Mercury approaching [star] Graffias

Event Date: December 10th

Time: 6:30 AM

Brief

We will continue to look east for the next couple of mornings, as (1) the Moon makes a tight fit with Venus and Mercury, while the latter also approaches a star we are more familiar with on late spring and early summer nights: Graffias (Beta Scorpii).  This is the most northern star of the southern hemisphere scorpion's head, and the two are slightly over 4º apart this morning.  As we see them below, Mercury has reached magnitude -0.5, which is a magnitude that it will maintain over the next several days, as it reaches aphelion near the time of superior conjunction.

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

Detailed

   Keep an eye on Mercury as it gets close to the star over the next few mornings.  The planet's ecliptic latitude is currently 1º 46' (while slowly dipping) and therefore, will get close to the star.  On the contrary, the Sun in its path, never crosses this star, since our star's angular size is only about 1/4º on either side of the ecliptic.  The Moon occasionally will occult the star when its orbit allows, and we will see just that during much of 2013, for every time that our satellite comes towards this precise part of the sky.  With that in mind, some parts of the world, specially the far-east, will see a (near-)occultation.

Let's get back to Mercury's apparent magnitude, which I glazed over at the end of the brief: I talked about its quick brightening when it was emerging from the Sun just past perihelion.  As it reaches aphelion, it moves far enough away from Earth, that its angular size changing nearly equals that of its rate of waxing.  This isn't quite the same concept as that as very bright Venus, which reflects much more Sunlight than Mercury, while much bigger than Mercury as well.  However as a result, we see Mercury at almost the exact same magnitude for part of this month.

   We will talk more about Mercury when it starts an excellent evening apparition early next year.  Until then, enjoy it while we still have it west of the Sun, as a winter-holiday target in the morning.