Tag: Mercury

Back issues, and backward motion

Just after the Moon backed away from totally eclipsing the Sun on April 20, Mercury will go into retrograde (backward) motion on April 21. Not an important event, but it gives a pretext to mention Back Issues.

Back issues, that is, of the Astronomical Calendar.

Many people like to have a full set of those annual books, with their cover pictures and stories, and some have had them bound in volumes by fives. Well, if you have a gap in your collection, you now have the chance to fill it. We have a few copies left for most of the years.

Any that you acquire will be autographed for you by the author.

 

Mercury turning back

As for Mercury’s stationary moment, when it begins to move back westward in relation to the starry background, here is the scene this evening, which for America is the nearest to that moment.

At the time and place of the picture, Mercury is 5° above the horizon. If you look closely, you can see that the arrow showing the planet’s motion over five days curves up and then back down. The arrow is very short, because the change of position near the stationary moment is slow.

This is not quite the same as the moment of maximum eastward elongation or angular distance out eastward from the Sun, which happened on April 11. In the time between those dates, Mercury was still appearing to move eastward on the map of the sky, but more slowly, so that it was beginning to fall back toward the Sun.

This space view, from 15° north of the ecliptic plane, shows the paths of the planets in April, and sightlines from Earth to Mercury at the dates of its easternmost elongation and of becoming stationary in longitude. The dashed line is the vernal equinox direction, or Earth-Sun direction on March 20, from which all sky positions are measured.

 

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