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Northern Rockies Skies for April: The Zodiac in April

March 26, 2014

A monthly look at the night skies of the northern Rocky Mountains, written by astronomers Ron Canterna, University of Wyoming; Jay Norris, Challis, Idaho Observatory; and Daryl Macomb, Boise State University.

April night skies present an easy solution to locate some of the constellations of the Zodiac. The most prominent constellation at sunset is Orion, with its bright stars Betelgeuse and Rigel, its three-star belt and its south-leading sword.

Once you find Orion, look west and slightly north for Taurus, a V-shaped constellation with a bright orangish star, Aldebaran. Then look to the east and there will be the two prominent stars of Gemini, Castor and Pollux (Jupiter is nearby this month). You now have two of the 12 zodiacal constellations.

The next constellation of the Zodiac, Cancer (the crab), is not so obvious. Halfway between Gemini and Leo (the lion), it is shaped like a very faint “X,” with its bottom leg stretched out.

Leo is more prominent and, at sunset, is located to the east. Its obvious head, body and legs shape, together with its bright star, Regulus, makes it easy to spot.

Finally, there is Virgo. To locate Virgo you want to find Spica, its brightest star. Go to the Big Dipper and its handle, and follow the imaginary arc to Arcturus and then to Spica, a modestly bright bluish star. Don’t confuse it this month with Mars, which is close by and red. Now, you should be able to locate these five constellations of the Zodiac.

This month, you will see Mars on the east horizon at sunset with Saturn close by, rising at 11 p.m. Venus is the morning star this month. Finally, there is a total lunar eclipse on income tax day, April 15.

Cosmology Problems

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The Big Bang theory addresses some fundamental, well-observed aspects of the expanding universe. These include the present rate of expansion and estimated age of the universe, about 13.8 billion years; formation and abundances of the lightest three elements (hydrogen, helium and lithium); and presence of the cosmic microwave background (CMB) radiation.

The latter is the relic signature of the "recombination era" when hydrogen nuclei (essentially, free protons) and electrons combined to form neutral atoms due to the universe's expansion and cooling. During the preceding hotter phase, lasting about 375,000 years, the universe was ionized -- in a plasma state.

Observations and inferences concerning the Big Bang are consistent with a "cosmological principle," namely that, on the very largest scales, the universe is roughly homogeneous and isotropic. That is, its physical characteristics averaged over very large volumes -- sizes larger than clusters of galaxies -- appear to be the same everywhere and in all directions.

To a high degree of accuracy, the laws of physics appear the same on Earth and for the most distant galaxies so far detected. Thus, the Earth and our Milky Way galaxy are not in any way special, nor in a special location. Uniformity of the CMB radiation, with a temperature of 2.7 degrees above absolute zero in all directions, also is in accord with the principle.

Notwithstanding, the consistency between observations and the original Big Bang theory, astrophysicists and particle physicists were led to deeper questions concerning causality and origins of fundamental particles. In particular, causality questions arise exactly because of the observed uniformity of the universe -- regions of space that were in the past out of communication with each other, due to the finite speed of light, nevertheless seem to have influenced their mutual conditions. This made the average density and temperature of matter in these separated regions nearly equal.

Questions about the generation of different particle species in the earliest fractions of a second of the Big Bang are at the heart of current particle physics research.

Augmented theories of the Big Bang, generically called "inflation," address some of these fundamental physics issues. Recent results from an experiment at the South Pole offer the first observational evidence that appears to support inflation -- the theory that the universe sprang from phase transitions (like the water phase turning into the steam phase when heated). Inflation would give rise to a universe much larger than our "observational horizon,” the limit to which we could see with a perfect telescope -- to be discussed next month.

To view this month’s sky chart, click here.

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