2021, May 16: Lunar Occultation, Mercury Best!

May 16, 2021: The moon occults a star in Gemini this evening, and Mercury reaches its greatest separation from the sun.  Bright Jupiter and Saturn are in the southeast before sunrise.  After sunset, Evening Star Venus, Mercury, Mars, and the lunar crescent are lined up above the western horizon.

2021, May 16: Morning planets Jupiter and Saturn are in the southeast before sunrise. Jupiter is in front of the stars of Aquarius, while Saturn’s background is Capricornus.
Chart Caption – 2021, May 16: Morning planets Jupiter and Saturn are in the southeast before sunrise. Jupiter is in front of the stars of Aquarius, while Saturn’s background is Capricornus.

by Jeffrey L. Hunt

Chicago, Illinois:  Sunrise, 5:29 a.m. CDT; Sunset, 8:05 p.m. CDT.  Check local sources for sunrise and sunset times for your location.

The planet exhibit continues as five planets are visible.  Jupiter and Saturn are in the southeast before sunrise.  Venus, Mercury, Mars, and the crescent moon are visible after sundown.

Morning Sky

Saturn appears above the east-southeastern horizon over 4 hours before sunrise, bright Jupiter follows less than an hour later.

By one hour before sunup, Jupiter, brighter than all the stars in the morning sky, is over 20° above the southeast horizon.

The Jovian Giant is slowly moving eastward compared to Aquarius.  Use a binocular to spot it between Iota Aquarii (ι Aqr on the chart) and Theta Aquarii (θ Aqr).  The plant is 1.7° to the upper left of Iota and 4.8° to the lower right of Theta.

Saturn, 16.7° to the upper right of Jupiter, is nearly 26° up in the south-southeast.  The planet is brighter than most stars this morning, except for Jupiter, Vega, and Arcturus.

The Ringed Wonder is slowly moving eastward compared to Capricornus.  Since it reappeared in the morning sky, after its solar conjunction, Saturn has been near Theta Capricorni (θ Cap).

In less than a week, Saturn appears to stop moving eastward compared to Theta and it begins to retrograde – move westward compared to the starry background.

This is an illusion of our faster moving Earth catching up it and passing between the sun and Saturn.

The planet exhibition is interrupted by the sun’s appearance in the sky.  During the daytime, Saturn and Jupiter lead the sun across the sky.  They set during the daytime.  After the sun sets, Venus, Mercury, Mars, and Moon are visible.

Evening Sky

This evening Mercury reaches its greatest separation (22.0°) from the sun.  This occurs shortly after midnight.  Observers in the Western Hemisphere see the planet several hours before the greatest elongation time.

2021, May 16: The star Kappa Geminorum emerges from behind the lunar crescent at 9:34 p.m. CDT as observed from Chicago in this binocular view.
Chart Caption – 2021, May 16: The star Kappa Geminorum emerges from behind the lunar crescent at 9:34 p.m. CDT as observed from Chicago in this binocular view.

Secondly, a large swath of North America sees the moon occult (cover) Kappa Geminorum (κ Gem).  For those interested, the link in the detailed notes below displays the times of the disappearance and reappearance of the star.

(The times provided on the web page through the link are in Universal Time (UT).  For Eastern Daylight Time, subtract 4 hours; Central Daylight Time (CDT), subtract 5 hours; and so on for other time zones in North America.  The UT date is May 17, but the CDT date in North America is May 16.)

In Chicago the occultation begins at 8:27 p.m. CDT, shortly after sunset, as the night side of the moon begins to cover the star.  The star reappears on the crescent side of the moon at 9:34 p.m. CDT.

An occultation is an eclipse of a star.  There is a distinction between and eclipse and an occultation.  During an eclipse, a shadow is involved, either Earth’s or the moon’s.  During an occultation, the moon covers a planet or a distant star.  Every celestial object either emits or reflects light, so in a sense there is a shadow of the moon during an occultation, but it is largely undetectable.  So, when the moon covers the sun, that’s an eclipse.  When it covers a planet or distant star, it is an occultation.  In some rare circumstances, a planet can cover distant stars. That’s an occultation as well.  When either Venus or Mercury pass in front of the sun as viewed from Earth, that’s a transit.

At 30 minutes after sunset, brilliant Venus 6.0° above the west-northwest.  Mercury, nearing its greatest elongation after midnight is 8.7° to the upper left of Venus.

Use a binocular to initially locate both worlds.

The crescent moon, 23% illuminated with the occultation in progress, is nearly 45° to the upper left of Venus.

Venus as an evening star article.

Read more about Venus in our summary document.

2021, May 16: Forty-five minutes after sunset brilliant Venus, Mercury, Mars, and the lunar crescent are along a diagonal line above the horizon.
Chart Caption – 2021, May 16: Forty-five minutes after sunset brilliant Venus, Mercury, Mars, and the lunar crescent are along a diagonal line above the horizon.

By 45 minutes after sunset, Mercury is visible to the unaided eye.  This is Mercury at its best evening appearance of the year.  At this hour it is about 12° above the horizon.  It is 5.4° below the star Elnath, the Northern Horn of Taurus.  The star’s name means, “the one butting with horns.”

Here’s more about Mercury during May 2021.

2021, May 16: An hour after sunset, with the lunar occultation in progress, Mars is 4.2° to the upper left of Epsilon Geminorum (ε Gem), 3.5° to the upper right of Zeta Geminorum (ζ Gem), and 4.7° to the lower right of Delta Geminorum (δ Gem, m = 3.5).
Chart Caption – 2021, May 16: An hour after sunset, with the lunar occultation in progress, Mars is 4.2° to the upper left of Epsilon Geminorum (ε Gem), 3.5° to the upper right of Zeta Geminorum (ζ Gem), and 4.7° to the lower right of Delta Geminorum (δ Gem, m = 3.5).

An hour after sunset, the lunar occultation is still in progress.  The moon is 3.5° to the lower left of Pollux and 9.5° to the upper left of Mars.

Against the stars, the Red Planet is 4.2° to the upper left of Mebsuta (ε Gem on the chart), “the outstretched paw of the lion,” and 4.7° to the lower right of Mekbuda (ζ Gem), “the folded paw of the lion,” and 4.7° to the lower right of Wasat (δ Gem), “the middle of the sky.”

Wasat is very close to the ecliptic – the plane of the solar system.  The moon and bright planets pass near this star when they move through this region.

Here’s more about Mars during 2021.

Find a detailed chart of the motion of Mars during the month here.

This evening, Venus sets 68 minutes after sunset, followed by Mercury 50 minutes later.  Tonight, the speedy planet sets at its latest time interval after sunset.  The gap between Venus and Mercury begins to close during the next several evenings.  Mercury dims each evening leading up to a very close conjunction with Venus on May 28.  Mars sets less than 4 hours after sunset.  The planet parade begins tomorrow morning when Saturn rises about 90 minutes after Mars sets.

Detailed Note: One hour before sunrise two bright planets, Jupiter and Saturn, shine from the southeastern sky.  Bright Jupiter is over 22° above the southeast horizon.  Saturn – nearly 26° up in the south-southeast – is 16.7° to the upper right of the Jovian Giant. In the starfield, Jupiter is on a line between ι Aqr and θ Aqr, 1.7° to the upper left of ι Aqr and 4.8° to the lower right of θ Aqr.  Saturn, nearing the beginning of its interval of retrograde motion, is 0.6° to the right of θ Cap.  The moon occults the star Kappa Geminorum (κ Gem, m = 3.6) as seen from a large swath of land across North America and eastern Asia.  For more details about your location, see http://www.lunar-occultations.com/iota/bstar/0517zc1170.htm. During the early evening, three bright planets and the moon are visible in the western sky.  Thirty minutes after sunset, brilliant Venus is 6.0° above the west-northwest horizon.  Use a binocular to locate Mercury (m = 0.3), 8.7° to the upper left of Venus. The moon (5.3d, 23%) is nearly halfway up in the sky above the west horizon.  Fifteen minutes later, Venus appears lower, nearly 4° up in the sky, while Mercury is nearly 12° above the horizon.  This speedy planet is 5.4° below Elnath.  As more stars appear, Mars, about one-third of the way up in the west, can be found, 9.6° to the lower right of the moon.  Pollux is 3.5° to the upper right of the lunar crescent.  By one hour after sunset, Venus is just above the horizon.  Mercury is 9.0° above the west-northwest horizon.  Use a binocular to observe Mars against the starfield.  It is 4.2° to the upper left of ε Gem, 3.5° to the upper right of ζ Gem, and 4.7° to the lower right of Delta Geminorum (δ Gem, m = 3.5).

Read more about the planets during May 2021.



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