two summer triangles: one rising, one setting

Event Date: August 15th
Time: 5:30 AM


Brief

   Earlier this month, I kiddingly mentioned a "second summer triangle" rising in the eastern morning sky; that being, as our familiar one consisting of the stars Vega, Deneb and Altair, sets in the west-northwest.  Venus, Jupiter and Sirius make up the second one that I teased about, and not starting to slowly reach a more recognizable shape.
Here is a reminder of where both triangle shapes can be seen shortly before Sunset.  Altair is on its way down below the horizon before the Sun comes up, and we will start to see the entire triangle for less minutes each day between now and when we finally lose Altair to the horizon late this year before darkness.  First the real, then the Venus/Jupiter/Sirius (V/J/S) one, shown below.

click on images to enlarge: courtesy of Starry Night Pro Plus, version 6.4.3, by Simulation Curriculum Corp.

Detailed

Unlike the Vega/Deneb/Altair (V/D/A) trio, the V/J/S triangle has its stars change configuration slightly each morning.  Why?  Simple.  Just remember the definition of the word planet: wandering star.  Venus is picking up prograde speed against the stars each morning as it moves further from us in its orbit, while Jupiter is slowing down in prograde, approaching stationary in upcoming weeks.  Sirius is the only "fixed" star of course, and as for the other triangle, all three of those stay fixed, disregarding proper motion.  Of course, Altair also dips into heavy atmospheric pollution before the sky starts to get lighter, making it harder to see.  Since Venus and Jupiter rise earlier by about four minutes earlier each day, as does Sirius, we see V/J/S higher in the dawn sky each day, and V/D/A lower each day, setting earlier.   Although the software does not let me draw lines between the planets and Sirius, the shape can be seen even if I left out the labels.  As Venus separates itself further from Jupiter, the shape of V/J/S will soon resemble that of the V/D/A.  Both are seen with all three stars each above the horizon at the same time for a limited number of days between now and later in the year; eventually, Venus comes close enough to the Sun again before superior conjunction, that by the time it rises, Altair is already below the horizon.  By then also, Venus is separated enough from Jupiter and Sirius that the three don't even form a triangle anymore; instead, more of a large arc-shape that we do not see until Venus rises.