2021, August 21: Jupiter, Blue Moon

August 21, 2021: Bright Jupiter is near the Blue Moon this evening.  The lunar term has several definitions. 

2021, August 21: As night falls, the nearly-full moon is 5.4° to the lower right of Jupiter. Saturn is to the upper right of Jupiter.
Chart Caption – 2021, August 21: As night falls, the nearly-full moon is 5.4° to the lower right of Jupiter. Saturn is to the upper right of Jupiter.

by Jeffrey L. Hunt

Chicago, Illinois:  Sunrise, 6:05 a.m. CDT; Sunset, 7:41 p.m. CDT.  Check local sources for sunrise and sunset times for your location.

This evening the bright moon is near Jupiter.  Nearing its Full phase, occurring tomorrow morning, the bright moon is 5.4° to the lower right of Jupiter as night falls.

This Full phase is the third one for this season.  Tomorrow’s full moon is named the “Sturgeon Moon” by the Farmer’s Almanac. Next month’s Harvest Moon is the fourth.

Usually, an astronomical season (solstice to equinox or equinox to solstice) has three full moons.  Following the traditions of naming a full moon after nature’s events in a season, there was no name for the extra full moon.  It was named “Blue.”

An article in Sky & Telescope magazine traced the origins of the term to the Maine Farmers Almanac in the 1930s.

The blue moon name has been expanded over time to indicate the second full moon in a calendar month, a mistake by Sky & Telescope.  The error has caught on in popular culture. 

The official time for the full phase is 7:02 a.m. CDT tomorrow, after the moon has set for most of the Americas. Whatever factors you use to determine the blue moon, find the nearly-full moon close the Jupiter this evening.  Follow them across the sky during the night.

Saturn is 18.2° to the upper right of Jupiter.  Both planets are retrograding in Capricornus.

2021, August 21: Venus steps toward Spica in the west after sunset.
Chart Caption – 2021, August 21: Venus steps toward Spica in the west after sunset.

About 45 minutes after sunset, brilliant Venus is about 8° up in the western sky.  It is stepping toward the star Spica for a conjunction on September 5.

2021, August 21: Through a binocular, Mercury is 2.9° to the upper left of Mars.
Chart Caption – 2021, August 21: Through a binocular, Mercury is 2.9° to the upper left of Mars.

If you’ve been tracking Mercury and Mars in the west after sunset, they are still in the same binocular field.  Mercury is 2.9° to the upper left of Mars.  This is a very challenging observation.

Detailed Daily Note:One hour before sunrise, bright Jupiter is about 10° above the west-southwest horizon. Twenty-five minutes after sunset, Mercury (m = −0.3) is less than 4° up in the west, 2.9° to the upper left of Mars.  Again, this evening, the five bright planets are in the sky simultaneously, if you can find them in this bright twilight.  Twenty minutes later, brilliant Venus is about 8° up in the west, 2.0° to the left of Zaniah, 4.2° to the lower right of Porrima (γ Vir, m = 3.4), and 17.3° to the lower right of Spica that is over 12° above the west-southwest horizon.  Farther eastward, Saturn is over 15° up in the southeast.  The bright moon (13.5d, 100%) is over 5° above the east-southeast horizon.  Jupiter is 5.4° to the upper left of the lunar orb.  As midnight approaches, Saturn is over 29° up in the south. In the starfield, it is 1.2° to the lower left of υ Cap.  The moon, 29.0° above the south-southeast horizon, is 4.8° below Jupiter.  The Jovian Giant and Ringed Wonder are retrograding in Capricornus.  Use a binocular to see the starfield with the planets.  At this hour, Jupiter is 2.2° to the upper right of ι Aqr, 0.9° to the lower left of μ Cap, and 3.4° to the upper left of Deneb Algedi.

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