Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2015 December 15
Colorful Arcs over Buenos Aires
Image Credit & Copyright: Sergio Montúfar (Planetario Ciudad de La Plata, pna)
Explanation: What are those colorful arcs in the sky? Like rainbows that are caused by rain, arcs of sunlight broken up into component colors can also result when ice crystals floating in Earth’s atmosphere act together as a gigantic prism. The top color arc is more typical as it is part of the 22 degree halo surrounding the Sun when hexagonal ice crystals refract sunlight between two of the six sides. More unusual, though, is the bottom color arc. Sometimes called a fire rainbow, this circumhorizon arc is also created by ice, not fire nor even rain. Here, a series of horizontal, thin, flat ice crystals in high cirrus clouds refract sunlight between the top and bottom faces toward the observer. These arcs only occur when the Sun is higher than 58 degrees above the horizon. The featured sky occurred to the northwest in the early afternoon last month over a street Diagonal of La Plata City, Buenos Aires, Argentina.
Tonight, look for the Northern Cross. It isn’t as famous as its counterpart – the Southern Cross – visible from the Southern Hemisphere or the northern tropics. But the Northern Cross also looks like a cross, and it’s pretty easy to spot. It’s a large, noticeable star pattern.
The star Deneb marks the top of the Northern Cross, and the star Albireo marks the bottom. Tonight you can find the Northern Cross shining fairly high in the west at nightfall, as seen from mid-northern latitudes. It sinks downward during the evening hours, and stands proudly over the west-northwest horizon around mid-evening.
The Northern Cross is what’s known as an asterism. In other words, it’s not a constellation but simply a noticeable pattern of stars. It’s part of the constellation Cygnus the Swan.
As an added bonus, if you have a pair of binoculars, break them out this evening and point them toward the Northern Cross and its larger constellation Cygnus the Swan. In this direction, you’ll find a part of our Milky Way galaxy that is called the Cygnus Star Cloud. It is part of the spiral arm of our galaxy that also contains our sun, and you should be able to pick out stars from it if the night is clear in your area.
By the way, we get many questions from people in northern latitudes about if and when they can view the Southern Cross in their portion of the sky. The truth is that unless you live close to the tropics (Hawaii, or the southernmost parts of Texas or Florida for those of us in the U.S.), you will not be able to view the Southern Cross, also known as the constellation Crux. To find out how to locate Crux in Hawaii at this time of year, look here.
Bottom line: Learn to recognize a famous star pattern known as the Northern Cross, which stands upright over the west-northwest horizon in December.
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About the author:
Deborah Byrd created the EarthSky radio series in 1991 and founded the website EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website and blogs frequently about astrophysics, the night sky and other topics related to Earth, space and the human world. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. “Being an EarthSky editor is like hosting a big global party for cool nature-lovers,” she says.
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