THE SOUTHWORTH PLANETARIUM 207-780-4249 www.usm.maine.edu/planet 70 Falmouth Street Portland, Maine 04103 43.6667° N, 70.2667° W Founded January 1970 "This sentence no verb." THE DAILY ASTRONOMER Wednesday, February 3, 2016 The Astronomy of Winter Yesterday, we marked an important annual milestone here in the far flung hinterlands. We passed the "coldest day" and now are progressing glacier-like toward the warmer season. Perhaps we should explain. Every day has its own high and low temperatures. These temperatures can vary widely from year to year. For instance, Jan 4, 2017 could be bitterly cold, while Jan 4, 2018 could be mild and pleasant. However, when the temperatures are averaged over many decades, each day will have its own average high and low temperatures. A graph of these temperatures over a year describes a beautifully smooth undulating curve which reaches its maximum around August 2 and a minimum around February 2. Both of these dates are about six weeks after the summer and winter solstices, respectively. The coldest day, on average, is February 2 and so now the average will increase until August 2nd. Granted, February can still be a frigidly cold month, just as August can be stiflingly hot. This momentous occasion gives us the opportunity to discuss the astronomy of weather: why we experience cold weather now and will, in theory, experience hot weather six months from now. Our planet Earth is tilted on its axis by 23.5 degrees, a value known as its "obliquity." As the planet revolves around the Sun, its poles are constantly changing their orientation relative to it. Around the summer solstice, the North Pole is aligned toward the Sun as closely as possible. Six months later, during the winter solstice, the North Pole is aligned away from the Sun. Consequently, in the winter, the Sun is low in our sky; in the summer, its altitude is higher. This changing altitude explains why we have more daylight in June than we have in December. However, the seasonal changes are not merely the result of the daylight duration difference. If daylight duration were alone responsible for the heating, the North Pole would be a roiling furnace between March and September, when the Sun is always visible there. We know that even around the solstice, the North Pole remains quite cool. What causes our seasonal temperature change is the atmosphere, or, more precisely, atmospheric absorption. When the Sun's angle above the horizon is low, its radiant energy passes through a great amount of atmospheric gases. When the Sun is higher, its heat energy s less impeded. After all, there is forty times the amount of atmospheric gases between you and the horizon as exists between you and the zenith (point directly overhead). In winter, the Sun never attains a high altitude and its heat is blocked by the thicker atmosphere close to the horizon. In summer, the Sun's angle is much higher and more of it copious heat flow penetrates to the ground which, when warmed, then imparts its heat onto the air above it. We know, of course, that we can have unseasonably warm periods in deep winter and despicably cold days in mid summer. This is because weather is a complex, sub-chaotic mess. All weather results from Earth attempting to reach a state of thermal equilibrium. Our planet is doing precisely what a cup of hot tea does: it moves the heat energy from excess to deficiency so that everything will be balanced between it and its surroundings. The difference is that the tea will eventually achieve thermal equilibrium once the heat source is removed from it. Earth is constantly receiving prodigious amounts of thermal energy and therefore our world will not achieve a state of thermal equilibrium until the Sun stops imparting energy onto it billions of years from now. Not, of course, that we're complaining.