March 2013

Tony Martinez

                                  Tony Martinez

Each month I will describe sights of interest in the night skies of South Carolina. These sights will be broken down into three sections; what you can see with the naked eye, with binoculars, and with a small telescope. The best time to view the night sky is at and around the times when the Moon is not visible, what is known as a New Moon; which will occur this month on March 11th. For March, your best viewing nights will be from March 1st to the 15th. The Star chart below is set for Florence, SC on March 15th at 9 pm.


Good Comet, Bad Comet?  

This is the year of the two comets, and we hope that one or both will be exciting. This month we await the first comet, Comet Pan-STARRS.  The comet was named after the telescope group called, Panoramic Survey Telescope and Rapid Response System. If it is a “good comet,” we should begin to see it in the west after sunset on March 6th to the 8th, and it may be visible to the naked eye for several days after that.

A comet is often described as a giant frozen snowball several miles across, containing water, gases and rocks. It is believed that comets are objects leftover after the formation of our solar system, and reside in an area orbiting the Sun beyond the dwarf planet Pluto. This area contains two regions known as the Kuiper Belt and the Oort cloud, shown below.


Note, the symbols AU on this image mean astronomical units. One AU is the distance from the Earth to the Sun; 93 million miles. Therefore, the Oort Cloud is 93,000,000 X 55,000 miles away!

The Kuiper Belt is like the asteroid belt in that it lies in the plane of the planets in our solar system. It is unlike the asteroid belt because it contains mainly snowball-like objects, and not rocky asteroids. It also contains three dwarf planets; Pluto, Haumea, and Makemake.

The above image does not give an accurate representation of the Oort Cloud. The Oort Cloud is not a belt of objects, but a sphere. A better image is as shown below.


If a comet’s orbit is near the plane of our solar system, then it is likely that it originated from the Kuiper Belt. If a comet’s orbit is well out of the plane of the solar system, then it originated from the Oort Cloud.  Comet Pan-STARRS’s out of plane orbit proves that it came from the Oort Cloud. In addition, Pan-STARRS is traveling in a parabolic path around the Sun; see image below.


Comets traveling in elliptical orbits are periodic comets, which return to the Sun many times. A periodic comet may come back every few years, in less than one hundred years such as Comet Halley, or can be like Comet Hale-Bopp which appeared in 1997, but won’t return for over 4000 years. Parabolic comets only pass by the Sun once.

Let’s return to the origin of the comets. As leftovers from the formation of our solar system, comets always resided in this distant place never feeling the heat of the Sun. Periodically, some unknown object hits or disturbs their orbit, and they “fall” toward the Sun. As they hurdle past the orbit of Jupiter, for the first time in 3-4 billion years they feel; heat! Once exposed to heat, comets now start to boil away gases such as carbon dioxide. As the journey toward the Sun continues additional gases and then water-ice starts to boil off.  Actually the ice does not boil away; instead it is converted directly from solid ice into gas, a process known as sublimation. As this sublimation continues, solid debris imbedded in the ice begins to crumble off, and once the comet passes by the Earth’s orbit, and approaches its closest approach to the Sun, called perihelion, the comet has now reached its most unstable state. The heat and energy of the Sun can tear the comet apart, but the most impressive effect is the comet can form a huge trail of rock, ice and gases millions of mile long; which we call the comet’s tail. An interesting property of the tail is that it always points away from the Sun. Outer space is devoid of almost all matter, so as the comet moves in its orbit, there is no friction from space itself, so the force of the solar wind can easily blow the comet’s debris away from the Sun.


Also note that most comets have two tails. The really bright one, called the dust tail, curves away from the Sun just as you would expect. The second is the gas tail. The gas tail is not as bright, and the gas molecules have been ionized by the Sun. These ionized gases are charged, and the magnetic field of the Sun forces them to always point directly away from the Sun much like iron filings near a magnet. The blue tail below is the gas tail. It is quite common that the faint gas may not be readily visible to the naked eye.

Gas tail

So let’s hope for a “good” comet this month. Since this is the first time Pan-STARRS has visited the Sun we may have a surprise cometary display, no naked eye display, or the Sun may tear it apart.

Naked Eye Sights: Comet Pan-STARRS, we hope, beginning about March 6th in the west after sunset.

Binocular Sights (7 to 10 power): Comet Pan-STARRS

Telescope Sights (60-100mm): Comet Pan-STARRS, best viewed at wide field and at low power through a reflecting telescope.

Experiment to try: To “see” ice sublime; changing directly from solid ice to a gas, fill an ice tray to the top with water, and place it out of the way in the back of your freezer.  Check the ice tray again when Comet ISON arrives at the end of November, and you will see how much ice sublimed in seven months.

See you next month!


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