A diagram of the First quarter Moon on July 17, 2021. | SkyNews
First quarter Moon on July 17, 2021 (Chris Vaughan, Starry Night Education)

This Week’s Sky: July 11 to 17, 2021

Venus and Mars meet in a very close conjunction on Monday evening, while the Lunar X makes an appearance on Friday.

Monday, July 12 – Venus meets Mars (after sunset)

A diagram of Venus meeting Mars on July 12, 2021, after sunset. | SkyNews
Venus meets Mars on July 12, 2021 (Chris Vaughan, Starry Night Education

On the evenings surrounding Monday, July 12, extremely bright Venus and much fainter Mars will meet in a very close conjunction quite low in the west-northwestern sky. While both planets have been traveling eastward in their orbits (red tracks with labelled dates:times), the faster motion of inner planet Venus will cause it to catch up to and pass slower-moving Mars from tonight to tomorrow. Look closely! Magnitude +1.84 Mars will be nearly 200 times fainter than magnitude -3.87 Venus, and positioned just 34 arc-minutes (equal to about the full moon’s diameter) to the lower left of Venus. From a location with an unobstructed horizon, start to look for the planets after about 9 p.m. local time, when they’ll sit a fist’s diameter above the horizon. They’ll set by 10 p.m. local time. Binoculars (red circle) will help – but use them only after the sun has completely set. The two planets will share the view in binoculars from about July 4 to 21, but they’ll only be telescope-close (yellow circle) from July 11 to 14.

Friday, July 16 – Lunar X in early evening (peaks at 9 p.m. EDT)

A diagram of the Lunar X in the early evening on July 16, 2021. | SkyNews
Lunar X on July 16, 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). On Friday, July 16 the ‘X’ is predicted to start developing by about 7 p.m. EDT (or 23:00 GMT), peak in intensity at around 9 p.m. EDT (or 01:00 GMT on July 17), and then gradually fade out. The 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 will be visible anywhere on Earth where the moon is shining, especially in a dark sky, between 23:00 and 03:00 GMT on July 17.

Saturday, July 17 – First quarter Moon (at 10:10 GMT)

A diagram of the First quarter Moon on July 17, 2021. | SkyNews
First quarter Moon on July 17, 2021 (Chris Vaughan, Starry Night Education)

When the moon completes the first quarter of its orbit around Earth at 6:10 a.m. EDT on Saturday, July 17 (or 10:10 GMT) its 90 degree angle away from the sun will cause us to see the moon 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. (Image: Jul17-2021 at 1010 GMT – First Quarter Moon.jpg)

Saturday, July 17 – Pluto at opposition (all night)

A diagram of Pluto at opposition on July 17, 2021. | SkyNews
Pluto at opposition on July 17, 2021 (Chris Vaughan, Starry Night Education

On Saturday, July 17, the dim and distant dwarf planet designated (134340) Pluto will reach opposition for 2021. On that date, the Earth will be positioned between Pluto and the sun, minimizing our distance from that outer world. While at opposition, Pluto will be located 3.10 billion miles, 4.98 billion km, or 277 light-minutes from Earth — and it will shine with an extremely faint visual magnitude 14.3. That’s far too dim for visual observing through backyard telescopes. Pluto will be located in the sky about midway between Saturn and the bright star Nunki in Sagittarius’ Teapot asterism. Telescope-owners (inset, yellow circle) can focus on a magnitude 7.8 star named HIP97602, which will be sitting 9.3 arc-minutes directly below Pluto on opposition night. Even if you can’t see Pluto directly, you will know that it is there.

Sunday, July 18 – Asteroid Pallas pauses (overnight)

A diagram of asteroid Pallas pausing on July 18, 2021. | SkyNews
Asteroid Pallas pauses on July 18, 2021 (Chris Vaughan, Starry Night Education)

On Sunday, July 18, the main belt asteroid designated (2) Pallas will halt its regular eastward motion in front of the distant stars, and begin a retrograde loop that will last until early November (red path). Pallas’ visual magnitude of 9.7 will allow it to be seen in amateur telescopes starting in late evening. On July 18 Pallas will be positioned in the eastern sky, less than half a degree to the right (or celestial south) of the magnitude 6.65 star HIP116417 and the magnitude 7.35 star HIP116431, which sit near the ring of stars that forms the western fish in Pisces. The asteroid and those stars will appear together in the eyepiece of your telescope.

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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