![[April, Looking North]](4-01-north.gif) |
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241  APRIL 8, 2001:
East--Ursa Major Moving Cluster
Looking east at about 10:00 p.m. your eyes gaze on one of the blandest regions of the night sky. However, this area, if not visually stimulating from an urban locale, holds a bonanza of objects, mostly galaxies, for professional astronomers to study. Here, we can see farther into space because we are looking away from the Milky Way’s galactic plane into a region where there is less dust. Well-studied clusters of galaxies are found in Ursa Major (Big Dipper), Coma Berenices, and Virgo, all in the east and above the horizon by 10 p.m. There is another interesting cluster which is currently visible over a large portion of the sky. It’s called the Ursa Major Moving Cluster, and it is a group of approximately 100+ stars which were all born about the same time. The Pleiades, currently in the NW, are probably the best example of this type of star clustering. You might guess that the stars of the Dipper (Ursa Major) are involved. Only the two ends of the Dipper are not part of the cluster. Also included are Sirius, the brightest star of the night, now low in the southwest, Beta Aurigae or Menkalinan (see April’s western map at the web address below), and possibly Alphecca, the brightest star of Corona Borealis, the Northern Crown. To find Alphecca, follow the arc of the Dipper’s three handle stars to orangey Arcturus, of Bootes, the Bear Driver. Alphecca will be below and to Arcturus’ left, appearing about as bright as the Dipper stars. With the cluster’s center near the faintest star of the Dipper, the main area of the Ursa Major Cluster is currently well-placed for viewing. Our sun is located at the edge of this cluster which is approaching us at about 6 miles per second and is centered about 75 light years from Earth.
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242 APRIL 15, 2001:
Southeast--Lizard Lady
There are four women which grace the night: Cassiopeia, the Queen of Ethiopia; Andromeda, the Chained Lady and daughter of Cassiopeia; Coma Berenices, the Hair of Bernice; and Virgo, the Virgin, or the Lizard Lady, as I like to call her. Her lizard head seems to be poking out from a sleeping bag that she is nestled within. Particularly well-placed at present are Coma Berenices and Virgo in the southeast around 10 p.m., but their stars are all dim, except for Virgo’s blue-white Spica, which is supposed to represents a sheaf of wheat which she holds in her hand. To find Spica, follow the arc created by the handle of the Big Dipper to orange Arcturus, of Bootes, the Bear Driver. Then continue southward to spike Spica. Spica, the 15th brightest star of the night, is right in the middle of Virgo’s sleeping bag pattern. Even with its entourage of faint stars, Virgo was important to the ancients because it was one of the 12 zodiacal signs that the sun passed through during the course of a year. Mythologies about Virgo, however, are scarce. She is best associated with Ceres, the goddess of agriculture. When her daughter Persephone was abducted by Pluto to live in Hades and become his queen, Ceres forbid that anything should grow on the Earth. With starvation facing all of humankind, Ceres enlisted Jupiter’s aid to get her daughter back, and a compromise was bargained with Pluto. Persephone would live with Ceres for half of the year and with Pluto for the other half. You might have already guessed that it is in the spring and summer, when food is growing and bountiful, that Persephone resides with her mother. Download a map for today’s StarWatch at the ASD Planetarium’s web address found below.
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243 APRIL 22, 2001:
West--The Majors and Minors
The winter constellations in the west are almost gone. Along with them, Jupiter and Saturn are setting earlier each evening too. The Big Dipper, and Leo, the Lion, along with lonely stars like red Arcturus of Bootes, and blue Spica of Virgo, are now dominating our spring nights. The sky has gone from the glorious to the subdued, but there are still some interesting footnotes to the heavenly pageant. Only in the Northern Hemisphere do we have the majors and the minors. Nearly setting in the west is Canis Minor, the smaller of the two hunting dogs of Orion. You can still see Sirius of Canis Major very low in the southwest right after dark. It will be gone in several weeks. Above it will be white Procyon, one half of the two stars which comprise Canis Minor. See this week’s StarWatch map posted at the ASD Planetarium’s web site. Near the top left of the map you’ll see the head of Leo, the Lion. To its right you’ll notice a triangle of three dim stars. That’s Leo Minor, the Lesser Lion. Of course, the most famous of the minors is Ursa Minor, the Little Bear, which has as its principal luminary the North Star. Although the pattern is unimpressive unless you are in an extremely dark sky, the North Star is bright enough to be seen from most metropolitan areas and has served as our Pole Star since the times of the Greeks and Romans. The sky repeats itself in another interesting way. There are three duplicate sets of constellations between the Northern and Southern Hemispheres. We have Corona Borealis, the Northern Crown, and Corona Australis, the Southern Crown; Pisces, the Fish in the northern sky, and Piscis Austrinus, the Southern Fish; and Triangulum, the Triangle, and Triangulum Australe, the Southern Triangle.
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244 APRIL 29, 2001:
Astronomical Refraction
It appears as if the amount of sunlight which we receive is in perfect balance. In the summer the sun is visible for a longer period of time each day, but during winter just the opposite is true. Overall, there appears to be symmetry between day and night. However, at our latitude of 40 degrees north, the sun is above the horizon for thousands of minutes longer than it is below the horizon because of a curious little effect called astronomical refraction. As the sun moves towards the horizon, it must pass through denser layers of atmosphere. This causes its rays to be bent upward ever so slightly, making the sun appear higher in the sky than it actually would be if the Earth did not possess an atmosphere. Astronomical refraction raises the positions of all objects in the sky except those which are directly overhead. If we see a perfect western horizon and observe the center of the solar disk on the horizon, astronomical refraction is elevating the sun by more than its full diameter. Without an atmosphere the sun would have set about 2 minutes, 58 seconds before its observed time of sunset. Keep in mind that at our latitude the sun also rises 2 minutes, 58 seconds ahead of schedule each day. That’s nearly six minutes of extra sunlight provided to us everyday by astronomical refraction. In a year’s time that adds up to over 2000 minutes of extra sun. We have all seen the effects of astronomical refraction in photographs of the sun which show it looking squished on the horizon. The portion of the sun closer to the horizon is "lifted" more than the upper limb, giving the sun its oval shape. See the author’s photo of how astronomical refraction affected the sunrise solar eclipse of August 11, 1999 at the web site below.
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![[April, Looking East]](4-01-east.gif)
![[April, Looking Southeast]](4-01-se.gif)
![[April, Looking West]](4-01-west.gif)
![[Astronomical Refraction]](99-eclipse1.jpg)
