Mercury’s vertical leap

Mercury stands farthest out from the Sun, in the evening sky.

See the end note about enlarging illustrations.

Technically, we say that the planet is at its greatest eastward elongation.  The extreme is reached on February 27 at 1h by Universal Time.  This is back in Feb. 26 by American clocks – 8 PM for the Eastern time zone, 7 PM for the Central, and so on.  It is less than an hour after the time of our scene.

The arrows through the planets and Sun show their movement (against the starry background) from 2 days before to 2 days after the present date.  Maximum elongation occurs because Mercur’s movement changes from slightly faster than the Sun’s to slightly slower.

The broad arrow on the celestial equator shows how far the sky appears to rotate in one hour. So Mercury will be down to the horizon in less than an hour.

This evening, and a few evenings around, offer one of the best opportunities to see the elusive little planet, if the low sky is clear.

Here are the year’s “apparitions” of Mercury compared: three swings out from the neighborhood of the Sun into the evening sky (gray) and three into the morning sky (blue).  The top figures are the maximum elongations, reached at the top dates given beneath.  Curving lines 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).

You can see that for Feb. 27 the elongation is not particularly great (18.1°).  However, the thick curve almost fits up to the gray area.  That is, the sunset altitude, for north-hemisphere observers, is almost as great as the elongation.

The reason for this favorabie geometry is clear in the sky scene.  Mercury’s travel away from the Sun appears vertical.  It roughly follows the ecliptic, which at this season is, for this latitude, nearly vertical.  And it also happens that Mercury, in this part of its orbit, is sloping slightly northward from the ecliptic.

It’s said that Copernicus, who made us understand that the planets revolve around the Sun, was never able to see little Mercury from misty Poland.  Perhaps you can research this for me and tell us whether it is just a Polish joke or an urban-pollution myth.

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DIAGRAMS 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).  I am grateful to know of what methods work for you.