.5 hours later, Earth will occupy that same location in space. The week of moonless evening skies that follow last quarter will be ideal for observing deep sky targets.

Friday, September 11 post-midnight – Crescent Moon meets Messier 35

Crescent Moon meets Messier 35 (Chris Vaughan, Starry Night Education)

When the waning crescent Moon rises shortly after 12 a.m. local time on Friday, September 11, it will be positioned several degrees to the celestial west of the large open star cluster in Gemini known as Messier 35 or the Shoe-Buckle. During the rest of the night, the Moon’s orbital motion (green line) will carry it closer to the cluster — bringing it just a finger’s width to the right of Messier 35 before dawn. To see the cluster’s stars more easily, hide the bright Moon just beyond the right edge of your binoculars’ field of view (red circle).

Friday, September 11 overnight – Neptune at opposition near Phi Aquarii

Neptune at opposition near Phi Aquarii (Chris Vaughan, Starry Night Education)

On Friday, September 11, Neptune will be directly opposite the Sun in the sky. At opposition, Neptune will be closest to us for this year — 4 light-hours or 28.9 Astronomical Units from Earth. It will shine at a slightly brighter magnitude 7.8, and will be visible all night long in good binoculars (red circle) and backyard telescopes in a dark sky. Around opposition, Neptune’s disk size will grow to 2.4 arc-seconds. Throughout September, Neptune will be located among the stars of northeastern Aquarius, about two degrees to the left (or celestial east) of the naked-eye star Phi (φ) Aquarii.

Saturday, September 12 evening – Jupiter completes its retrograde loop

Jupiter completes its retrograde loop (Chris Vaughan, Starry Night Education)

On Saturday, September 12, Jupiter will appear to stop moving with respect to the distant stars — marking the end of a westward retrograde loop (red path with labelled dates:times) that began in mid-May. After tonight, Jupiter will resume its regular eastward motion in northeastern Sagittarius, and will begin to reduce its eight-degree separation from Saturn.

Sunday, September 13 at 9:08 p.m. EDT – Algol at minimum brightness

Algol at minimum brightness (Chris Vaughan, Starry Night Education)

Algol, also designated Beta Persei, is among the most accessible variable stars for skywatchers. Its naked-eye brightness dims noticeably for about 10 hours once every two days, 20 hours and 49 minutes because a dim companion star orbiting nearly edge-on to Earth crosses in front of the much brighter main star, reducing the total light output we receive. On Sunday, September 13 at 9:08 p.m. EDT (or 1:08 GMT on Monday), Algol will reach its minimum brightness of magnitude 3.4. At that time, for observers in the Eastern Time Zone, the star will sit 12 degrees above the northeastern horizon. Five hours later, at 2:08 a.m. EDT (or 6:08 GMT), Algol will be high in the eastern sky, and will have brightened to its usual magnitude of 2.1.

Monday, September 14 pre-dawn – Old Moon meets Venus near M44

Old Moon meets Venus near M44 (Chris Vaughan, Starry Night Education)

When the waning crescent moon rises at about 3:15 a.m. local time on Monday, September 14, it will be positioned a few finger widths to the left (or four degrees to the celestial north) of the bright planet Venus. The pair, which will remain visible in the east until sunrise, will fit together into the field of view of binoculars (red circle) and will make a lovely wide-field photograph when composed with some interesting landscape. Your binoculars might also reveal the large open star cluster known as the Beehive or Messier 44, sitting just above and between the Moon and Venus. To see the cluster’s stars more easily, hide the Moon and Venus just below your binoculars’ field of view.

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 to tour the universe together.

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