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The Sky This Week, 2016 April 19 - 26

A bright and speedy star.
Jupiter, with Europa and its shadow, 2016 April 16
Imaged with USNO's 12-inch Clark/Saegmüller refractor telescope

The Moon brightens the evening sky this week, passing through the springtime constellations before diving to the southern reaches of the ecliptic and the rising stars of summer. Full Moon occurs on the 22nd at 1:24 am Eastern Daylight Time. April’s Full Moon is popularly known as the Pink Moon, Grass Moon, Seed Moon, and Egg Moon. I’d like to propose "Yellow Moon" as an appropriate name based on the pollen that changes everything to this springtime color! Look for the Moon about five degrees north of the bright star Spica in the late evening of the 20th. On the morning of the 25th the waning gibbous Moon helps provide a great photo opportunity as she poses with the planets Mars and Saturn and the bright star Antares.

As the weather warms and the nights grow shorter, the evening sky is in the transition period between the bright stars of winter and those of the summer sky. The spring constellations lack the multiple bright stars characteristic of the winter sky, which you can still see for a short while in the west as evening twilight ends. Summer’s bright stars will rise at around midnight. The common thread between summer and winter is the presence of the Milky Way. The brighter stars tend to concentrate along this luminous band, and in the spring we are looking directly away from it. In fact, the North Galactic Pole is located almost directly overhead at local midnight, which means that when we look into space in this direction we’re looking through the thinnest veil of "foreground" stars toward the depths of intergalactic space. While bright stars are few and far between during these evenings there is one notable exception in the form of a rose-tinted luminary now climbing in the east. This is the star Arcturus, brightest star in the Northern Hemisphere sky and fourth-brightest in the heavens. Arcturus is an evolved star that belongs to a vast halo of older stars that orbit through the plane of the Milky Way. Thanks to its relatively close distance of 37 light years and its motion relative to the Sun it has the highest "proper motion" of all of the stars visible to the naked eye. This motion was first detected by Edmond Halley in 1718, and the star will move about the apparent diameter of the Moon over the next 1000 years.

You still have an opportunity to track down elusive Mercury this week before he begins his plunge toward the Sun. The recent spate of clear weather has allowed me to glimpse the innermost planet each night for the past week as a solitary star-like glimmer some 10 degrees above the horizon just to the north of due west. Mercury will hover in this position for most of the week, but he will gradually fade by a bit over a magnitude by the week’s end. He will make a precipitous dive toward the Sun as April ends, and on May 9th we’ll get a chance to see his tiny disc cross the face of the Sun in a rare transit event.

Jupiter is perfectly placed for the evening skywatcher, riding high in the south and crossing the meridian at around 10:00 pm. The giant planet continues to be the showpiece object for small telescopes, whether you enjoy watching the comings and goings of his four bright Galilean moons or tracking the ever-changing features in Old Jove’s turbulent cloud tops. You can watch the famous Great Red Spot rotate across Jupiter’s disc on the evenings of the 20th and 22nd. On the latter night the innermost moon, Io, crosses the planet’s face, dragging its inky shadow behind it.

Mars and Saturn can now be glimpsed low in the southeastern sky as midnight approaches, but the planets are best placed in the sky at around 4:00 am. Mars is now a month away from opposition and is rapidly brightening, far outshining the nearby star Antares. Saturn is also brighter than the star but is a full magnitude fainter than the red planet. Both of the planets form an attractive triangle with Antares, and on the morning of the 25th the waning Moon pays a courtesy call.

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