October 21, 2017

AstroEvent(s): Of Heliacal Risings and Slender Moons.

Can you see the ultra-thin crescent? (Photo by Author).

Dust off those Sothic cycle calendars again; the Dog Days of summer are upon us; yes, itís time this first week of August to try that feat that the Ancient Egyptians depended on, with a sighting of the dawn heliacal rising of the bright star, Sirius. [Read more...]

Event of the Week: Happy Winter Solstice!

Time again to reset those sundials... (Photo by Author).

Time again to reset those sundials... (Photo by Author).

Brace yourselves; the Winter Solstice is upon us this week. This is the point at which the Sun reaches its lowest southerly declination and begins its long march northward. This makes for shortened days and long nights in the northern hemisphere and the reverse in the southern. Of course, its not the Sun that’s moving, but the Earth with its 23 degree 26′ minute tilt that causes this variation. Several cultures mark this celestial turn of events, not the least of which is modern day Christmas, which is fixed on December 25th, the solstice date on the old Roman Calendar. Modern reform by Pope Gregory gave us an offset solstice that falls on or around the 21st each year, and will eventually move by one day every 3,000 years. The solstice is always a good time to check out any local chance alignments at sun rise or sunset, as well as note the length of shadows cast at local noon. The precise timing of the winter solstice this year is Monday, December 21st at 5:47 PM Universal Time. Merry Saturnalia/Christmas!

This week’s astro-term is the Chandler Wobble. This is one of the many complex movements of our planet that causes the complex motions of the Earth’s axis to shift slowly. But unlike larger effects such as our friend, the precession of the equinoxes, the Chandler Wobble is much more subtle. First discovered by Seth Carlo Chandler in 1891, this movement amounts to 0.7 arc seconds or about 15 meters of axial shift over a period of about 433 days. This wobble is caused by the “sloshing” motion of Earth’s fluid core and even the drag created by the friction of our oceans. Think of the Earth as a sort of egg with a liquid center dragging us about† as we orbit about the Sun. This amount can vary (it was greatest in 1910) and is enough that modern off the shelf GPS devices can measure it and must take it into account. It can also cause the poles, equator and lines of longitude and latitude to change perceptibly. Along with the drag created by our Moon and Sun, the Chandler Wobble is also responsible for variations in Delta T, causing an occasional tweaking of our clocks by the addition or subtraction of an occasional second!