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The Sky This Week, 2014 July 8 - 15

A tale of two red stars.
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Conjunction of the Moon and Mars, 2014 July 6, 02:19 UT

The Moon brightens the evening sky this week, passing through the full phase as she crosses the heart of the summer Milky Way.  Full Moon occurs on the 12th at 7:25 am Eastern Daylight Time.  July’s Full Moon is popularly known as the Thunder Moon, but it is also known as the Buck Moon, Hay Moon, or Hot Sun Moon.  Look for the Moon about two degrees northwest of the star Graffias, the northernmost of the three stars that form the head of Scorpius, the Scorpion on the evening of the 8th .  On the 9th she stands just under 10 degrees northeast of the bright star Antares.  She ends the week drifting eastward among the faint starfields of the rising autumnal constellations.

The bright light of the Moon keeps us from enjoying the spectacle of the summer Milky Way, but the season’s bright stars still offer some interesting sights and stories.  Last week we discussed Antares, the bright reddish star in the heart of Scorpius.  There are some very interesting parallels between this star and its counterpart, Betelgeuse, in the winter constellation of Orion.  Both stars are highly-evolved “red supergiant” stars, far along on their evolutionary tracks.  As the hydrogen in their cores is exhausted, it begins to fuse into helium in an ever-expanding shell around the core.  This forces the stars’ girths to expand dramatically.  If either star occupied the Sun’s position in our solar system the Earth would be located inside the tenuous outer layers of these swollen monsters.  As their size increases their surface areas grow exponentially and become much cooler, giving them their characteristic reddish hue.  The other stars in their respective constellations are hot young “blue giant” stars that recently formed from huge clouds of interstellar dust and gas; the three stars that form the “head” of the Scorpion are thus very similar to the three stars that form the “belt” of the Hunter.  Despite the fact that the constellations are opposite each other in the sky, they are also linked by sky lore.  The boastful mortal Orion claimed domination over all animals on the Earth, a claim also made by the goddess Diana.  To teach Orion a lesson, Diana sent a scorpion to remind the Hunter of his mortal status.  The scorpion stung Orion on the heel with what proved to be a fatal wound just before Orion dispatched it.  Diana, secretly in love with Orion, placed both combatants in the sky but made sure they were opposite each other.  Indeed, here in Washington, you’ll find that Betelgeuse sets about 20 minutes after Antares rises.  The one-time adversaries now perpetually chase each other around the sky.

Mars is now steadily pressing eastward along the ecliptic, and you can get an idea of how fast he’s moving by watching him overtake and pass the bright star Spics.  As the week opens the red planet is just under three degrees from the star, and he closes the gap to just a bit over one degree on the night of the 13th.  By the end of the week he’ll be northeast of the star and setting his sights on distant Saturn.  While not quite as eye-catching as last week’s close lunar conjunction, this should be a fun event to observe.

Saturn is still putting on a good show in the evening sky, but he’s now well west of the meridian as darkness finally falls.  His slow retrograde motion is now grinding to a halt, and he’ll become stationary on the 21st.  For the next several weeks he will stay just over two degrees northeast of the star Zubenelgenubi before gathering eastward momentum in the late summer sky.  The ringed planet is now the sole telescopic showpiece planet in the sky, so be sure to catch a glimpse of him while you can.  I spent the recent holiday weekend with my little three-inch travel scope, and even at a paltry 26 power the planet’s rings were easily visible.

Bright Venus continues to drift eastward among the rising stars of winter as she keeps pace with the Sun.  This week she passes between the horns of Taurus, the Bull, but you’ll be hard-pressed to see this against the brightening sky of morning twilight.  Venus herself will stand out like a beacon.

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