A diagram of the old Moon near Uranus, pre-dawn on July 4, 2021. | SkyNews
Old Moon near Uranus, pre-dawn (Chris Vaughan, Starry Night Education)

This Week’s Sky: June 28 to July 4, 2021

As the third quarter Moon wanes, it passes by Uranus. Also, Mercury reaches greatest western elongation.

Thursday, July 1 – Third quarter Moon (at 21:10 GMT)

A diagram of the third quarter Moon on July 1, 2021. | SkyNews
Third quarter Moon (Chris Vaughan, Starry Night Education)

The Moon will officially reach its third quarter phase at 5:10 p.m. EDT (or 21:10 GMT) on Thursday, July 1. At third quarter, our natural satellite always appears half-illuminated, on its western side, towards the pre-dawn Sun. It rises in the middle of the night and remains visible in the southern sky all morning. The name for this phase reflects the fact that the Moon has completed three quarters of its orbit around Earth, measuring from the previous new Moon. The ensuing week of moonless evening skies will be ideal for observing deep sky targets.

Sunday, July 4 – Old Moon near Uranus (pre-dawn)

A diagram of the old Moon near Uranus, pre-dawn on July 4, 2021. | SkyNews
Old Moon near Uranus, pre-dawn (Chris Vaughan, Starry Night Education)

When the waning crescent Moon rises in the east at about 2 a.m. local time on Sunday, July 4, it will be positioned a slim palm’s width to the right (or 5 degrees to the celestial southwest) of the magnitude 5.8 planet Uranus, close enough for them to share the view in binoculars (red circle). Try to find the planet before about 4:30 a.m. local time. After that, the brightening dawn sky will overwhelm it, but will leave the Moon visible.

Sunday, July 4 – Mercury at greatest western elongation (pre-dawn)

A diagram of Mercury at greatest western elongation, pre-dawn on July 4, 2021. | SkyNews
Mercury at greatest western elongation, pre-dawn (Chris Vaughan, Starry Night Education)

During much of July, Mercury will be visible in the pre-dawn sky. On Sunday, July 4, the swiftly-moving planet will reach a maximum angle of 22 degrees west of the Sun, and peak visibility for its morning apparition. The best time to see the planet will come just before 5 a.m. in your local time zone, when Mercury will sit very low in the east-northeastern sky. In a telescope (inset) the planet will show a 36-per-cent-illuminated, waxing crescent phase. Mercury’s position well below the morning ecliptic (green line) will make this apparition a poor one for Northern Hemisphere observers, but a good showing for those located near the Equator and farther south.

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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