Dud events

Mercury will on August 16 stand out far on the evening side of the Sun – the farthest for the year.  And yet it is the year’s worst apparition of Mercury!

Mercury graph for 2016

What this diagram from Astronomical Calendar 2016 is trying to say is that we see Mercury, in its small orbit, swing out into our view six times in the year, three times eastward into the evening sky (the gray curves), three times westward into the pre-dawn sky.  And you can see that its August elongation of 27.4° is the greatest not only for the evening performances but for the whole year.

But the thicker black line shows how far out Mercury seems to reach, as seen from our north-hemisphere part of the Earth.  And for August it’s only 11.7° – the smallest distance.

It’s a classic instance of this paradox about Mercury’s orbit.  The planet is farther out from the Sun than at other times because it was at the aphelion of its elliptical orbit on Aug. 15.  But it is, at this time of year, far out ahead of a Sun that is descending southward along the ecliptic; so it is out a low angle to the horizon.  And this is exacerbated, this time, because Mercury has moved south of the ecliptic (it was at the descending node on Aug. 5).  The result:

sky scene 2016 August 16

Then on August 18 (Thursday) comes another phenomenon that sounds major and is minimal: the Full Moon is eclipsed –

Penumbral eclipse of Moon 2016 August 18

– but this is only a penumbral eclipse, and an extremely slight examples of this slight kind of eclipse.  The Moon will barely touch the outermost and faintest part of Earth’s shadow – for a matter of seconds, at 9:43 Universal Time which is 5:43 EDT and 2:43 in the Pacific zone.

You’ll be lucky if you see Mercury, and you’ll be kidding yourself if you see the eclipse.  But tell me if you do; I’ll be away on a trip from now till then.

2 thoughts on “Dud events”

  1. I had an errand in Berkeley yesterday afternoon, Sunday August 13, so I had an early supper there and then drove up to Grizzly Peak Boulevard, which has a dramatic view to the west over San Francisco Bay to the San Francisco Peninsula, the Golden Gate, and Marin, to watch the Sun set and to look for the planets. The bay was blanketed with fog, but the air above the fog was clear. Venus, Jupiter, Mars, and Saturn were all clearly visible to the unaided eye within half an hour after sunset. I needed binoculars to see Mercury. About 45 minutes after sunset Mercury was dipping down into horizon haze, but there were moments when I thought I could see a faint Hermetic glimmer.

    There were other folks parked along the side of the road watching the sunset. I enjoyed pointing out the planets to them, explaining where we all are in our mutual orbits around the Sun (with a digression into Kepler’s laws of planetary motion and Newton’s explanation of them, for those who were interested), and suggesting they keep watching for Venus’ continuing climb into the evening sky and her conjunction with Jupiter, as well as Mars’ slide between Saturn and Antares.

    A lovely serendipitous excursion.

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