Luna offers a Daytime Astronomy opportunity today for much of the western hemisphere, with the eastern U.S. being favored for the best views.

At about 20:30 UT, the waxing first-quarter Moon will occult the first magnitude star, Antares; and with a little optical aid, you can witness this daytime event!

Antares is constellation Scorpius‘ brightest star.  It is a class-M super-giant located some six-hundred light-years away, towards the center of our galaxy.  During summer nights, Antares is distinct as a bright reddish-hued star on your south horizon.  While this star is typically only visible after sunset, today’s occultation will assist observers in locating Antares in a sunlit sky.

At the time of today’s occultation, Luna will be 54% illuminated.  In other words, that side facing us will appear a little more than half-full; and because it is a waxing moon, that visible portion appears as the right-half of the Moon.  With a waning, or third quarter, Moon, it is the left-half that we see.

Now, here’s something to think about.  Knowing that the celestial field appears to move from east to west, and that the right-side of the Moon will be illuminated, one might assume that it will be that illuminated portion that first appears to move in front of Antares.  Some might even assume that because the Moon currently rises before Antares, that an occultation isn’t even possible.  However, the relative distance and motion of the Moon, versus that of the background starfield, creates just the opposite effect.

Compared against the movement of the stars, the Moon appears to move more slowly across the sky.  So, rather than the Moon appearing to pass in front of Antares, Antares will appear to slip behind the Moon; thus it will be that darker portion of the Moon that first occults Antares.  To think of it, with the Moon rising ahead of Antares, and its darker half being the first to “touch” Antares, it’s almost as though the Moon will move backwards against the background stars.

Of course, your location will determine exactly what you see and when you will see it.  You can use this table, from the International Occultation Timing Association to plan your observing session.  U.S. locations begin at #401, about one-third of the way down the page.  The times are noted in UTC, so be sure to convert those to your local time zone.  You can also use a desktop planetarium, like Stellarium, to plan your session.

Using these tools, I can see that I should begin my observing at about 5:00 pm local time, when the Moon is about 25 degrees above my SSE horizon.  So, to plan, I’ll find a location with a clearer horizon (yard trees typically obscure up to about 45 degrees); and I’ll be equipped with a telescope, two pairs of binoculars (large and small), and my digital camera with a telephoto lens.

If you, too, plan to take advantage of this daytime astronomy opportunity, we’d love to hear about it!  Just leave a comment with your observation notes or image links.  Even better, consider joining the Daytime Astronomy Group and sharing your observations there!