Mercury’s first evening act of 2021

Mercury will be farthest out in the evening sky on January 24 (or 23, by American clocks).

See the end note about enlarging illustrations.

In other words, it will be at greatest eastern elongation (angular distance from the Sun).  More exactly, the peak of elongation is at 2h Universal Time, which is 5 or more hours earlier  in American time zones, therefore back in Jan. 23.

In that sky scene, the arrows through the planets and the Sun show their movement, in relation to the starry background, from 2 days ago to 2 days later.  Mercury is at greatest elongation because its eastward movement becomes at this moment slower than the Sun’s.  Some other features not visible in the twilight sky are shown to give a feeling of where we are in relation to the solar system and beyond.  At the time and place of the scene, Mercury is 7° above the horizon.

In this space view, from 15° north of the ecliptic plane, the paths of the planets are shown for January 2021, with the sightline from Earth to Mercury at the date of greatest eastward elongation.  The inner planets are exaggerated 300 times in size, Jupiter 50, the Sun 4.  The dashed line, with ram’s-horns symbol, is at longitude 0°, the vernal equinox direction, where the Sun will appear for us at the March equinox.

This graph compares Mercury’s sorties into the sky in the year.  Gray areas represent evening apparitions (eastward elongation); blue, morning apparitions (westward).  The top figures are the maximum elongations, reached at the top dates shown beneath.  Curves show the altitude of the planet above the horizon at sunrise or sunset, for latitude 40° north (thick line) and 35° south (thin), with maxima reached at the parenthesized dates below (40° north bold).

This year, Mercury swings three and a half times into the evening sky and three times into the morning sky.  The January excursion reaches a fairly low elongation, and is very unfavorable for southern observers, yet is fairly favorable for northerners.  The reason is the angle of the ecliptic to the sunset horizon in this part of Mercury’s orbit.  As you can see in the sky scene, Mercury is almost vertically above the Sun, so that it gets most of the benefit of its elongation.  The effect is enhanced because Mercury happens also to be in the steepest northward part of its own orbit: it is at its ascending node across the ecliptic on the same date, Jan. 24 (at 10 UT).

Here is the arc that Mercury makes into the sunset skies of January and February, as seen from latitude 40° north.

For comparison you can also see its lower performance in September, and the beginning of an evening apparition in December that will climb on into next January.  The May apparition, being farther north, is off the right edge of this picture.  The planet’s disk is exaggerated 480 times in size.  From the direction of its sunlit part you can judge the position of the Sun along the horizon at each date.

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ILLUSTRATIONS in these posts are made with precision but have to be inserted in another format.  You may be able to enlarge them on your monitor.  One way: right-click, and choose “View image”, then enlarge.  Or choose “Copy image”, then put it on your desktop, then open it.  On an iPad or phone, use the finger gesture that enlarges (spreading with two fingers, or tapping and dragging with three fingers).  Other methods have been suggested, such as dragging the image to the desktop and opening it in other ways.

Sometimes I make improvements or corrections to a post after publishing  it.  If you click on the title, rather than on ‘Read more’, I think you are sure to see the latest version.

This weblog maintains its right to be about astronomy or anything under the sun.