A diagram of the Lunar X on May 18, 2021. | SkyNews
Lunar X on May 18, 2021 (Chris Vaughan, Starry Night Education)

This Week’s Sky: May 17 to 23, 2021

The show stealers this week are the Moon and the gas giants. Look for the Lunar X, Saturn standing still in our sky and a double shadow transit on Jupiter.

What’s up in This Week’s Sky? From eclipses and meteor showers to planetary conjunctions, this weekly forecast written by Chris Vaughan gives you observing updates for the night sky as seen from Earth, focusing on the Solar System, Sun, Moon, planets and more.

Tuesday, May 18 – Lunar X in Twilight (peaks at 8:34 p.m. EDT)

A diagram of the Lunar X on May 18, 2021. | SkyNews
Lunar X on May 18, 2021 (Chris Vaughan, Starry Night Education)

Several times a year, for a few hours near its first quarter phase, a feature on the Moon called the Lunar X becomes visible in strong binoculars and backyard telescopes. When the rims of the craters Purbach, La Caille and Blanchinus are illuminated from a particular angle of sunlight, they form a small, bright X-shape. The Lunar X is located on the terminator, about one third of the way up from the southern pole of the Moon (at 2° East, 24° South). The “X” is predicted to peak in intensity at 8:34 p.m. EDT on Tuesday, May 18. That 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 will be visible anywhere on Earth where the Moon is shining, especially in a dark sky, between 23:00 on May 18 and 03:00 UT on May 19.

Wednesday, May 19 – First Quarter Moon (at 19:12 GMT)

A diagram of the first quarter Moon, May 19, 2021. | SkyNews
First quarter Moon on May 19, 2021 (Chris Vaughan, Starry Night Education)

When the moon completes the first quarter of its orbit around Earth at 3:12 p.m. EDT (or 19:12 GMT) on Wednesday, May 19, 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 for seeing the lunar terrain when it is dramatically lit by low-angled sunlight.

Sunday, May 23 – Double Shadow Transit on Jupiter (15:17 to 16:19 GMT)

A diagram of the double shadow transit on Jupiter on May 23, 2021. | SkyNews
Double shadow transit on Jupiter on May 23, 2021 (Chris Vaughan, Starry Night Education)

From time to time, the small round black shadows cast by Jupiter’s four Galilean moons become visible in amateur telescopes as they cross (or transit) the planet’s disk. Before dawn on Sunday, May 23, observers located in Japan, New Zealand and Australia can see two of those shadows on Jupiter at the same time. At 15:17 GMT (or 12:17 a.m. JST), Callisto’s large shadow will join Io’s smaller shadow already in transit. The two shadows will cross Jupiter together for an hour until Io’s shadow moves off the planet at 16:19 GMT. Callisto’s shadow will continue to transit Jupiter until 20:00 GMT.

Sunday, May 23 – Saturn stands still (pre-dawn)

A diagram of Saturn standing still on May 23, 2021. | SkyNews
Saturn stands still on May 23, 2021 (Chris Vaughan, Starry Night Education)

On Sunday, May 23, Saturn will cease its regular eastward motion through the distant stars of Capricornus and begin a retrograde loop (red curve with dates/times) that will last until mid-October. The apparent reversal in Saturn’s motion is an effect of parallax produced when Earth, on a faster orbit, passes the Ringed Planet on the “inside track”. You can observe the planet’s direction change by noting how Saturn’s distance from the nearby bright star Theta Capricornus varies over several days.

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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