This evening’s sky. The beautifully crescent Moon has passed south of Pollux and is about to come into a straight line with the two Twin stars.
The appulse – the moment of smallest apparent distance between Pollux and the center of the Moon, as viewed from Earth’s center – is May 12 23h by Universal Time, which is 4, 5, 6, 7 hours earlier by clocks on “daylight-shifting time” in North America’s Eastern, Central, Mountain, and Pacific time zones.
And the minimum separation between Pollux and the Moon’s center was 1.60°. But that, too, was geocentric – as if viewed from the Earth@s center, the way calculations have to be made. From our position on Earth’s northern hemisphere, the Moon is displace somewhat southward, so the separation is greater (the effect called parallax).
Much of this – what viewpoint you are talking about, whether you are talking about angular or linear distances between objects, the Moon’s sunlit shape and its solid shape – is part of “The Apparent versus the Real,” a distinction so useful to keep in mind that we are making it the opening chapter in the new Astronomical Companion. Just now I’m trying to think up a sketch to illustrate it.
Maybe one of the many cartoons on the theme of “Objects in mirror are closer than they appear”?
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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” or ”Open image in new tab”, 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.