Summer Triangle: high near midnight

Event Date: July 25th
Time: 12:37(.33) AM


Brief

   The Summer Triangle's transit time is nearing midnight for our longitude.  It will transit closest to the exact minute August 2nd and 3rd, about two minutes later and earlier respectively.  Of course, for a few hours before and after transit during the night, we can still enjoy seeing the three stars very high up.  During the two hours sandwiching transit, Vega and Deneb don't seem to change altitude, since they are far enough north on our celestial dome.  Take a look at the alt/az and celestial grid (grey) overlap with the Triangle in the image below, to see what I are referring to.

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

Detailed

Vega (upper right), which transits a little over an hour before the center of the triangle transits.  I talked about the center and how to find it earlier in the season; review here if you forget.  For now, look at Vega's position with the celestial gridline just below it (red-south): the space between the meridian and where Vega is shows that there is very little change in altitude.  As seen from our mid-northern latitude, Vega only changes in altitude by 12º between transit and the time above.  This may not seem like a small amount for an hour elapsed, yet remember that Vega transits less than a degree from zenith, and has several hours to set after transit.  The rate of altitude change varies as it circles the north axis.  Deneb, which transits a little north of zenith and further from it than Vega, spends an even longer time setting after transit.  Altair spends the least time above the horizon, although still a decent amount of time, being about 9º in northen declination.