2018: December’s Morning Planet Dance


Three bright planets appear low in the southeast sky during late December.  Watch their movement during the five days highlighted to see them move in a celestial dance against the starry background, especially with Mercury passing Jupiter. Both planets are among fourth and fifth magnitude stars in southern Ophiuchus.  Mercury reaches greatest elongation (21°) on December 15, rising nearly 110 minutes before sunrise.  It stands about 20° above the horizon at sunrise. After its greatest elongation, Mercury rises about 2 minutes later each morning.  While the planets are moving eastward compared to the stars, Mercury appears lower each morning when viewed at the same time, as it heads back into bright twilight toward its solar conjunction.   This morning elongation of Mercury bookends the year, nearly matching rising intervals during its apparition in January.  Mercury moves fastest.  Watch it as it moves past Psi Ophiuchi and onward toward Jupiter for a conjunction on December 21.  Venus, above the Claws of the Scorpion (Zubenelgenubi and Zubeneschamali), rises about 230 minutes before sunrise.  It moves closer to the Southern Claw during the mornings described below. A few weeks ago, Venus was at its phase of greatest brilliancy and greatest illuminated extent.  It appears about 25° above Jupiter and Mercury.   In comparison, Jupiter, about rising 90 minutes before sunrise, creeps against the sidereal scene. It is near Omega Ophiuchi.  Use binoculars to track the motion of the planets against the positions of the stars and to initially locate Antares which is very low in the sky.  The motion of Venus and Jupiter are described in detail in accompanying articles. The following describes the mornings at 45 minutes before sunrise:

December 19:  Brilliant Venus (m = −4.7) is 28° up in the southeast, 4.3° above Zubenelgenubi (α Lib, m = 2.8).  Bright Jupiter (m = −1.8) is 27° to the lower left of Venus.  Jupiter is 0.8° to upper right of Omega Ophiuchi (ω Oph, m = 4.4) and 5.3° to the upper left of Antares (α Sco, m = 1.0), although the star is only 3° in altitude.  Use binoculars to find it.  Mercury (m = −0.5) is 2.5° to the upper right of Jupiter and 1° to the upper right of Psi Ophiuchi (ψ Oph, m = 4.5).

December 20: This morning Jupiter is 1.6° below Mercury and 0.6° to the upper right of Omega Ophiuchi. Mercury is 0.6° to the lower left of Psi Ophiuchi.

December 21: Mercury, Jupiter and Antares are nearly in a line, spanning 6.1°; the Jupiter-to-Antares gap is 5.2°.  Jupiter is 0.9° to the lower right of Mercury, their closest separation, and 0.4° to the upper left of Omega Ophiuchi.

December 22: Jupiter is 1.2° to the right of Mercury (m = −0.4) and 0.2° to the upper left of Omega Ophiuchi.

December 23: Jupiter is 2° to the upper right of Mercury, which has an altitude of 5°.  The giant planet is 0.22° to the upper left of Omega Ophiuchi.  It passes 5.2° to the upper left of Antares and Mercury passes 6.1° to the upper left of the star.  Venus is nearly 25° to the upper right of Jupiter and 2.9° to the upper left of Zubenelgenubi.

Jupiter-Antares conjunctions become more difficult to see at the time of their next two conjunctions.  On December 4, 2030, Jupiter is only 6.5° west of the sun when it passes 5.1° from Antares.  At the November 23, 2042, conjunction, Jupiter is 8.8° east of the sun, setting 30 minutes after the sun, and passes 5.1° north of Antares. Mercury passes between them on November 20.  The November 8, 2054, conjunction occurs when Jupiter sets 75 minutes after the sun.  It is 24° east of the sun and 5.1° above Antares.  The next conjunction that has the pair perfectly-placed in the evening sky is July 13, 2090, when they are on the meridian at 10:00 p.m. CDT, Jupiter is 5.2° north of Antares.

Back to the current apparition of Jupiter:  After the Jupiter-Mercury activity, Venus moves between the Scorpion’s claws, heading for a widely-spaced conjunction with the giant planet in January.  This is followed by a conjunction with Saturn in February.  More details are in the accompanying articles; look for a focused article about the Venus conjunctions in the Winter issue.

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