By Andy Veh, for the Redoubt Reporter
The starry sky is now at its best with the most prominent stars being well-placed high in the south — blue Rigel and red Betelgeuse in Orion, Sirius beneath it, Procyon to its left, Pollux and Castor higher up, Capella almost in the Zenith, and Aldebaran and the Pleiades completing the splendor. The Big Dipper starts out close on the Northern horizon, but Cassiopeia, Perseus and Andromeda are close to the zenith.
In the east, Cygnus and Pegasus are about to set while bright Vega, being circumpolar in Alaska, stays close to the horizon. Leo’s Regulus rises in the evening, trailing Gemini and Cancer low in the east.
Several planets are visible in the evening and all night. You can’t miss bright Jupiter as it rises in the northeast in the early evening, moving through the south and setting in the northwest when school starts in the morning. This winter it appears in Gemini, making a nice triangle with Pollux and its fainter twin, Castor.
Jupiter is in retrograde motion this month (an optical illusion caused by Earth’s orbit), seemingly moving from Pollux to the fainter Alhena (one of Gemini’s feet) to its right, being smack in the middle of the twins’ heads on Jupiter’s left and their feet early in the month. That motion continues through January and February. In March and April we will have passed Jupiter enough that we can see its regular motion, back toward Pollux. Look for the full moon right next to the giant planet Jan. 14.
I apologize for not mentioning Venus in last month’s column. I erroneously thought it was too close to the horizon — and then was surprised to see it sitting nicely in the southwest early evening sky when driving through town. I’m sure that you saw it, too, despite me goofing. It will appear there for a few more days, then get too close to the sun and set simultaneously with it starting around Jan. 10. It might be visible again in late January in the southeast shortly before sunrise, joined by a very crescent moon Jan. 28.
Uranus and Neptune can still be seen in the evening but they require finder charts. I recommend looking them up on the Internet.
In the morning, Mars rises in the east around 1 a.m., gaining altitude during the night and shining prominently reddish in the south near Virgo’s Spica by the time we get our kids to school. The rusty planet and the bluish star appear close to each other starting mid-January. The third-quarter moon joins the two Jan. 23.
Spica appears as the 15th brightest star as seen from our solar system, a blue giant 260 light years away, 10 times more massive and 12,000 times brighter than our sun, with a surface temperature of 40,000 degrees Fahrenheit. After it has fused hydrogen to helium, then helium to carbon, then carbon and helium to oxygen, neon, magnesium all the way to iron, for about 30 million years, it will probably explode as a supernova.
Saturn rises around 5 a.m. in the southeast. It is just left of Mars and Spica. These three and red giant Arcturus high in the south form a large triangle. The third-quarter moon joins Saturn Jan. 23.
Earth is at perihelion on its closest approach to the sun Jan. 4 at 91.4 million miles, but only 3 percent closer compared to its aphelion, its farthest distance from the sun, July 5, at 94.5 million miles, which means that about 6 percent more of the sun’s energy reaches Earth.
That in itself is not the reason for the seasons. After all, perihelion and aphelion occur during the wrong seasons on the northern hemisphere, the maximum difference of 6 percent in solar radiation is way too small to account for the large temperature fluctuations between the seasons and the varying distance doesn’t explain the variation in the lengths of the days and the altitudes of the sun above the horizon.
Due to the 23-degree tilt of the Earth’s axis to its orbital plane, the length of day (which at its extreme is 70 percent shorter in winter than in summer — 5.7 hours compared to 19 hours) and the angle of the sun above the horizon are much more important. On the Kenai, the noon sun is only 7 degrees above the southern horizon in January, versus 53 degrees in July. These angles are important in how much energy is actually absorbed. Due to that large difference, any given ground surface area receives 85 percent less energy.
For example, on a winter day around noon, only 8 watts per square foot of solar radiation may reach the ground on the Kenai Peninsula. In contrast, on a summer day around noon, we may receive 49 watts per square feet of solar radiation. (This is computed as the above minus 85 percent, plus 6 percent). The latter, combined with the length of day, yields an insolation — short for in-coming solar radiation — of 54 watt-hours per square feet per day in the middle of winter, versus 1,070 watt-hours per square feet per day in the middle of summer (see http://www.applet-magic.com/insolation.htm for more information). This compares to 760 watt-hours per square feet per day, averaged for the entire Earth, in any season, of energy radiated by Earth into space.
We can see why the temperatures are decreasing during winter and increasing during summer. In contrast, while the effect of the varying distance to the sun on the temperature in a location is rather insignificant, it may affect seasons slightly, making southern hemisphere’s summers slightly warmer. However, that’s hard to determine because the topography and the distribution of landmass and ocean between the two hemispheres is so different.
Andy Veh is an associate professor of physics, math and astronomy at Kenai Peninsula College.