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The Sky This Week, 2016 May 24 - 31

Wisps in the night.
Markarian's Chain, central region of the Virgo Galaxy Cluster,
imaged with an 80mm (3.1-inch) f/6 refractor telescope
on 2013 April 14 from Vaucluse, Virginia

The end of May finds the Moon waning in the morning sky, spending the early morning hours among the faint stars of the rising autumnal constellations. Last Quarter occurs on the 29th at 8:12 am Eastern Daylight Time. Luna has little competition for your attention this week, passing no stars that are brighter than third magnitude.

With the seemingly endless rain and cloudiness we’ve been experiencing here in Washington lately, it may surprise you to see the changes that have taken place in the sky over the past few weeks. First there is the time of sunset, which now occurs at around 8:30 pm. From a dark location, this means that astronomical twilight now ends after 10:00 pm, so observing the sky under full darkness is now a late-night proposition. That said, if you own a modest telescope, you can look well past the boundaries of our Milky Way galaxy and peer into vast reaches of deep space. This time of the year is the best to observe an area of the sky that was known to classical astronomers of the 19th Century as the "Realm of the Nebulae". The area of the sky bounded by the bright stars Regulus, Spica, Arcturus, and the asterism of the Big Dipper hosts hundreds of faint smudges of light which were diligently cataloged and described by many observers, but it wasn’t until the early 20th Century that the true nature of these objects was discovered. The early observers speculated that these wispy patches of light were nascent solar systems condensing out of gaseous clouds, but Edwin Hubble changed all of that with his pioneering work with the 100-inch Hooker Telescope on Mount Wilson as World War I was winding down. The combination of the immense light-gathering power of this then-new telescope and Hubble’s diligence in making long-exposure photographs of these "nebulae" revealed them to be distant external galaxies, analogs of our home galaxy, scattered among the springtime sky. There are thousands of these external galaxies concentrated in this part of the sky, and many of them are visible to modest instruments under dark skies. These faint wisps are members of a vast group of galaxies known as the Virgo Cluster, since the greatest concentration is in that constellation. The average distance to these galaxies is between 50 and 60 million light-years, but their influence stretches over these vast distances and affects our Milky Way. Yes, our galaxy is a far-flung member of this galaxy cluster, slowly orbiting around several massive elliptical systems that form the cluster’s core. Sweep the sky with your telescope on the next clear dark evening and you should see several of our far-flung neighbors.

By 10:00 pm Jupiter is west of the meridian, but the giant planet still offers a few hours of decent observing time before he settle toward the western horizon. You can start observing him as soon as he pops out of twilight, which should gain you an hour of "extra" time before full darkness. He is still the best planet to look at through a small telescope, and his four bright moons provide a different view every night. On the evening of the 26th you can watch Europa and Ganymede pass each other between 9:30 and 10:30 pm EDT while the Great Red Spot rotates off the disc. On the 28th the Red Spot crosses the planet’s central meridian at 10:00 pm.

Mars approaches the meridian, low in the southern part of the sky, as midnight approaches. The red planet reached opposition last week, and this week he makes his closest approach to Earth on May 30th. At his time he’ll be a tad under 75 million kilometers (46.6 million miles) from our fair planet’s shores. This is the closest Mars has been since 2005, and even though his apparent disc is about half that of Jupiter’s, he’s still worth taking a look at. You should be able to see some of his dark surface features and white polar caps. You might even see faint white clouds that form around the summit of the solar system’s largest volcano, Olympus Mons!

Saturn tags along behind Mars, crossing the meridian around an hour after the red planet. Saturn will reach opposition next week, and despite his low altitude is well worth a look before turning in for the night. Saturn’s rings are tipped at close to their widest presentation toward Earth, so even in a small telescope the planet looks like a baleful eye staring across the immense gulf of space toward us.

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