Today, June 21, is a concaltion. New word, like “conjunction,” for a gathering of several calendar events:
The solstice (as we can feel from the rusung heat); about 14 hours later, the first-quarter phase of the Moon. Oh, and Fathers Day, which may be noticed on the western but less on the eastern side of the Atlantic.
At first quarter, the “signpost” Moon is near the point where the Earth was about three and a half hours ago. How near depends, as Fred Schaaf has pointed out, on how near the quarter-phase moment is to another kind of Moon event: its passage through the ecliptic (the plane in which the Earth moves). The nearest such events this time are June 19, descending node, and July 4, ascending node. So at first quarter the Moon is not exactly at the point where Earth was, but a little south of it.
Working on how to calculate and illustrate this, I realized that, instead of comparing with the times of node passage, it is easier to use the ecliptic latitude of the Moon.
This is a view in the ecliptic plane, and from behind in Earth’s orbit, of Earth and Moon at the moment of the quarter phase. The arrow represents Earth’s travel from the moment when it was nearest to where the Moon now is. The “ghost” Earths are the Earth 1, 2, and 3 hours ago.
Because the Moon descended through the ecliptic plane a few days ago, it is, at the quarter-phase moment, a few degrees, and a few thousand miles, sout of the point where the Earth was.
A view from above – from ecliptic north – would in a way show the Earth’s travel better, but would not show the difference in latitude.
I’ll work on this to compare other examples.
Here is the evening scene for an American location near to the first-quarter time.
The point we call the antapex of Earth’s way is the direction away from which we are moving at this point in our orbit around the Sun. You can see that the Moon passed south of it about 5 hours ago.
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