December 18, 2017

AstroEvent: The Autumnal Equinox.

An African Sunset. (Credit: Art Explosion).
An African Sunset. (Credit: Art Explosion).

 

   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. [Read more...]

14.04.10: Milankovitch Cycles…On Titan?

NASA’s Cassini spacecraft has revealed an elusive mystery on the surface of Titan; namely, why does the northern hemisphere of the large moon contain numerous lake basins, while in the south they’re relatively scarce? Now, scientists at Caltech working with JPL think they may have an answer. These lakes show up as bright (empty) and dark (filled) patches as the Cassini spacecraft pings them with its Synthetic Aperture Radar (SAR). Of course, on Titan, the hydrologic chemical of choice is liquid ethane and methane, and it is thought that some transport mechanism results in a net flow imbalance between the two hemispheres. Seasons on Titan last roughly 15 years as it dances around Saturn in its 29.5 year orbit about the Sun. But simple seasonal drainage of about a meter per year couldn’t empty the 100 meter-plus deep basins in a single season. This also doesn’t account for the overall disparity in number of basins seen, both filled and unfilled. Instead, scientists point towards the eccentricity of Saturn’s orbit as the possible cause. Saturn’s eccentricity is 0.055, or a little over 5% deviation from a perfect circle. This would make for periodic inequalities in the seasons, much like what occurs on Earth. For example, the perihelion of Earth actually occurs in northern hemisphere winter, somewhat ameliorating the severity of the seasons. But the variation of eccentricity coupled with the obliquity of the planetary spin axis and the precession of the equinoxes can vary over geologic time scales, causing variations in the climate. This is known as the Milankovitch cycle, and is thought to be a major contributing factor to the onset of Ice Ages. On Titan, a similar process is thought to occur, resulting in a net imbalance over thousands of years in the methane flow cycles between the two hemispheres. We may now simply be observing Titan during an epoch when seasonal methane pooling favors the northern hemisphere. Whatever the case, Titan is proving to be a fascinating and changing world deserving of further scrutiny.