Arcturus and Vega at same altitude

Event Date: March 15th

Time: 5:43(.17) AM

Brief

   I mentioned Vega yesterday with a reminder of the Summer Triangle.  At magnitude 0.0, it is the 5th brightest in apparent magnitude.  Although the 1st through 3rd brightest are all below the horizon, being Sirius, Canopus and Alpha Centauri respectively, the fourth brightest is well above the horizon, being Arcturus. 

UPDATE: these latter two have been flipped in brightness order, according to the Wikipedia list, and Arcturus being a variable star.

  Although Arcturus' and Vega's nearly identical magnitudes make them seem "twin-like, they are very different in another way: Arcturus is a an old red-orange giant, while Vega is a much hotter blue-white star.

   The image below shows the alt-az grid, and both stars at nearly the same altitude.  They are just slightly over 60º 4' above the horizon. 

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

Detailed

Although it requires a close look, the software is set up to show each star's color when zoomed out.  As mentioned yesterday, Vega is 38º north, while Arcturus about half that declination.  As a result of that, we see Arcturus less time above the horizon.  For southern hemisphere observers, those in locations where the stars clear the horizon, would see Arcturus longer than Vega during certain times of the year.  Every day, about four minutes earlier, these two stars reach the same altitude.  The only thing that also factors into this is their proper motion, which means a change in about one arc-second every six-seven weeks (declination) and about a 10th of a second (s) in r.a. every three weeks.  Despite that, we will see them at the same altitudes for the rest of our lives, and they will continue to for years to follow.