Greetings stargazers.
Everything we know about things outside of our solar system is from the light we receive. I am using the term “light” very broadly to include all types of electromagnetic radiation, since visible light is just a small part of the entire spectrum. From very long wavelength radio waves to very short wavelength gamma rays, they all share similar characteristics and travel at the same speed through the vacuum of space.
Sometimes the light we receive from distant sources is polarized. There are no political connotations here – polarization simply means that the electric field components of one photon and the next are aligned in the same direction. A photon is the smallest possible packet of light. Polarized sunglasses let photons of just one orientation through and block those that happen to be perpendicular. If you have worn polarized sunglasses, you may have experienced the inability to read the numbers on a gas pump or see some computer monitors without turning your head just right. That is because light from those sources is polarized and your sunglasses preferentially block horizontally polarized light.
You can also try going outside on a clear sunny day with some polarized sunglasses. The sky looks bright because light from the sun is scattered off molecules of air. The light scattered 90 degrees away from the sun is partially polarized. When you turn your head so your sunglasses are parallel to the polarized sky it will appear brighter. When you rotate your head so your sunglasses are perpendicular to the light, the sky appears darker.
For a little background information, electromagnetic waves are called that because their energy is split evenly between an electric field component and a magnetic field component. Those fields are oscillating in directions perpendicular to each other and perpendicular to the direction the light wave is traveling. This is like a water wave, where the surface of the water is moving up and down, while the wave itself is moving horizontally. The theory describing this phenomenon was developed by James Clerk Maxwell and is one of the greatest accomplishments of 19th century physics.
When light is radiated from a thermal source, such as the surface of a star, the orientations of the electric fields between one photon and the next are completely random. When light hits and bounces off some material object like the surface of a lake, a grain of dust or a molecule of gas, there is a chance that some of the photons will have their electric fields aligned.
Studying the polarization of the scattered or reflected light can give clues about the material that is doing the scattering. Other astronomical sources of polarized light can give information about interstellar magnetic fields.
Useful links
The January full moon was last weekend, so the next two weeks surrounding the new moon are the best for having an extra dark sky for stargazing. It is more important to be aware of the moon phase for stargazing during the winter than it is during other times of the year. This is because the full moon is higher now than at any other time of year. This makes sense because the full moon is directly opposite the sun, which is at its lowest point during the winter. And if we ever get a layer of snow on the ground this year, the reflected light can make the nighttime very bright indeed.
Jupiter was at opposition Jan. 10, which was its closest approach to Earth this year. However, Jupiter is so far from the sun, that the difference between when it's extra close and extra far away is not typically noticeable. It will be due south and at its highest point in the sky at solar midnight, which is about 12:15 a.m. this month. The next three or four months will be the best time to look at Jupiter through a telescope.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Reach him at hakes_c@fortlewis.edu.
