THE SOUTHWORTH PLANETARIUM 70 Falmouth Street Portland,Maine 04103 (207) 780-4249 usm.maine.edu/planet 43.6667° N 70.2667° W Altitude: 10 feet below sea level Founded January 1970 2021-2022: XX "Heaven is under our feet as well as over our heads." -Henry David Thoreau THE DAILY ASTRONOMER Wednesday, September 29, 2021 Vertical Earth ________________ *Here's to you, JV!* ________________ __________________________________ *From the Department of Redundancy Department* Correction to yesterday's article,which included the following sentence: *First of all, if we could construct a scale-model map of the Milky Way that covered all of North America, our solar system would fit neatly into the solar system* Well. The sentence should have read *First of all, if we could construct a scale-model map of the Milky Way that covered all of North America, our solar system would fit neatly into a coffee cup.* A careless and clumsy error. However, in our defense, the terms "solar system" and "coffee cup" are easily confused. _____________________________________ We were initially going to begin today's article by instructing you to close your eyes. Then, we realized -or, actually, a more sensible person told us- that if your eyes were shut, you couldn't continue to read to obtain further instructions. So, please close one eye! While the opened eye remains focused on the dynamic world around you, the closed eye should be looking into the infinite catacombs of alternate Universes. That is one particularly delightful feature of your remarkable brain which, we needn't remind you, is the most wondrously complex object in the cosmos.* At thought speed, one can modify any part of the cosmos and then work out how that modification might alter physical reality. Scientists refer to this delightful form of mind play as a "thought experiment." We'll enjoy a thought experiment today by addressing a question sent to us by long-time subscriber *JV:* *"Have any astronomers or earth scientists addressed the question of what Earth’s weather would be like if it did not have its tilt? Would the Northern and Southern Hemispheres have, based on latitude, the same temperature year round?"* Although this splendid question opens pores in the impermeable membrane separating astronomy and meteorology, we shall do our best to address the issue. First, a quick reminder that Earth is tilted by 23.5 degrees relative to the vertical, or to the ecliptic plane, defined as the plane of Earth's orbit around the Sun. [image: diagram-showing-effect-earth-axis-illustration-171149103.jpg] As Earth revolves around the Sun, the poles continuously change orientations relative to it. On the summer (June) solstice, the north pole is aligned as close to the Sun as possible while the south pole is pointed away from it. The reverse situation occurs on the winter (December) solstice. Neither pole is directed toward the Sun on the two equinoxes, spring (March) and autumn (September). If Earth were not tilted relative to its orbital plane, we'd experience an equinox every single day of the year. How would that change our weather and sky? This is where we can combine our collective scientific knowledge and imaginations to work out the ramifications. If Earth had no tilt, or *obliquity*: - The Sun would always rise due east and set due west - The Sun would always be directly overhead at the equator - Our day length would be close to, but not exactly, 12 hours - The Sun would remain visible along the horizon at the north and south poles forever. The stars would never be visible there or within a 6 - 8 degree region away from either pole due to twilight effects. Those are a few of the astronomical ramifications. The meteorological consequences would be profound. First, we wouldn't experience the same seasonal variations we now all enjoy. The Sun would neither ascend to the summer heights nor plunge into the winter lows as it does now. The prevalent conditions at any given location would indeed depend on latitude. The tropics, defined as the region between 23.5 degrees N and 23.5 degrees S would still remain quite hot. The tropics are so warm because the Sun's angle is sufficiently high so as to heat the surface quite efficiently. Even though the Sun could only occupy the zenith at the equator, the noon time sun at either tropic would attain an altitude of 66.5 degrees, slightly less than the Sun's noon time altitude in Portland on the June solstice. Moreover, the Sun would reach that same height at both tropics every day. Also, the Sun's noon altitude at latitudes closer to the equator would be ever higher. The tropics will still remain extremely hot and enchanting. Regions farther to the north and south would become progressively cooler with increasing distance from the tropics. However, here at the mid latitudes we'd experience little change in temperature. We know that in the real world, the spring equinox is generally colder than the autumnal equinox because the former equinox follows winter while the latter one is preceded by summer. Were Earth not tilted, the temperature would be somewhere between that of the frigid March equinox and the much warmer September equinox. We would still experience weather, however. Although weather system dynamics is complexity squared,cubed and inscribed in hieroglyphics, all weather is ultimately driven by one phenomenon: the planet attempting to reach a state of thermal equilibrium. While the tropics receive an abundance of heat energy, the poles suffer from an utter lack of it. Consequently, some of that tropical heat will migrate to cooler regions. This heat exchange helps to produce convection cells and drive ocean currents. So, Earth wouldn't become a meteorologically stagnant planet. Just a boring one for most of us. A meteorologist would be able to offer a far more detailed answer because there are most assuredly aspects that I have missed. However, I hope the answer was sensible. We should end by mentioning that Earth's obliquity doesn't remain constant. It slowly vacillates between 22.1 degrees and 24.5 degrees over a 40,000 year period. Earth's obliquity reached its maximum value approximately 8,000 years ago and has been decreasing ever since. The tilt will be at a minimum in about 12,000 years. These axial tilt variations are important factors in climatic Milankovitch cycles, a brain-liquefying concept that we'll cheerfully avoid. [image: obliquity.jpg] *Well, yes, we acknowledge that the Universe likely teems with an incalculable number of life forms. It is also quite likely that many species have attained logarithmically higher levels of both technological and intellectual sophistication. However, until those exalted beings arrive at our shores to expose themselves, we will safely assume that the human brain sits comfortably atop the complexity pyramid. To subscribe or unsubscribe from the Daily Astronomer: https://lists.maine.edu/cgi-bin/wa?SUBED1=DAILY-ASTRONOMER&A= <https://lists.maine.edu/cgi-bin/wa?SUBED1=DAILY-ASTRONOMER&A=1>