2021, August 17: Mercury Targets Mars


August 17, 2021:  One evening before their close conjunction, Mercury moves toward Mars.  The gap between them is 1.1° this evening.

2021, August 17: Through a binocular, Mercury is 1.1° to the lower right of Mars.
Chart Caption – 2021, August 17: Through a binocular, Mercury is 1.1° to the lower right of Mars.

by Jeffrey L. Hunt

Chicago, Illinois:  Sunrise, 6:01 a.m. CDT; Sunset, 7:47 p.m. CDT.  Check local sources for sunrise and sunset times for your location.

This evening, Mercury closes in on Mars before their close conjunction tomorrow evening.  The impending conjunction is the closest until 2032.  Eighteen conjunctions between the two planets occur in the interim.

The pair is difficult to see as they are low in the sky during bright twilight.

Use a binocular 25 minutes after sunset, to look low in the western sky.  The pair is only about 3° above the horizon.  Mercury is bright enough to be seen.  Mars, much dimmer, is to Mercury’s upper left.

This is a challenging observation because of the planets’ altitude and the brightness of the sky. 

This evening the planets appear to be close, but they are over 129 million miles in space.

Conjunctions between the two planets occur often, although many occur when both planets are near the sun.  From the mid-northern latitudes, Mercury rarely (almost never) appears in the sky when the sky is completely dark.  At times it is bright enough to be seen without the aid of a telescope, but mostly sky watchers are chasing it through a binocular.

The easier seen conjunctions when Mercury is reaching its greatest elongation, when it is farthest from the sun in our view.  The maximum angular separation of the sun with the planet is about 27°.

The Earth’s tilt is another factor.  Sometimes when Mercury is near its greatest elongation, it stands higher in the sky (spring evenings, fall mornings) and at other occasions it is very low.

The planet’s orbital tilt is a third factor.  When it is below the plane of the solar system it is a little more difficult to observe than when it is above the plane of the ecliptic.

The current apparition of Mercury is very unfavorable for viewing.  The ecliptic is poorly inclined from our tilt, although the planet is slightly above the plane.  Currently, the planet is over 15° east of the sun.  Its greatest elongation occurs next month.

Finding and following Mercury during this appearance requires assistance from a binocular or telescope.

The next conjunction occurs during October, when they are hiding in bright twilight and only 2° from the sun.

The following conjunction occurs November 10, when the planets are only 11° from the sun.

Tomorrow evening’s conjunction is a close one!

Detailed Daily Note:The moon is at perigee at 4:16 a.m. CDT, 229,375.8 miles away.  One hour before sunrise, Jupiter is over 13° above the west-southwest horizon.  It is 1.7° to the lower right of ι Aqr, 1.5° to the upper left of μ Cap, and 3.9° above Deneb Algedi. Twenty-five minutes after sunset, Mercury (m = −0.5) is over 3° up in the west, 1.1° to the lower right of Mars.  This is a challenging observation.  All five bright planets span the sky from the western horizon to Jupiter low in the east-southeast.  Twenty minutes later, Venus is about 8° up in the west, 4.7° to the upper left of Zavijava, 3.0° to the lower right of Zaniah, and 22.0° to the lower right of Spica that is nearly 14° up in the west-southwest. Venus sets at azimuth 270° (West), 88 minutes after sunset.  The bright moon (9.5d, 77%), over 22° above the southern horizon, is 5.9° to the upper right of Alnasl (γ Sgr, m = 3.0), at the end of the spout of the Teapot of Sagittarius.  Use a binocular to find the dimmer stars with the moon’s brightness.  Saturn – retrograding in Aquarius – is about 14° above the southeast horizon.  Jupiter – 18.4° to the lower left of Saturn – is about 6° up in the east-southeast. As midnight approaches, Saturn is about one-third of the way up in the south, The Ringed Wonder is 1.4° to the lower left of υ Cap. Jupiter is to Saturn’s left.

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