A diagram of the Lunar X on September 13, 2021. | SkyNews
Lunar X on September 13, 2021 (Chris Vaughan, Starry Night Education)

This Week’s Sky: September 13 to 19, 2021

See the Lunar X this week as the Moon waxes past first quarter. Also, watch as it passes gas giants.

Monday, September 13 – First quarter Moon (at 20:39 GMT)

A diagram of the First quarter Moon on September 13, 2021. | SkyNews
First quarter Moon on September 13, 2021. (Chris Vaughan, Starry Night Education)

When the Moon completes the first quarter of its journey around Earth on Monday, September 13 at 4:39 p.m. EDT, or 20:39 GMT, its 90 degree angle away from the Sun will cause us to see the Moon exactly half-illuminated – on its eastern side. At first quarter, the Moon always rises around mid-day and sets around midnight, so it is also visible in the afternoon daytime sky. The evenings surrounding first quarter are the best ones for seeing the lunar terrain when it is dramatically lit by low-angled sunlight, especially along the terminator, the pole-to-pole boundary between the lit and dark hemispheres.

Monday, September 13 – Lunar X in early evening (peaks at 7 p.m. EDT)

A diagram of the Lunar X on September 13, 2021. | SkyNews
Lunar X on September 13, 2021 (Chris Vaughan, Starry Night Education)

Several times a year, for a few hours near its first quarter phase, features on the Moon called the Lunar X and Lunar V become visible in strong binoculars and backyard telescopes. The Lunar X is located on the terminator south of the crater La Caille, about one third of the way up from the southern pole of the Moon (at 2 degrees east, 24 degrees Ssouth). The “V” is located near the crater Ukert (at 1 degree east, 14 degrees north). On Monday, September 13, those letters are predicted to start developing by 5 p.m. EDT (or 21:00 GMT), peak in intensity around 7 p.m. EDT (or 23:00 GMT) and then gradually fade out. That peak will be during waning daylight for observers in the eastern Americas, but you can observe the Moon in a telescope during daytime, as long as you take care to avoid the Sun. The Lunar X and V will be observable anywhere on Earth where the Moon is visible, especially in a dark sky, between about 21:00 GMT on September 13 and 1:00 GMT on September 14.

Monday, September 13 – Mercury at greatest eastern elongation (after sunset)

A diagram of Mercury at greatest eastern elongation on September 13, 2021. | SkyNews
Mercury at greatest eastern elongation on September 13, 2021. (Chris Vaughan, Starry Night Education)

After sunset on Monday, September 13, Mercury (orbit shown in red) will be just hours away from its widest separation, 27 degrees east of the Sun, and its maximum visibility for the current apparition. On Tuesday evening it will be almost as elongated. With Mercury positioned well below the evening ecliptic (green line) in the west-southwestern sky, this appearance of the planet will be a poor one for Northern Hemisphere observers, but will offer excellent views for observers near the equator and in the Southern Hemisphere. The optimal viewing times at mid-northern latitudes fall around 7:30 p.m. local time. Viewed in a telescope (inset) the planet will exhibit a waning, half-illuminated phase.

Tuesday, September 14 – Neptune at opposition (all night)

A diagram of Neptune at opposition on September 14, 2021. | SkyNews
Neptune at opposition on September 14, 2021. (Chris Vaughan, Starry Night Education)

On Tuesday, September 14, Neptune will reach opposition. At that time Neptune will be closest to Earth for this year – a distance of 2.69 billion miles, 4.33 billion km, 4 light-hours, or 28.9 Astronomical Units. The blue planet will shine with a slightly brighter magnitude 7.8. Since it’s directly opposite the Sun in the sky, it will be visible all night long in good binoculars (red circle) if your sky is very dark, and backyard telescopes. Your best views will come after 9 p.m. local time, when the blue planet has risen higher. Around opposition, Neptune’s apparent disk size will attain 2.4 arc-seconds and its large moon Triton will be most visible (inset). Throughout September, Neptune will be located among the stars of northeastern Aquarius, about 4 degrees to the left (or celestial east) of the naked-eye star Phi Aquarii (φ Aqr).

Wednesday, September 15 – Watch Algol brighten (at 9:40 p.m. EDT)

A diagram of Algol on September 15, 2021. | SkyNews
Watch Algol brighten on September 15, 2021. (Chris Vaughan, Starry Night Education)

Algol, also designated Beta Persei, is among the most accessible variable stars for skywatchers. During a ten-hour period that repeats every two days, 20 hours, and 49 minutes, Algol’s visual brightness dims and re-brightens noticeably. This happens because a companion star orbiting nearly edge-on to Earth crosses in front of the much brighter main star, reducing the total light output we receive. Algol normally shines at magnitude 2.1, similar to the nearby star Almach (a.k.a. Gamma Andromedae). But when fully dimmed, Algol’s brightness of magnitude 3.4 is almost identical to Rho Persei (or Gorgonea Tertia or ρ Per), the star sitting just two finger widths to Algol’s lower right (or 2.25 degrees to the celestial south). On Wednesday, September 15 at 9:40 p.m. EDT (or 01:40 GMT on September 16), Algol will be at its minimum brightness, and low in the northeastern sky. Five hours later, Algol will shine at full brightness nearly overhead in the eastern sky.

Thursday, September 16 – Bright Moon below Saturn (evening)

A diagram of the Bright Moon below Saturn on September 16, 2021.  | SkyNews
Bright Moon below Saturn on September 16, 2021. (Chris Vaughan, Starry Night Education)

After the Sun sets on Thursday evening, September 16, look in the lower part of the southeastern sky for the bright waxing gibbous Moon shining a slim palm’s width below (or 5 degrees to the celestial southwest of) the yellowish dot of Saturn, with much brighter Jupiter positioned well off to their left (east). While the pair is crossing the sky during the night, both objects will just fit into the field of view of your binoculars (red circle). Meanwhile the diurnal rotation of the sky will lift the Moon to Saturn’s left before they set together in the west-southwest at about 2:30 a.m. local time.

Friday, September 17 – Bright Moon joins giant planets (overnight)

The bright Moon joins giant planets on September 17, 2021. | SkyNews
Bright Moon joins giant planets on September 17, 2021. (Chris Vaughan, Starry Night Education)

After 24 hours of eastward orbital motion, on Friday, September 17, the bright, waxing Moon will sit below and between Jupiter and Saturn among the faint stars of Capricornus. The trio will shine in the southeastern sky after dusk, and then cross the southern sky overnight. They’ll make a nice wide-field photo when composed with some interesting scenery – immediately after dark and then again before Saturn sets, ahead of the other two, shortly before 3:30 a.m. local time on Saturday morning. The Moon will be tucked a little closer on Jupiter’s lower left (celestial southwest) by that time.

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.

Get a Free Digital Issue