October 14, 2019

AstroEvent: See Mercury at its 2011 Morning Best.

Never seen the planet Mercury before? This coming week offers a good time to try, as the inner-most world undergoes its best morning apparition for northern hemisphere viewers. The tiny world reaches a greatest elongation of 18.1° degrees west of the Sun on September 3rd at a brightness ranging from about -1 magnitude to 0.0 on the dime. What’s ironic is that Mercury is very near perihelion, and thus is offering us the worst elongation that can occur, which would be 17.9° degrees! What helps us northern hemisphere viewers out is the relatively high angle of the morning ecliptic as we approach the autumnal equinox, shooting Mercury “out of the weeds” about a half an hour to an hour before local sunrise. Keep an eye out and watch on successive mornings as Mercury approaches within a degree of +1.4 magnitude star Alpha Leonis (Regulus) on September 9th before vanishing in the dawn twilight by mid-month. In a telescope, Mercury will show a tiny “half-moon” phase 7.2” arc seconds in size headed towards Full as it reaches superior conjunction with the Sun on September 28th… what will be the last date that you can spy this fleeting world?

Mercury in the dawn through mid-September. (Created by Author in Starry Night).

Another unique event transpires on the evening on September 3rd, when the 42% illuminated waxing crescent Moon occults the bright +2.3 magnitude star Delta Scorpii. Lying very close to the ecliptic, this star is frequently occulted by the Moon and is undergoing a series of lunar occultations throughout  2011. However, this week’s offering  is the best of the year for US East Coast viewers. The occultation is centered around 01:36 UT on September 4th, and a fine graze line runs from West Texas to Pennsylvania. If the name Delta Scorpii sounds familiar, that’s because this star, also known as Dschubba, also underwent an unexpected outburst raising its brightness by a full magnitude in 2002, markedly changing the familiar appearance of the constellation of the Scorpion.

The Astronomy Word of the Week is the Scorpius-Centaurus Association. This is the largest grouping of OB-class stars near our solar system which extends through the constellations Scorpius, Centaurus, Lupus, and the Crux. Approximately 380 to 470 light years distant, we know these hot young stars are all related because of their convergent proper motion as we all rotate about our Milky Way galaxy. Famous alumni of the Scorpius-Centaurus Association include Antares, (itself a class M supergiant) the stars that form the Southern Cross, and yes, our friend Dschubba. Total membership in the Scorpius Centaurus Association is estimated at about 2,000 stars. The stars are about 5-20 million years old (re: young) and the entire association appears to be part of a very recent wave of stellar formation in our galactic neck of the woods!

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