Over in the morning sky, Venus –
is brightest on September 20 at 21 Universal Time, and shows its greatest illuminated extent on Sep. 21 at 15 UT.
“Brightest” means that its magnitude reaches -4.5, and “illuminated extent” means the angular area of sunlit surface visible in a telescope. Numbers such as these are calculated from formulae that take into account things like the phase angle from the Sun, and they are what I call “soft” extremes: they are like the rounded summits of hills. The quantity isn’t changing much; there isn’t perceptible difference from day to day.
There is yet another quantity that could be calculated (I don’t) and could climax at yet another slightly different instant: the surface brightness, which means the intensity of light per area rather than the total light – this is more important for extended objects such as nebulae and galaxies and comets, which can be much less visible than their quoted magnitudes suggest because their light is spread palely over an area.
And there is Venus’s greatest westward elongation, its extreme angular distance outward from the Sun, which is traditionally the most quoted of all these measures of its observability, and which doesn’t come till October 26.
What you’ll like right now, though, is lady Venus presiding over the court of the morning. Down below her, their bejeweled costumes progressively obscured by veils of light as if from an open door, are her redfaced brother Mars, a Roman general named Regulus, and her father Jupiter.