By Andy Veh, for the Redoubt Reporter
The starry sky is now at its best with the most prominent stars being easily visible 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 west, 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.
Of the planets at night, only Jupiter is visible, next to Taurus’ red giant Aldebaran and with the star cluster Pleiades nearby. It appears all night long, moving from southeast to the northwest throughout the night, setting just prior to dawn.
Jupiter is joined by the waxing gibbous moon Jan. 21. Seen from South America, the moon even covers the giant planet on that date (this is called an occultation).
Jupiter is now the brightest wanderer in the sky because Venus is in superior conjunction (on the other side of the sun) and will not be visible again until fall of this year. Once it emerges from behind the sun, it’s already late spring and the evenings are too bright for too long to see it.
Saturn is prominent in the morning sky all winter long, forming an acute triangle with Spica and red Arcturus. Look for them near the southeastern horizon. The waning crescent moon was close on Jan. 6 and 7.
All other planets — Mercury, Mars, Uranus and Neptune — appear too close to the sun, so that they can’t be seen against the bright daytime sky.
Earth is at perihelion, on its closest approach to the sun Jan. 2 (91.4 million miles), but only by 3 percent closer compared to its aphelion, its farthest distance from the sun on July 4 (94.5 million miles). That means that about 6 percent more of the sun’s energy reaches Earth.
The distance’s effect on temperature is rather insignificant. Much more important is the length of day, which at its extreme is 70 percent shorter in winter than in summer (5.7 hours compared to 19 hours). Also more important is the angle of the sun above the horizon. On the Kenai Peninsula, the noon sun is only at 7 degrees above the southern horizon in January, versus 53 degrees in July. These angles are important for 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 foot of solar radiation (this is computed as the above minus 85 percent and plus 6 percent).
Andy Veh is an associate professor of physics, math and astronomy at Kenai Peninsula College.