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The Sky This Week, 2014 April 29 - May 6

Don't overlook the Moon!
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The Moon Before First Quarter

The Moon returns to the evening sky this week, quickly passing through the setting stars of the Great Winter Circle before moving into springtime’s constellations.  First Quarter occurs on May 6th at 11:15 pm Eastern Daylight Time.  She shares the evening spotlight with Jupiter on the evenings of the 3rd and 4th.    

This is another great week to get to know our nearest celestial neighbor.  She is very well-placed for telescopic examination of her crescent phases, and you can watch the line between her daylight and night hemispheres, the terminator, slowly advance across her battered landscape.  During the first few days of each lunation her visage is dominated by a number of the vast lava plains first identified by early observers as “seas”.  These great expanses of relatively smooth terrain are pock-marked with many small to medium-sized craters from impacting meteoroids.  Although they look small in the telescope, typical bowl-shaped craters in these plains are tens of miles across, dwarfing the famous Meteor Crater that’s not far from our Flagstaff, Arizona observing station.  When the sun angle is low you can also see the meandering, frozen ridges of solidified lava known as “wrinkle ridges”, which look like static waves in some frozen arctic ocean.  By the time the First Quarter phase rolls around, the terminator reveals the much older and battered terrain of the lunar highlands.  Here the craters are generally much larger and more densely packed together, mute testimony to the ferocious assault of infalling planetessimals that formed the proto-planets some four billion years ago.  Almost all of these features have names, and if you spend a few nights at the eyepiece with a good lunar atlas you will get to know them as landmarks for subsequent telescopic tours.  Even though the same features are revealed month after month as Luna goes through her phases, observing these familiar places is always a bit different from night to night as subtle changes in the lighting highlights hidden detail that you may not have seen before.  It is often said that the Moon is “looked over, then overlooked” by new telescope owners, but she is still the only other world in our solar system that we can see in great detail.  She always gets a number of looks from my telescope each passing month.

Looking at the Moon may set you up for some disappointment when you turn your attention to the bright planets that inhabit the evening sky.  Jupiter is the next logical target since you’ll want to catch him before he gets too low for detailed viewing.  Jupiter’s apparent disc is about the size of an average lunar crater, but if you spend a few minutes and wait for moments of steady air you can begin to detect some features on his cream-colored face.  Almost any telescope will show his light brown equatorial cloud belts, and if you have a scope that’s four inches or more in aperture you should be able to spot some structure in these bands.  Any telescope will show the planet’s four bright Galilean moons, and a good six-inch scope will resolve their tiny discs.  For a sense of scale, the two innermost of these worlds, Io and Europa, are comparable to our fair Luna in size.

You’ll now find Mars in the southeastern sky as evening twilight fades.  The red planet is quite conspicuous due to his brightness and distinctive tint, which contrasts nicely with the bright star Spica some 10 degrees away.  Mars’ apparent disc is less than half the size of Jupiter’s, but a steady four-inch telescope should reveal some details on that disc.  Early this week the planet’s most prominent albedo feature, the Syrtis Major, is well placed for viewing between 9:00 pm and midnight.  This feature looks a bit like the Indian subcontinent against the pink backdrop of martian dessert.  You will also probably see a large whitish area bordering the Syrtis that looks like a polar cap.  In reality this is a huge impact basin called Hellas, and the white stuff is formed by clouds trapped within the basin’s walls.  The actual polar cap is on the limb opposite the “pointy” end of Syrtis Major and marks the planet’s north pole. 

Saturn becomes visible in the southeast by the end of evening twilight.  This far-flung world is located in the obscure constellation of Libra, the scales and outshines all of that constellation's bright stars.  Saturn will reach opposition on May 10th, so we’re now right in the middle of its prime observing season.  If you get eyestrain trying to eke out detail on Mars, swing the scope over to Saturn before retiring.  You’ll be treated to one of the most exotic sights in all of Nature, the gaseous orb of the planet spinning inside its icy rings. 

Early risers can take in the view of Venus in the southeastern sky as morning twilight brightens the horizon.  The planet is steadily marching through the rising constellations of autumn, which are quickly overwhelmed by the first faint rays of dawn.  Venus, however, will blaze away until the Sun comes up, and if you know just where to look you can follow her in broad daylight for a time.

Next week the planet Mercury becomes visible in the evening sky, so we’ll have a chance to glimpse all of the planets known to the ancients.

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