A diagram of the Sun at June solstice on June 20, 2021. | SkyNews
June solstice on June 20, 2021 (Chris Vaughan, Starry Night Education)

This Week’s Sky: June 14 to 20, 2021

It’s a rather slow week in the skies, as the Moon reaches first quarter and the June solstice passes.

Thursday, June 17 – First quarter Moon (at 03:54 GMT)

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

When the Moon completes the first quarter of its orbit around Earth at 11:54 p.m. EDT on Thursday, June 17 (or 3:54 GMT on June 18) its 90-degree angle away from the Sun 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 ones for seeing the lunar terrain when it is dramatically lit by low-angled sunlight.

Sunday, June 20 – June solstice (at 03:32 GMT on June 21)

A diagram of the Sun at June solstice on June 20, 2021. | SkyNews
June solstice on June 20, 2021 (Chris Vaughan, Starry Night Education)

On Sunday, June 20 at 11:32 p.m. EDT (or 03:32 GMT on Monday, June 21), the Sun will reach its northernmost declination for the year, resulting in the longest daylight hours of the year for the Northern Hemisphere and the shortest daylight hours of the year for the Southern Hemisphere. The solstice marks the beginning of the summer season in the Northern Hemisphere, and winter in the Southern Hemisphere.

Monday, June 21 – Jupiter reverses direction (wee hours)

A diagram of Jupiter reversing direction on June 21, 2021. | Starry Night Education
Jupiter reversing direction on June 21, 2021 (Chris Vaughan, Starry Night Education)

During the early morning hours of Monday, June 21, Jupiter will pause in its regular eastward motion in front of the distant stars of western Aquarius and then begin a retrograde loop (red curve with dates) that will last until mid-October. The apparent reversal in Jupiter’s motion is an effect of parallax produced when Earth, on a faster orbit, begins to pass Jupiter on the “inside track”. Starting this week, Jupiter will rise before midnight local time, and its 19-degree angular separation from Saturn will slowly decrease.

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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