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The Sky This Week, 2013 April 23 - 30

Travels in the springtime sky, and Saturn reaches opposition.
SatOppositionEffectsmall.jpg

Saturn showing the Seeliger Opposition Effect, 2006


The Moon slips into the morning sky this week, diving southward along the ecliptic as she goes. Full Moon occurs on the 25th at 3:57 pm Eastern Daylight Time. April’s Full Moon is popularly known as the Grass Moon, Fish Moon, Growing Moon, or Pink Moon among various cultures. This month’s Full Moon is characterized by a very small partial lunar eclipse which will be visible throughout Europe, Asia, Africa, and Australia. At its maximum just over one percent of the Moon’s disc will be covered by Earth’s umbral shadow. This is the final umbral eclipse in saros number 112, which began in the year 859 and will end in the year 2139. Those of us here in North America can watch the Full Moon rise about four degrees away from golden-hued Saturn on the evening of the 25th. On the previous evening Luna’s bright disc may be found just one degree south of the bright star Spica.

Bright moonlight all but washes out the fainter constellations of spring, but there are still a few starry sights to capture your gaze as we wait for darker nights to return. This is now the prime time of the year to spot the Big Dipper high in the northern sky during the late evening and early morning hours. The seven stars that make up this distinctive asterism are bright enough to be seen from urban skies and make a convenient guide to other interesting objects. Five of the seven stars form a physical association known as the Ursa Major Moving Group. Fainter stars in the constellation bring the group’s core population to about 14 members, and another two dozen stars scattered around the northern sky also seem to share the same characteristics of motion and composition. The two stars at the end of the Dipper’s "Bowl" point the way to Polaris, the North Star, while the curve generated by the stars in the Dipper’s "handle" invite you to "follow the arc to Arcturus". This star is the fourth-brightest star in the sky and the brightest in the northern hemisphere. Its rosy tint tells us that it is an evolved star with a relatively cool red surface. It is one of the closer stars to Earth just under 37 light-years distant. Light from Arcturus struck a photo-sensitive tube which triggered the ribbon-cutting mechanism to open the 1933 Chicago World’s Fair. At the time the star was thought to be about 40 light-years away, which was the elapsed time since the 1893 World Columbian Exposition that was also staged in the Windy City. If we continue the "arc" from the Big Dipper past Arcturus, we can "speed on to Spica", another first-magnitude star of the springtime sky. Spica offers a nice color contrast to Arcturus, beaming with an icy-blue tint. Spica receives a close visit by the Moon on the night of the 24th.

Bright Jupiter is still visible in the early evening sky, but his altitude above the horizon is being whittled away with each passing night. The giant planet now sets at around 11:00 pm, so getting a good look at him with a telescope is becoming problematic. Low-power views will still show his four bright moons, but higher-power examination of his turbulent cloud belts will be thwarted by the turbulence in Earth’s atmosphere. The best time to look is just after sunset when the planet pops out in the gathering twilight sky.

Saturn is ready to take Jupiter’s place, although his position in the sky isn’t as favorable as it was for Old Jove. The ringed planet reaches opposition on the morning of the 28th when the Earth passes between Saturn and the Sun. This is a very interesting time to observe the planet with a telescope. For about a day on either side of opposition you’ll notice that the planet’s rings seem anomalously brighter than they do at other times. The smaller Saturnian moons also seem a bit easier to see, gaining perhaps half a magnitude in brightness. This effect is known as the "Seeliger Opposition Effect" and is caused by the lack of shadows on the rough surfaces of the moons and rings as seen from our vantage point around the time of opposition. This is why the Moon appears so bright at the time of Full Moon and why we sometimes see a "halo" around our heads when we look at our shadows on a sandy beach. The effect on Saturn is particularly noticeable when the rings are tilted at a wide angle toward us, as they are this year. See if you can spot it for yourself this week!

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