March 24, 2019

AstroEvent: The Autumnal Equinox.

The Balance between the forces of darkness and light are restored on Thursday in the form of the†autumnal equinox. This is the point that the position of the Sun along the ecliptic intersects the equator, causing the length of daylight and darkness to be approximately equal from pole to pole. This can vary slightly in reality, owing to respective positions across time zones, the sun fast, and the equation of time. Keep in mind; like the Full or New Moon, the equinox is also a precise moment in time. From here on in until the December solstice, nights will get shorter in the northern hemisphere and days will get longer in the southern. Catch that sunrise or sunset today, and you are looking exactly at the point which is due east or due west of your locale, respectively.†The precise moment of equinoctial crossing is September, 23rd, 03:09 universal. Geosynchronous satellites also tend to experience disruption on days leading up to and proceeding the equinoxes, as the radio noisy Sun passes directly behind them for a period each day. Bring on the darkness!

The astroword of the week is Milankovitch Cycles. Every Earth Science student learns that this combination of orbital and axial variations is connected to long term climate change on the Earth. First postulated in 1920 by Milutin Milankovic, Milankovitch cycles comprise changes in the Earthís axial tilt from 22.1 to 24.5 degrees, the precession of the pole over 26,000 years, changes in eccentricity from 0.005 to 0.028, and apsidal precession of the Earthís orbit over 25,000+ years. This can lead to mild seasonal variations when these aspect ratios are at a low value (i.e. say a low eccentricity, low value axial tilt, and perihelion/aphelion coinciding with the equinoxes), to wide seasonal variations when these perimeters increase. Presently, aphelion falls in July during northern hemisphere summer, eccentricity is at a value of 0.017 (about median) and axial tilt is 23.44 degrees (a dash on the high side) and decreasing. This should set us up for ameliorated temps in the northern hemisphere and harsher summer and winter climates in the southern hemisphere, that is, IF the effects of anthropomorphic climate change and global dimming arenít taken into account. These recently introduced effects stand out drastically against the longer climate trends, and one fears that any natural safety valves such as global dimming may mask the true impacts of greenhouse gas emissions until itís too late. †Currently, northern hemisphere fall is one of the shorter seasons, clocking in at not quite 90 days. †Itís a complex world out there, and understanding the interplay between Milankovitch cycles and the impact of human activity will go a long way in our development as a mature, forward planning species.

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