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.
We’ve been having clear weather in San Francisco for the past few days, and I have a clear view to the east outside the back door of my flat, so I’ve been able to enjoy this stately planetary pavane. Mars is approaching Regulus noticeably from morning to morning and Jupiter is rising a bit earlier each morning. I would add that the real star of the show, the Sun, is rising farther south each morning as our own Earth approaches equinox. And, for the astrologically inclined, Mercury is making mischief in the wings.
I was amazed at how quickly Venus swings past through inferior conjunction to where it is now, and so bright that even two days ago I spotted it at 1PM EDT while I was standing next to to Cleopatra’s Needle in Central Park, NYC, even with nothing like the moon to be near it to use as a guidepost.
Sorry I’ve been a bit remiss on comments,,, but I’m making up for it now.