Skip to Main Content

Apply Now to the University of Wyoming apply now

Northern Rockies Skies for December: The Perseus Constellation, the Greek Hero

November 22, 2013

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.

Located between Andromeda and Auriga, and directly overhead around 10 p.m. in the December night skies, lies the constellation Perseus.

Usually depicted holding the head of the Medusa, the Perseus constellation is not easily recognized. The brightest star, Mirfak or Algenib, is a yellowish-red supergiant about 500 light years from the sun. Algol, the second brightest star in Perseus, is a triple-star eclipsing system. It was quite pivotal in deciphering the observational evidence that stellar mass is what determines a star’s evolution.

Perseus is full of star clusters (M 34 and Mel 20) and nebulae (Dumbbell), so get out your binoculars or small telescope and peruse the environs of the Greek hero, Perseus.

Planets and Meteor Showers:  Venus is the evening star this month, on the western horizon right after sunset. Jupiter is in Gemini and rises around 7 p.m. It can be viewed most of the evening. Mars is in Virgo and rises around 2 a.m. Saturn rises in the east about three hours before sunrise. The Geminid meteor showers peak around Dec. 13 and 14, and are usually rich in numbers of meteors.

The winter solstice, the shortest day of the year, occurs Saturday, Dec. 21.

December 2013 Interest:  Famous Astronomers:  Isaac Newton II -- Gravitation and Mechanics

(Best URL:

Edmond Halley, the famous English astronomer who observed comets and wished to understand the motions of their periodic orbits, approached Sir Isaac Newton in 1684 and asked for insight into the problem. Halley's interest spurred Newton to finish the mathematics of celestial mechanics that he had undertaken 16 years earlier.

With Halley's financial backing, Newton then published the single greatest leap forward in physics. Newton's resulting trilogy, "Philosophiae Naturalis Principia Mathematica" (1687; "The Principia" for short), addressed virtually every aspect of mechanics, from planetary orbits to moving objects on the Earth.

Halley's immediate concern was solved by Newton's demonstration that the laws of planetary motion established by Kepler -- fundamentally, that planets move around the sun in elliptical orbits -- were a consequence of the universal law of gravity. Simply put, a planet and the sun mutually attract each other in proportion to their respective masses, and inversely proportionally to the square of their distance of separation.

Newton even understood that the sun would move very slightly in response to the less massive attraction of each planet. That is, the sun is not exactly at the center of mass of our solar system. Further, he showed that comets with sufficient kinetic energy are marginally bound, or not bound -- are just "visitors" to the solar system -- having either parabolic or hyperbolic orbits, respectively. Newton's gravitation also explained the details of the moon's orbit around the Earth, and the orbits of the moons of the other planets.

In conjunction with his celestial mechanics -- overthrowing the long-standing physics of Aristotle -- Newton formulated his three laws of motion, which still comprise the foundation of classical mechanics.

His first, the "law of inertia," was that objects at rest, or in motion, tend to preserve their motion unless acted upon by a force. The second law states that force is the product of an object's mass times the acceleration imparted by the force. His third law expresses that every action has an equal and opposite reaction, as demonstrated when one dives off an unmoored boat in a lake.

Newton recognized that the law of gravitation was an incomplete statement of nature's action, in the sense that the force of gravity between two mutually gravitating masses does not act instantaneously across their separation. How gravity is conveyed between two masses is still an area of fundamental research.

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

1000 E. University Ave. Laramie, WY 82071
UW Operators (307) 766-1121 | Contact Us | Download Adobe Reader

Accreditation | Virtual Tour | Emergency Preparedness | Employment at UW | Privacy Policy | Harassment & Discrimination | Accessibility Accessibility information icon